A Professional’s Guide to Durable Home Design

Durability

byDesign2nd Edition

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A Professional’s Guide to Durable Home Design

Durability

byDesign2nd Edition

Prepared for

U.S. Department of Housing and Urban Development

Washington, DC 

Prepared by

Newport Partners

ARES Consulting

October 2015
Durability by Design

Acknowledgments 

This updated guide was written by Newport Partners and ARES Consulting, with support from the U.S. 

Department of Housing and Urban Development (HUD). The authors especially acknowledge the 

contributions of Dana Bres, P.E., of HUD, who provided thoughtful direction and feedback throughout 

the development of this guide. We also gratefully acknowledge these reviewers who offered guidance 

and insights to make this document more useful to the building industry: 

 John Jones, Building Performance Institute 

 Timothy A. Reinhold, Insurance Institute for Business & Home Safety 

 Gary J. Ehrlich, P.E., National Association of Home Builders 

 Abby Davidson, Qualtim, Inc. 

 Jack Armstrong, Structural Insulated Panel Association 

Finally, we wish to acknowledge both the U.S. Department of Energy’s Building America program and 

the U.S. Environmental Protection Agency’s ENERGY STAR Homes program, which both served as 

sources for current information on building durability and building science. 

Disclaimer 

The material presented in this report represents the views of the authors and does not necessarily 

reflect the views or policies of the U.S. Department of Housing and Urban Development or the U.S. 

Government. While the information in this document is believed to be accurate, neither the authors, 

nor reviewers, nor the U.S. Department of Housing and Urban Development, nor Newport Partners or 

ARES Consulting, nor any of their employees or representatives makes any warranty, guarantee, or 

representation, expressed or implied, with respect to the accuracy, effectiveness, appropriateness, or 

usefulness of any information, method, design, or material in this document, nor assumes any liability 

for the use of any information, methods, designs, or materials disclosed herein, or for damages arising 

from such use. The document’s content is not intended to be a substitute for professional design advice 

or product specification. Always seek the advice of a qualified design professional (architect or 

professional engineer) with any questions you may have regarding durable design for homes. Further, 

any products and building systems described in this guide are included only as examples of available 

choices. No endorsement or recommendation of these products or their use is given or implied. 
Durability by Design

Foreword 

America’s path to sustainable communities of quality, affordable homes rests upon durability. In the 

past 10 years, the methods and materials used to construct homes have changed remarkably, requiring 

designs that allow for those changes to contribute to the durability of the home from the foundation to 

the roof. These changes are part of a broader shift to more efficient and sustainable homes. While 

sustainability is important, the efficiency and environmental benefits of these homes will not be realized 

without design and construction that address moisture, insects, and other natural hazards that can 

undermine home quality and livability over time. 

The need for forward-thinking design and construction is why this updated publication, Durability by 

Design, is so timely and important. First published by the U.S. Department of Housing and Urban 

Development (HUD) in 2002, this guide has consistently been one of the Department’s most popular 

publications. With this updated guide, HUD is offering new and refined guidance for designing durable 

homes for today’s housing industry—addressing critical topics, including water vapor management, 

envelope design, and natural hazards. Although the primary audience for Durability by Design is 

designers and builders, the benefits of the topics reach families and communities throughout the 

country. Building more durable homes also means providing more comfortable, affordable, efficient, 

and sustainable homes for America’s families. 

Katherine O’Regan, Ph.D. 

Assistant Secretary for Policy Development 

and Research 
Durability by Design ix

Contents 

CHAPTER 1—INTRODUCTION .......................................................................................................................1

1.1 General ............................................................................................................................................1

1.2 Integrated Design—Making Durability Part of the Process ............................................................2

1.3 Guide Overview ...............................................................................................................................4

CHAPTER 2—DURABILITY CONCEPTS............................................................................................................6

2.1 General ............................................................................................................................................6

2.2 Durability Defined ...........................................................................................................................8

2.3 Common Durability Issues ...............................................................................................................9

CHAPTER 3—Ground and Surface Water....................................................................................................13

3.1 General ..........................................................................................................................................13

3.2 Recommended Practices ...............................................................................................................13

3.3 Additional Resources .....................................................................................................................21

Chapter 4—Rain and Water Vapor .............................................................................................................22

4.1 General ..........................................................................................................................................22

4.2 Recommended Practices for Rainwater Control ...........................................................................23

4.3 Recommended Practices for Water Vapor Management .............................................................55

4.4 Additional Resources .....................................................................................................................75

Chapter 5—HVAC and Plumbing .................................................................................................................77

5.1 General ..........................................................................................................................................77

5.2 Recommended Practices—HVAC ..................................................................................................78

5.3 Recommended Practices—Plumbing ............................................................................................91

5.4 Additional Resources .....................................................................................................................93

Chapter 6—Sunlight ....................................................................................................................................94

6.1 General ..........................................................................................................................................94

6.2 Recommended Practices ...............................................................................................................95

6.3 Additional Resources ...................................................................................................................101

CHAPTER 7—Insects ..................................................................................................................................102

7.1 General ........................................................................................................................................102

7.2 Recommended Practices .............................................................................................................104

CHAPTER 8—Decay and Corrosion ...........................................................................................................109

8.1 General ........................................................................................................................................109
x Durability by Design

8.2 Recommended Practices .............................................................................................................109

8.3 Additional Resources ...................................................................................................................113

CHAPTER 9—NATURAL HAZARDS .............................................................................................................114

9.1 General ........................................................................................................................................114

9.2 Recommended Practices—High Wind Regions ...........................................................................115

9.3 Recommended Practices—Flood Prone Regions ........................................................................118

9.4 Recommended Practices—Seismic Regions ................................................................................121

9.5 Recommended Practices—Wind-Driven Rain Areas ...................................................................122

9.6 Recommended Practices—Hail Prone Areas...............................................................................123

9.7 Recommended Practices—Wildfire Regions ...............................................................................124

9.8 Additional Resources ...................................................................................................................125

APPENDIX A—BUILDER/DESIGNER DURABILITY CHECKLIST .....................................................................127
Durability by Design xi

List of Figures

Figure 1–1: The Web of Durability ................................................................................................................3

Figure 2–1: Soil Grade Slopes Towards the Foundation, Causing Water to Pool.........................................9

Figure 2–2: Non-existent Flashing Around Window, Practically Inviting Water Intrusion .........................10

Figure 3–1: Bore Hole and Bore Log ...........................................................................................................15

Figure 3–2: Example Site Grading and Drainage Plan.................................................................................16

Figure 3–3: Concrete Flatwork Settlement.................................................................................................17

Figure 3–4: Basement Construction and Optional Enhancements for Wet Site Conditions......................19

Figure 4–1: Roof Overhangs........................................................................................................................25

Figure 4–2: Decay Hazard Index Map .........................................................................................................25

Figure 4–3: Typical Roof Drainage Problems to Avoid................................................................................26

Figure 4–4: Roof Ventilation Configurations ..............................................................................................32

Figure 4–5: Cross Section of an Eave Ice Dam ............................................................................................34

Figure 4–6: Example of a Severe Eave Ice Dam ..........................................................................................34

Figure 4–7: Eave Ice Dam Flashing..............................................................................................................37

Figure 4–8: Rainfall Intensity Map of the United States.............................................................................39

Figure 4–9: Gutter Design Example ............................................................................................................40

Figure 4–10: Wind-driven Rain Map of the United States..........................................................................46

Figure 4–11: Typical Locations Where Flashing Details Are Required........................................................51

Figure 4–12: Step and Kick Out Flashing.....................................................................................................51

Figure 4–13: Window Flashing Sequence on a Wall With Exterior Foam Insulation..................................52

Figure 4–14: Deck Ledger Flashing Detail ...................................................................................................53

Figure 4–15: The International Energy Conservation Code Climate Zone Map .........................................57

Figure 4–16: Examples of Exterior and Interior Continuous Air Barrier Strategies....................................59

Figure 4–17: Air Leakage Points Requiring Special Attention for a Continuous Air Barrier Installation ....60

Figure 4–18: Water Vapor Control “Triangle” and Two Strategies for a Cold Climate Application ...........62

Figure 4–19: Deformed Window and Ruptured Caulking due to Shrinkage of Wood Framing .................72

Figure 4–20: Water Streaming From Cavity Insulation as it is “Wrung Out”..............................................73

Figure 4–21: Example of Unacceptable Construction Material Storage.....................................................74

Figure 5–1: Impacts of Incorrect Inputs for a Loading Sizing Calculation...................................................79

Figure 5–2: Conditioned Attic .....................................................................................................................80

Figure 5–3: Example of Duct Joint Which Should be Air-Sealed.................................................................82

Figure 5–4: Whole-House Ventilation System Types..................................................................................86

Figure 5–5: Right and Wrong Exhaust Duct Installations............................................................................87

Figure 5–6: International Energy Conservation Code Climate Zones Map.................................................89

Figure 5–7: Spray Foam Insulation Protecting Pipes Installed in an Exterior Wall .....................................92
xii Durability by Design

Figure 6–1: Yearly Average Solar Irradiation Map ......................................................................................95

Figure 6–2: Building Latitude and its Impact on Overhang Effectiveness ..................................................96

Figure 6–3: Effect of Surface Coloration on Solar Heat Gain......................................................................98

Figure 7–1: Termite Probability (Hazard) Map .........................................................................................103

Figure 7–2: Use of Termite Shields...........................................................................................................107

Figure 7–3: Use of Concrete as a Termite Barrier.....................................................................................107

Figure 8–1: Details to Separate Wood from Ground Moisture ................................................................110

Figure 9–1: Examples of Roof and Building Damage From High Winds ...................................................115

Figure 9–2: Ring Shank Nail.......................................................................................................................116

Figure 9–3: Roof Pitches and Their Implications for Wind Forces............................................................116

Figure 9–4: Examples of Roof Covering Loss ............................................................................................117

Figure 9–5: Sheathing Seams Covered With Bituminous Tape...................................................................117

Figure 9–6: Diagram of Flood Zones and Base Flood Elevation................................................................119

Figure 9–7: Sump Pump Discharge Right Next to Foundation..................................................................120

Figure 9–8: Continuous Load Path Diagram .............................................................................................122
Durability by Design 1

CHAPTER 1—INTRODUCTION 

1.1 General 

A sustainable future for America requires sustainable 

buildings. And sustainable buildings must be durable 

buildings. And while the mention of the word 

“sustainability” is usually a trigger for designers and 

builders to think about energy efficiency and green 

building materials, building durability cannot be 

overlooked as a critical pillar of sustainability. Simply put, 

a home with a fantastic thermal envelope and high 

efficiency mechanical systems which is also riddled with 

prematurely failing building materials and systems is NOT

green or sustainable. Green and sustainable homes must 

be durable homes. 

But what do we really mean by the goal of a “durable” 

building? For this guide, “durability” is defined as the 

ability of a material, system, or building to maintain its 

intended function for its intended life-expectancy with 

intended levels of maintenance in intended conditions of 

use. Obviously this definition may take on different 

meanings for different groups (e.g., builders, 

homeowners, manufacturers), demonstrating that 

communication and education are also key aspects that 

affect durability. 

Addressing durability shouldn’t be a pursuit of extremes, 

but rather a cost-effectiveness strategy for both initial 

and longer-term (i.e., maintenance, replacement) costs. 

Designing a home to be “ultra” durable can add so much 

cost it makes the home unaffordable. Erring in the other 

direction can result in poor performance and loss of 

business reputation—including homeowner complaints, 

unsafe or unhealthy living conditions, and excessive 

maintenance and repair costs. 

Why Is Durability So Important?

 Avoidance of short-term durability or 

performance problems (i.e., callbacks) is

important to the builder’s and designer’s 

reputation, risk management, and business 

profitability. 

 The long-term durability of a home is important 

to retaining its investment value as well as its 

continued function as a safe, healthy, and 

aesthetic living environment. 

 Sustainability policies and programs focused on 

energy efficiency or green homes are wasted 

efforts if these homes aren’t durable. 

There’s nothing green about a home which ends up like 

this. Image Source: U.S. HUD 2005. New Orleans, LA.

 Poor durability increases the operating and 

maintenance cost of home ownership.

 Failure to meet reasonable expectations for 

durability increases liability exposure.

 People don’t like maintenance (i.e., high 

durability and low maintenance are important 

sales and purchasing factors).

 New products designed or installed without 

adequately considering durability can 

prematurely fail, leading to both customer 

dissatisfaction and manufacturer losses.
2 Durability by Design

But, “you get what you pay for!” Right? While this may often be the case, it’s important to realize that 

there are many design and construction practices that have zero or minimal construction cost impacts, 

while offering significant durability benefits. These benefits may be measured in terms of maintenance, 

repair, general function of the home and its component parts over time, enhanced business reputation, 

and customer satisfaction. Moreover, many such practices are well-known and don’t need to be reinvented, but only communicated effectively to the builder, designer, and consumer. 

This guide strives to reinforce both “tried and true” durability practices which apply today just like they did 

a generation ago (e.g., best practices for gutter sizing), along with measures that address the housing 

industry’s rapid evolution in terms of materials and construction practices (e.g., dew point management in 

high R-value walls). In both cases, the guide aims to focus on practical solutions to significant, recurring 

durability problems. 

1.2 Integrated Design—Making Durability Part of the Process 

Traditional homebuilding has had an approach similar an Olympic relay, where the baton is passed from 

one runner to the next until the finish line is crossed. With homebuilding, this used to be the method 

favored by most to accomplish the task of turning over a home to a new homeowner. With design and 

construction of homes becoming more complex, integrated design has quickly moved from a good 

practice to a necessity for homebuilders. 

Why? Like it or not, new homes today function as a system of interconnected parts. Driven in large part by 

energy requirements found in building codes, homes today simply “work” differently from they did before. 

For example, the air sealing and insulation levels of the building envelope greatly affect the HVAC design, 

as well as the potential for a small, chronic window leak to actually dry out. The strategy used to meet wall 

insulation requirements also affects the weather resistant barrier (WRB) selection; window flashing, air 

sealing, wall bracing methods; and even the potential for condensation to form in the wall. As Figure 1–1 

illustrates, a home’s durability is intricately wrapped up in these interconnected systems. To some extent 

these systems have always had connections. The difference is that in today’s new homes the “web” is 

woven much more tightly, and a change in one system “pulls” directly on other parts of the home. 
Durability by Design 3

Figure 1–1: The Web of Durability 

At the core of durability are three of the major systems in a house: envelope, structure, and mechanicals. Each of 

these affects the other. On the periphery are the forces that can also affect each of these core systems. Builders 

and designers must recognize that each decision made affects (and 

should inform) many other systems or decisions. 

Integrative design can bring order and predictability to this web. 

What does this mean to the builder? Providing leadership and 

facilitating effective communications. Similar to assembling a 

design team, assembling a construction team of like-minded 

individuals or companies interested in providing a superior product 

to the consumer would be one of the first and foremost tasks. 

Many integrated design issues which 

cascade across different trade 

partners are highlighted throughout 

this guide. Why? Because they can 

have a big impact on a home’s 

durability! Look for text boxes labeled 

“Integrated Design and Construction” 

throughout the guide.
4 Durability by Design

The builder gets the dialogue started before the project starts. As part of this dialogue the 

interconnections we see in Figure 1–1 are discussed, and implications for the contractors’ roles and work 

scopes are identified. The cast of trade partners involved in this process should reflect the project’s goals. 

For example, in a residential project with the objective of excellent building durability and a high level of 

energy efficiency, these goals necessarily involve trades like foundation, framing, HVAC, insulation/air 

sealing, windows/siding, and roofing. An energy consultant would also be involved. As a result of this 

deliberate communication process and adjusting work scopes and schedules, unexpected surprises such as 

delays or change orders are reduced, and durability and overall home performance improve. Applying this 

approach to both the design process and the construction process will help to ensure that a safe, strong, 

efficient, durable home will be the end result. 

1.3 Guide Overview 

This guide is arranged in the most practical and user-friendly way possible. However, as the “durability 

web” figure shows, there are many interrelated topics, which make any arrangement of information on 

durability a bit challenging. To the degree possible, redundancy in content is minimized and interrelated 

topics or discussions are crossreferenced so that readers can seek additional information needed with 

relative ease. Appendix A also contains a cross-cutting durability checklist for builders and designers. 

The scope of this guide cuts across the “durability drivers” shown listed below. Note that the building 

envelope and structure are not listed separately as they’re integrated across all of the drivers. 

An effort has also been made to include geographically based data and other technical information that 

allows the reader to quickly determine the relevance of a particular durability issue to local conditions or 

requirements. 

Lastly, it is vital for the reader to note that this guide is just that—a guide—and that all locally applicable 

building code regulations apply and take precedence. Likewise, manufacturer recommendations should be 

followed and take precedence. This guide is a collection of important durability measures and practices, 

however it is by no means comprehensive or all-inclusive. For that reason, each chapter also contains links 

to additional resources which will go deeper into a particular topic. 
Durability by Design 5

Ground and Surface Water Chapter 3 

Rain and Water Vapor Chapter 4 

Mechanical Systems Chapter 5 

Sunlight Chapter 6 

Insects Chapter 7 

Decay and Corrosion Chapter 8 

Natural Hazards Chapter 9 

Durability Measures—Minimum Code Requirements or Best Practices? 

As for the actual durability measures in Chapters 3 through 9, some are simply basic code requirements. 

Why? Because even basic code requirements can be critical details to get right, and simply focusing on 

them in the design and specification phase can offer a big durability payoff! In fact, many “above-code” 

programs which label and distinguish homes in the market include a number of requirements which are 

simply code requirements. In most cases, these programs include them as requirements because a third 

party is verifying that they are applied. 

Recommended measures may also cover issues not directly covered by codes or standards, yet are critical 

for durability. Despite the extensive framework of requirements found in building codes like the 

International Residential Code (IRC), there are often gaps in the details when applying the code to a 

specific application or local condition. 
6 Durability by Design

CHAPTER 2—DURABILITY CONCEPTS 

2.1 General 

In this chapter, some fundamental concepts of durability related to the design of residential buildings are 

addressed. This background information is intended to establish a baseline of understanding and to 

introduce concepts important to developing a realistic perspective on durability. It is important to 

appreciate the significance of durability when attempting to balance or optimize many factors that define 

the realities of construction and the service life of a building. 

Before discussing the concept of durability, some unrealistic notions surrounding the topic should be 

dispelled: 

 Durability does not mean perfection, but it does require diligent effort for continual improvement. 

 Durability does not mean that things should last forever, but it does require that reasonable life 

expectancies are achieved or exceeded. 

 Durability doesn’t mean that all problems are foreseeable for designers; there are many examples 

of problems beyond the prediction of designers, such as polybutylene plumbing and initial EIFS 

systems. 

 Durability doesn’t mean fail-safe installation, but it does demand a level of care that matches skill 

levels and quality of workmanship with the nature of the work being performed and 

consequences of installation defects. 

Simply put, durability is a multifaceted challenge. It involves nearly all aspects of construction, including 

planning, design, material specification, construction management, work force skill, and quality control. 

This particular document addresses the residential building design and specification phases of the 

construction cycle, although several topics naturally stray into work site issues and quality control. 
Durability by Design 7

To fail to plan for durability is a plan to fail. For 

example, durability problems are frequently 

associated with avoidable construction defects. A 

2007 ASHRAE study found that out of 17,000 

construction defect claims, 69% were the result of 

moisture-related defects in construction of the 

building envelope.1

 In recent years there have been 

settlements costing U.S. homebuilders tens of 

millions of dollars to repair and correct water 

damage caused by construction defects. 

Furthermore, the USDA Forest Product Laboratory 

reports that “termites invade more than 600,000 

homes and cause over $1.5 billion in damage 

annually.”2

 We could go on with more statistics and 

figures, but clearly there is plenty of evidence that 

the factors influencing durability really do matter, 

and that there is plenty of room for improvement. 

While builders and designers cannot foresee all 

durability issues, where they can affect better 

durability is in informed selection, integration, and 

application of components, materials, and building 

systems. This process is far from simple as an everchanging menu of products and systems is used to 

optimize cost, performance, aesthetics and 

consumer appeal, code compliance, and 

constructability. Further, a change to any one of 

these factors (e.g., a major uptick in material prices 

or a code change) can trigger a change from 

“Assembly A” to “Assembly B.” If Assembly B wasn’t 

vetted for how the new product would affect the 

home’s durability, the builder has just incurred 

additional risk. These material changes also cannot 

 

1

Grosskopf, K., Oppenheim, P., and Brennan, T. (2008). Preventing Defect Claims in Hot, Humid Climates. ASHRAE 

Journal, July 2008. www.ashrae.org

2

 “Wood You Believe: Insect and Marine Borer Edition”, U.S. Forest Products Lab Newsline, Vol. 13, Issue 2, 2013. 

www.fpl.fs.fed.us.

“Taking Credit” for Great Durability 

In today’s housing market builders and their design 

team often ask how they can get recognition in the 

market for going “beyond code” in energy 

efficiency, durability, or disaster resistance. These 

are critical attributes for long-term performance 

and the experience of living in the home—but 

they’re also not the shiny countertops or inviting 

floor plans buyers are drawn to....so how does a 

builder “get credit”?

This is a great question and one worth figuring 

out.

Leading builders and designers who go above and 

beyond code must be able to market the superior 

performance of their product. Here are some of 

their strategies for gaining this market recognition:

 Leverage third party labeling programs – so the 

builder doesn’t have to say “trust me” but rather 

the home has an independent certification. 

Examples of third party labeling programs with 

durability measures include ENERGY STAR 

Homes; DOE Zero Energy Ready Home; and 

Fortified Home. 

 If your homes include measures which reduce 

lender risk via lower energy bills, reduced 

insurance premiums, or other mechanisms –

provide as much documentation as possible to 

the lender and the appraiser. Also upload as 

much certification information as possible to the 

local real estate listing service (e.g., MLS).

 Use consumer-friendly tools to draw attention to 

the hidden features of the home. Example 

strategies include creating a checklist of moisture 

management measures for your home versus a 

minimum code or existing home, or creating a 

transparent wall cutaway, or “truth wall,”

showing the internal components of the 

envelope.
8 Durability by Design

be disassociated from changes that occur in the labor force. New materials or methods introduced into a 

construction process require a means of not only understanding the system or product, but also training of 

affected trades and quality control. Further downstream with the homeowner, these changes may also 

place new demands or expectations on home maintenance. 

How can a builder take action today and better prepare for the durability challenges of tomorrow? This 

chapter and the remainder of this guide offer answers to this question. Section 2.2 defines durability and 

provides resources to engage both the builder and consumer in the pursuit of durability. Section 2.3 

provides a brief overview of common durability problems in the present or recent past—which serve as 

lessons to apply better strategies in building the homes of tomorrow. 

2.2 Durability Defined 

Durability is the ability of a material, product, or building to maintain its intended function for its intended 

life-expectancy with intended levels of maintenance in intended conditions of use. There’s ample “wiggle 

room” in this definition admittedly, but ultimately what is built must work as expected, or as nearly so as 

practicable. 

What is a reasonable expectation or goal for durability? It depends. 

It depends on how much it costs. It depends on the expectations of the end user and the long-term 

investment value of the product. It depends on the local climate. It also depends on expected norms when 

the end user is not intimately involved with or knowledgeable of various design decisions and their 

implications. It also depends, of course, on the material or building system itself. 

For example, a house is expected to last for 75 years or more with normal maintenance and replacement 

of various components. Current building rates are actually at a pace where homes may need to be in use 

well beyond a 75 year replacement cycle. “Normal maintenance” is needed to reach these longevity goals, 

but what one person considers normal maintenance may be perceived differently by another. Durability is, 

therefore, an exercise in the management of expectations as well as an application of technology. 

For this reason, some builders and designers make significant efforts to inform their clients and trade 

partners about reasonable expectations for the durability, performance, maintenance, and operation of a 

home. Some references to help in this matter include: 

 Healthy Homes Maintenance Checklist: 

http://www.hud.gov/offices/lead/library/hhi/Healthy_Housing_CheckList.pdf 

 Home Maintenance Made Easy: What To Do, When to Do It, When To Call For Help: 

https://builderbooks.com/book/home-buyers-owners/home-maintenance-made-easy-what-todo-when-to-do-it-when-to-call-for-help.html 

 Moisture and Air, Householder’s Guide—Problems and Remedies, Canada Mortgage Housing 

Corporation, 2004 (rev. 2012), http://www.cmhc.ca 
Durability by Design 9

In addition to this essential task of helping homeowners understand basic maintenance responsibilities 

and expectations, a growing number of builders are also “taking credit” for the enhanced durability 

measures they incorporate (see text box on page 7).

2.3 Common Durability Issues 

Durability must be considered at all phases of a building’s life cycle, from the design phase (the focus of 

this guide) to field installation and then to the long-term maintenance of a home. While a home’s builder 

and designer cannot always assure that durability-enhancing practices will be upheld “downstream” with 

contractors and homeowners, many of the most common durability issues can be positively affected by 

the builder and designer through selecting appropriate products, calling out key details on plans, and 

applying integrated design. 

So what are these “common durability issues”? 

Figure 2–1: Soil Grade Slopes Towards the Foundation, Causing Water to Pool 
10 Durability by Design

Figure 2–2: Non-existent Flashing Around Window, Practically Inviting Water Intrusion 

According to the home inspection industry,3 these problems are among the “top 10” of home inspection 

issues: 

 The landscape slopes toward the house (see Figure 2–1). 

 Lack of proper exterior water control, e.g., gutters and downspouts. 

 Bathroom vents exhaust into the attic. 

 Lack of weep holes in brick, stone veneer and weep screed in stucco. 

 Lack of and/or improper deck flashing. 

 

3

 The 10 Most Common Home Inspection Issues and Gaffes, ASHI Reporter, American Society of Home Inspectors, 

March 2014. http://www.ashireporter.org/HomeInspection/Articles/The-10-Most-Common-Home-InspectionIssues-and-Gaffes/7596 (accessed September 2014). 
Durability by Design 11

 The furnace, air conditioner, fireplace, and/or dryer vent have not been serviced in the past 12 

months 

 Mortar missing in between the brick/stone of exterior chimney. 

Thus, 7 of the 10 most common home inspection issues have direct durability implications! 

Key durability-related findings from a survey of code officials conducted jointly by the National Association 

of Home Builders (NAHB) and the International Code Council (ICC) include: 

 Among grading/site drainage code provisions, inadequate grading and downspout/draining 

controls were two of the three most common code violations 

 In the area of flashing, window flashing problems were the most common area for code violations 

(see Figure 2–2). Further, code officials responded that 66% of the flashing problems were related 

to installation, as opposed to the product itself or both the product and the installation. This is 

actually an improvement; in the original 2006 study, 82% of code officials felt that flashing 

violations were primarily installation-related. 

These results on durability-related code provisions are further evidence of commonly occurring issues. And 

considering that the building code doesn’t give us all of the durability measures to ensure good 

performance, getting the code provisions right is just a starting point. 

Field surveys of older and newer homes also reveal both problems and some useful “lessons learned” for 

improved durability. In a 2001 durability assessment sponsored by U.S. HUD,4

 findings included: 

 Foundation Materials & Methods: In field assessments of 1970s and 1990s houses (homes that 

were roughly 25 years and 5 years old at the time of the study), 57% and 78% had basement 

foundations, respectively. In addition, 51% of the 1970s basement foundations used block 

construction whereas 73% of the 1990s basement foundations used concrete. While age is a factor 

in developing foundation cracks, 65% of the block foundations experienced visible cracking while 

only 10% of the concrete foundations exhibited similar cracks. Furthermore, 28% of the sites with 

surface depressions next to the foundation (commonly due to poor backfilling practices and 

settlement over time) accounted for 44% of the sites with visibly cracked foundation walls. 

o Lesson learned: Build a strong foundation and ensure proper backfill compaction and 

surface water drainage. 

 Roof Overhangs: While only 40% of 1970s homes had roof overhangs of 12” or less, 82% of the 

1990s homes fell into this category. 

 

4

“Assessing Housing Durability: A Pilot Study,” U.S. Department of Housing and Urban Development, November 

2001. http://www.huduser.org/portal/publications/destech/housing_durability.html (accessed September 2014). 
12 Durability by Design

o Lesson learned: A trend toward less roof overhang coupled with greater frequency of 

two-story construction in newer homes is leading to less protection of wall assemblies 

from rainwater intrusion. Greater attention to the proper execution of water-resistive 

barrier and flashing practices is important to offset the durability consequences of this 

trend. 

Clearly, there are a wide range of durability problems in America’s homes. And while moisture is certainly 

the dominant driver of durability issues, other factors such as insects, sunlight, and mechanical system 

performance can also make or break a home’s durability over time. Thus, the chapters which follow have a 

strong emphasis on moisture and also take into account the other major durability drivers. 

Beyond these chapters, Appendix A provides an abbreviated durability checklist which designers and 

builders should use to confirm they’re addressing key issues.

 
Durability by Design 13

CHAPTER 3—GROUND AND SURFACE WATER

3.1 General 

Nearly all building sites have the potential to experience problems with ground moisture, particularly 

when the water table is high or drainage is poor. Poor 

site drainage and difficult site conditions, such as “loose”

soils or fills, can contribute to eventual building 

settlement, foundation wall cracking, and aggravated 

moisture problems. As residential development 

continues in many markets and development space is at 

a premium, the luxury of selecting sites which offer good 

soil and surface conditions is often an amenity that’s 

simply not available. Thus, this section gives 

recommendations that recognize the need to be 

resourceful with the building lots which are available. 

The objective of a foundation is to separate the building 

materials and the indoor environment from the earth 

while also providing adequate structural support. The 

following rules of thumb and recommended practices 

should serve to minimize the potential for durability 

problems related to foundations (which are among the 

most common durability issues listed in Section 2.3). 

3.2 Recommended Practices 

3.2.1 Conduct a Preliminary Site Investigation 

The following actions help to identify potential site problems that can be accounted for in planning and 

design. While an expense to the overall project, the preliminary site investigation will ultimately prove to 

be a cost saving measure as potential problems will be addressed prior to becoming an issue for the home 

that is eventually built. If the site is part of a larger development, those costs could be spread over many 

dwelling units. 

As part of a preliminary site investigation, typically bore holes or test pits are used to verify subsurface soil 

conditions. An illustration of a typical bore hole used to explore subsurface conditions is shown in Figure 

3–1. Test pits are another way to identify subsurface conditions on a site but usually require an excavator 

or back hoe. If either piece of equipment is available, a test pit can be a quick and informative way to 

understand the local soils and should be dug to 2’ below the footing level. 

 Survey the surface conditions and local plant species for signs of seasonal or constant high ground 

water levels. (See USDA resources at end of chapter.) 

Ground Moisture Rules of Thumb

 Most damp foundations are caused by 

improper surface drainage and poor 

backfill procedures. Ensure ground water is 

directed away from the home and proper 

backfill soils and techniques are used.

 “Waterproof” basement walls per code; 

use a sump pump when site is wet. All

basements should be built in this manner 

unless a site is very dry.

 Do not build below-ground space lower 

than the highest seasonal water table 

level. 

 When in doubt, seek advice from a 

qualified geotechnical engineer.

 Moisture entering a house through the 

foundation can contribute to moisture 

problems in the above-ground portions of 

the building through added water vapor 

loading.
14 Durability by Design

 Consider the lay of the land and surface water flow onto and off of the site to ensure that proper 

surface water drainage can be achieved around the building site. 

 Check soil maps from USDA’s Natural Resources Conservation Service. (See additional resources 

listed at the end of the chapter.)

 Use a hand auger to bore one or more test holes at the proposed building location and determine 

general soil type/characteristics and ascertain the water table level; be sure to factor in any 

seasonal or recent climate conditions such as the amount of precipitation over the previous month 

or so (see Figure 3–1). At least one hole should be at the building location and extend at least a 

couple of feet below the proposed footing elevation. If deeper subsurface problems are expected 

(as by local experience), then a geotechnical engineer may need to use special drilling equipment 

to explore deeper below grade to ensure that adequate support and stability exists. 

 If possible, test the soil for bearing capacity at the depth and location of proposed footings. A 

simple hand-held penetrometer (e.g., a standardized metal rod and drop weight) used in 

accordance with the manufacturer’s instructions serves this purpose. 

 If fill or questionable soil conditions are suspected (as on a steep slope), the services of a 

geotechnical engineer and knowledgeable foundation contractor may be needed to appropriately 

prepare the site (e.g., compaction) or design a suitable foundation system. 

 Do not use basement foundations on sites with high ground water table. If a basement is a must, 

build the basement using waterproof construction methods. 

 Observe other homes being constructed in the area and talk with those performing that 

construction to garner a better understanding of the local conditions. 

 Review the conditions of the site during or immediately after a large rainfall to observe the runoff 

patterns. 
Durability by Design 15

Figure 3–1: Bore Hole and Bore Log 

 3.2.2 Plan Ahead for Sufficient 

Site Grading and Surface Water 

Drainage 

Site grading plans should be 

developed to direct water away 

from the building foundation, 

particularly if the building is 

located down-slope from a hill or 

similar land formation that may 

produce significant rainfall runoff. 

Figure 3–2 illustrates key 

elements of a site grading and 

drainage plan. 

Use of grassy swales is a common 

and cost-effective practice when 

the potential water volume is not 

large, wetting is not constant, and 

the swale is not sloped steeply 

enough to produce high water 

velocities. The range of acceptable 

swale slope depends on many 

Integrated Design and Construction: On-site Infiltration 

and Foundation Drainage 

Local storm water management and site development regulations often 

require the use of on-site water retention systems (e.g. infiltration ponds, 

rain gardens). Further, many green rating programs also offer credits for 

this practice as a means to improve water quality, recharge aquifers, and 

reduce streambed erosion from upstream and development activities. 

Be aware that this sustainable design idea, if not approached through an 

integrated design and construction (ID&C) concept, can result in retention 

or infiltration areas that are also sources for future foundation moisture 

issues which conflict with functional goals of the building code and 

building sustainability. In fact, site surveys have revealed elevated water 

tables as far as 30’ out from these components on building sites even 

though regulations may require only a 10’ separation (see picture below).
16 Durability by Design

factors, but slope should not be less than about 1% to prevent ponding, nor more than about 15% unless 

rip-rap (4” to 8” stone) with a filter cloth underlay is used to line the swale. 

Model building codes typically require grading of 6” of fall, or drop, out 10’ from the foundation of the 

building or as far as practical. Care should be taken in the backfill process to avoid settlement of the grade 

closest to the foundation as this is one of the most likely places a drainage failure can develop. Compaction 

during the backfill process is a direct solution to this issue. Additional soil may also be added to help offset 

a moderate amount of backfill settlement, but not so much as to reduce the clearance between grade and 

the bottom plate to < 6”. Self-compacting backfill such as pea gravel may also be used to avoid settlement 

which occurs over time and results in insufficient grade away from the foundation. 

Figure 3–2: Example Site Grading and Drainage Plan 

Source: Moisture Resistant Homes, U.S. HUD, 2006. 
Durability by Design 17

 

Concrete flatwork, such as walkways, driveways, and patio slabs, which is adjacent to the building should 

be sloped ≥ 2% (about 1⁄4” in 12”) away from the building. The soil beneath the flatwork should be properly 

compacted when installed and is critical to minimize differential settling of the walkway or driveway that 

can frequently reverse slope and bring water toward the house. Exterior flatwork settlement is a very 

troublesome durability mistake as it will create messy puddles, icy spots, high step-ups to front porches, 

etc. that homeowners must endure for years to come (Figure 3–3). In addition, gutters and gutter drains 

should be used to further remove roof run-off from the foundation area (see Section 4.2.6).

Figure 3–3: Concrete Flatwork Settlement 

3.2.3 Design Foundations for Moisture Protection 

Foundation options generally include basement, slab-on-grade, crawl space, or a mix of these foundation 

types (e.g., split level construction). One thing is common in all foundation construction: ground moisture 

will find its way “in” unless appropriate measures are taken. 

An important measure to include is a ground vapor barrier under all basement, slab-on-grade, or crawl 

space construction. This will eliminate (or suitably minimize) a large potential moisture source to a house 

that can create or aggravate above-ground moisture problems (see Chapter 4). 
18 Durability by Design

The ground vapor barrier should be placed directly below the concrete slab. There are still some 

reservations regarding curling of the slab with this technique, which can be addressed with a low water-tocement ratio (less than 0.5). Curling of the slab describes when the concrete slab distorts by either upward 

or downward bending, typically at the edges of the slab. This is typically the result of moisture differentials 

that occur while the slab is curing. If curling still seems to be an issue with this ratio, continue to use it but 

wet cure the top concrete using burlap or some type of fabric that will enable wet curing (see Section 3.3 

Additional Resources from ACI and Building Science Corp). 

For slab-on-grade and crawl space foundations (which also require a vapor barrier), moisture protection 

usually involves placing the building on a slight “mound” relative to the surrounding site. If the site is 

properly graded, a perimeter drain system is unnecessary in mounded foundation systems. If the site has 

potential for wetting, a perimeter drain system should be incorporated 

Typical basement construction practice for waterproofing, 

which is recommended for all sites unless they are extremely 

dry, is illustrated in Figure 3–4. However, “waterproofing” is not 

meant to resist water from flooding or a high water table; it is 

merely able to resist water and vapor movement more so than 

damp proofing measures. It should be noted that concrete has a 

considerably lower vapor permeability (i.e., can stop water 

vapor better) than masonry. However, available data seems to 

suggest no significant difference between concrete and 

masonry relative to the potential for basement water problems 

in actual practice when proper waterproofing measures are 

taken. 

Backfill and grading specifications should be shown on the 

construction documents, and also clearly stated in the foundation contractor’s agreement. The backfill 

and grading should be inspected for compliance with these specifications. Important foundation 

measures and best practices are listed below: 

• For backfill avoid silt, heavy clay, or expansive clay, particularly for basement walls. Granular 

soils are preferable that allow water to penetrate to the foundation drainage system and move 

the water away from the building. 

• Backfill should be compacted to avoid flat or negative grades from developing around the 

foundation. Alternatives to compaction include providing extra soil to offset settlement, using 

self-compacting backfill material such as pea gravel, or adding more soil after 6–12 months. 

• Use minimum 3,000 psi concrete in slabs with welded wire fabric, and foundation walls with 

reinforcement per code (at a minimum) to control cracking. In both cases it’s critical to ensure 

Managing Expectations:

Concrete Slabs

Even with the best slab designs and 

proper construction, it is unrealistic to 

expect crack–free and curl-free floors. 

Every owner should be advised by the 

designer and contractor that it is 

normal to expect some cracking and 

curling on every project. This does not 

necessarily reflect adversely on the 

adequacy of the floor’s design or 

quality of construction.

- American Concrete Institute
Durability by Design 19

proper positioning of the welded wire fabric and the reinforcement. If CMU foundation walls are 

used, provide proper code minimum reinforcing and specify coatings to create a monolithic 

barrier to water penetration. 

• Vibrate poured concrete walls for good consolidation in forms. 

• Use high quality urethane caulks to seal all penetrations through the foundation wall prior to 

applying waterproofing measures 

• Six-mil polyethylene sheeting as a waterproofing layer provides the added benefit of being a 

Class I vapor retarder. This greatly limits water vapor diffusion from the soil into the foundation 

wall. Alternative waterproofing treatments should likewise be evaluated for their ability to limit 

water vapor migration into the wall. 

Figure 3–4: Basement Construction and Optional Enhancements for Wet Site Conditions 
20 Durability by Design

3.2.4 Specify Finished Basement Assemblies Which Manage Moisture 

The below-grade living space created by finishing a basement 

space creates a number of challenges: moisture concerns from 

below the slab and through the exterior walls, possible radon 

contamination, newer more stringent code-required insulation 

levels, and creating assemblies which can provide comfortable 

spaces which stay dry. This is another location where ID&C 

becomes paramount as basements have a higher probability for 

issues to arise. 

Finished basements in new construction that are insulated on 

the exterior of the foundation wall offer a number of 

advantages. From a moisture control perspective, this type of 

assembly creates a warmer surface temperature of the foundation wall. As a result the foundation wall is 

less prone to condensation, and doesn’t need to be covered up with interior-facing insulation. This gives 

the assembly to the ability to dry towards the interior, allowing moisture from initial curing of the concrete 

or other sources to escape. 

This approach is infrequently used however, due to challenges with integrating the exterior foam with the 

above-grade wall, protecting the above-grade portion of the foam over the long-term, and other 

challenges. 

Instead, if we assume that the foundation walls have exterior waterproofing (but not insulation), then the 

inside of the foundation walls should be insulated and designed to dry to the interior. A set of 

recommendations to accomplish these goals is shown in Figure 3–4. Note that this assembly includes 

redundant moisture management strategies to increase the robustness of the assembly. In some cases a 

particular method or material could be substituted and work acceptably (e.g., use of an impermeable floor 

covering material), assuming that the related systems (e.g., capillary breaks and foundation drainage 

system) are installed and work well. In the interest of durability it’s better to use a belt and suspenders 

approach whenever possible. 

Key elements of the basement wall assembly shown in Figure 3–4 include: 

 As indicated by the blue drying arrows in the figure, this assembly is designed to allow any 

moisture accumulation to dry to the home’s interior. As such, avoid the use of a Class I vapor 

retarder like poly in the framed section of the wall. 

 The layer of rigid foam insulation (typically EPS or XPS) against the foundation wall is a critical layer 

in the assembly. This layer is continuous, the foam is air impermeable and the joints are taped or 

sealed to prevent air leakage, and it keeps the cold foundation wall isolated to prevent 

Integrated Design and Construction: 

Below-Grade Living Spaces 

Finishing underground spaces warrants 

much forethought for ventilation, 

comfort, lighting, safety, and envelope 

requirements. Much like the abovegrade construction, each of these 

systems affects the other with the design 

and construction of finished 

underground habitable space. 
Durability by Design 21

condensation. In the case that the basement will not be finished immediately, keep in mind that 

exposed foam must meet code requirements for thermal protection (IRC R316.4). 

 Prevent indoor air (and the water vapor it holds) from leaking into the wall assembly by using the 

non-paper-faced gypsum board (increased moisture/mold resistance) layer as an air barrier. Seal 

the drywall to the studs and top/bottom plates, and caulk any penetrations. Don’t forget to seal at 

the ceiling level as well. 

 Create a separation layer from the basement slab to the wall finishes. Moisture will wick into 

gypsum, wood, or other materials in direct contact with the slab so hold finishes up about 1⁄2” from 

the slab surface. Use sill seal or a similar product to create a thermal and capillary break between 

the slab and the bottom plate. 

 To minimize the food source for mold, consider using cold formed metal framing and fiberglass 

faced gypsum board for basement finishes. 

 To allow the basement slab to dry upward towards the homes interior (because it has a vapor 

barrier below), allow the slab to dry before finishes are applied. Flooring product manufacturers 

offer guidance on acceptable moisture levels. Further, ensure that floor finishes do NOT create a 

second vapor barrier on the top side of the slab, because there is already one below and the 

basement needs to be able to dry to the INSIDE. Higher permeance finishes (e.g., carpet and 

padding above 1 perm) provide increased drying potential to the inside. 

3.3 Additional Resources

American Concrete Institute. The Institute is a leading authority and resource for the development and 

distribution of consensus-based standards, technical resources, and proven expertise for individuals and 

organizations involved in concrete design. www.concrete.org 

Building Science Corp.; Joe Lstiburek, Building Science Insights, BSI-003 Concrete Floor Problems, 

5/20/2008 (accessed June 2014). This article addresses the issues concerning slab-on-grade with vapor 

barrier construction. 

ENERGY STAR Water Management System Builder Checklist. This checklist provides a comprehensive 

reference for site, foundation, wall, roof, and building material guidelines for effective water management 

for homes. Several of its provisions, such as not using Class I vapor retarders on air-permeable insulation 

on below grade exterior walls, are drawn in part from this source.

EPA Moisture Control Guidance for Building Design, Construction and Maintenance. A well-written guide 

to provide insight into keeping indoor air quality (IAQ) at healthy level. 

USDA Natural Resources Conservation Service. This service provides a wide array of soil and plant-related 

information, including soil and plant classification by state provided at the county level. 
22 Durability by Design

CHAPTER 4—RAIN AND WATER VAPOR 

4.1 General 

Chapter 4 presents actionable design guidance important to an effective and durable moisture control 

strategy for above-grade building assemblies—roofs and exterior walls. Nearly all matters addressed in this 

chapter are climate-dependent solutions because moisture is largely a climate-dependent durability 

hazard. Section 4.2 focuses on control of rainwater and Section 4.3 focuses on control of water vapor 

diffusion. 

Integrated Design and Construction: Moisture Control 

Moisture control is a matter of integrated design of the “whole building” because it requires the careful 

consideration of several interrelated and contributing factors. Rainwater control can be affected by 

building configuration (height, width of overhangs, etc.), roofing and siding material selection and 

installation method, roof slope, proper sizing of gutters, execution of flashing, use of sealants where 

needed, material tolerance of moisture, maintenance, etc. All of these factors contribute to a building’s 

resiliency and durability over a lifetime of rainwater exposure. 

For water vapor control, the interrelationships are no less complex. The insulation material and 

methodology, degree of air-sealing of assemblies, application of vapor retarders, water vapor properties 

of other materials (including structural sheathing, water-resistive barrier, air barrier, and interior finishes), 

height of the building, construction moisture exposure, and indoor relative humidity control, ventilation, 

and HVAC design all affect the performance outcome. The strategy for water vapor control can also affect 

the building’s ability to tolerate any minor imperfection in execution of the rainwater control strategy. 

This chapter strives to help break down these interdependencies into coordinated, yet separate actionable 

topics to facilitate an overall integrated design and construction vision for moisture control. Otherwise, the 

challenge can seem overwhelming and success very uncertain when, in fact, all that is needed is an 

informed and effectively executed plan. 

While simple in concept, the achievement of durable, moisture-resistant buildings is challenging because it 

is often the “little things that make a big difference”; and, many of these “little things” are hidden after 

construction is completed or may not be obvious to those with a casual understanding of modern 

construction materials and methods. For example, some of the most important flashing elements are 

hidden after siding is installed. Also, water vapor diffusion relates to intrinsic or “hidden” properties of the 

materials which may vary significantly even among similar-looking material types. Thus, durability as it 

relates to moisture control must involve a well-informed and integrated design and construction approach 

(refer to text box). Quality control during construction is also important to supplement normal building 

department inspections, which may tend to focus on building safety issues and therefore may not address 

the “little” details that can make a big difference for durability.
Durability by Design 23

Why is the guidance in Chapter 4 so important? Failures to appropriately address the effects of rain and 

water vapor have contributed to varying degrees of moisture durability problems—including rot, 

corrosion, collapse, insect infestation, and mold—in as much as 1⁄4

th of the housing stock in some of the 

most vulnerable regions of North America.5, 6, 7

 Particularly severe and accelerated moisture durability 

failures have occurred when good practices for control of rain and water vapor have not been properly 

integrated into the design and construction of a home.8

 Therefore, moisture management is not just a 

matter of building durability; it also is a risk management concern with serious potential liability 

implications (refer to Chapter 2). 

4.2 Recommended Practices for Rain water Control 

4.2.1 General 

The objective of designing an effective weather-resistant building envelope is simple: keep rainwater away 

from vulnerable construction materials. Keeping these components dry will maintain a building’s structural 

integrity and help prevent moisture-related problems as mentioned in Section 4.1. 

Rainwater is widely acknowledged as the most problematic external source of moisture for 

buildings.9, 10, 11, 12 Sections 4.2.2 through 4.2.9 provide actionable guidance for designing durable, 

moisture-resistant roofs and exterior walls; refer to Chapter 3 for similar guidance on foundations and site 

design. Other forms of precipitation that cause water intrusion also are not neglected. For example, snow 

is considered in relation to its role in the formation of roof eave ice-dams. In Section 4.4, additional 

resources are provided for further study and guidance. 

4.2.2 Size Roof Overhangs and Projections to Add Moisture Protection 

Roof overhangs and projections such as porch roofs or overhanging floors provide a primary means to 

deflect rainwater away from building walls. Thus, the potential for water penetration through siding, 

 

5

Tsongas, G. (2009), “Chapter 13- Case Studies of Moisture Problems in Residences”, ASTM MNL 18, ASTM 

International, West Conshohocken, PA. 

6

 Rousseau, J. Rain Penetration and Moisture Damage in Residential Construction, Building Science Insight, 

National Research Council Canada / Institute for Research in Construction (NRC-IRC), http://irc.nrccnrc.gc.ca/bsi/83-1. Canadian Mortgage Housing Corporation, Ottawa, Canada, June 1983. 

7

 Assessing Housing Durability: A Pilot Study, U.S. Department of Housing and Urban Development, Washington, 

DC. November 2001. www.huduser.org 

8

Crandell, J.H. and Smart, J. (2004). “Lessons from EIFS: Past, Present, and Future Challenges”, Wood Framing 

Housing Durability and Disaster Conference, 2004. 

9

 Healthy and Affordable Housing: Practical Recommendations for Building, Renovating and Maintaining Housing, 

U.S. Department of Housing and Urban Development and U.S. Department of Energy, Washington, DC. 2004. 

10 ASTM E 241-00, Standard Guide for Limiting Water-Induced Damage to Buildings, American Society of Testing 

and Materials, West Conshohocken, PA. 2000. 

11 Treschel, H. (ed.) “Moisture Control in Buildings”, American Society for Testing and Materials, West 

Conshohocken, PA. 2009 

12 Kerr, D. Keeping Walls Dry, Part 1 of 2, Canada Mortgage Housing Corporation, http://www.cmhcschl.gc.ca/en/imquaf/himu/himu_002.cfm 
24 Durability by Design

windows, and doors is minimized. With proper site grading (Chapter 3) and use of gutters (Section 4.2.6), 

roof overhangs can also help protect foundations from rainwater. The recommendations in this section are 

generic in nature to provide a simple and practical minimum recommendation. They do not consider all 

the pros and cons of roof overhangs as related to an integrated design approach (refer to Integrated 

Design and Construction text box). For example, larger overhangs than those recommended in this section 

may be important for tall buildings. 

Integrate minimum roof overhang widths into a home’s design, as shown in Table 4–1 and Figure 4–1. In 

addition, porch roofs or floor overhangs should be considered to protect lower story walls, particularly 

doors and windows. A Decay Hazard Index map is provided in Figure 4–2 to assist in using Table 4–1.

Integrated Design and Construction: Roof Overhangs 

Roof overhangs have multiple benefits in relation to building durability. Roof overhangs can be used to 

shield walls to reduce the likelihood of water intrusion problems and, thus, permit a broader selection of 

exterior wall covering approaches with acceptable performance (refer to Section 4.2.7). Roof and porch 

overhangs can also provide solar shading for glazing to reduce air-conditioning energy consumption and 

help reduce the degrading effects of solar radiation (refer to Chapters 5 and 6). But, overhangs add wind 

uplift load to the roof and may require stronger uplift load path connections from the roof to the 

foundation, particularly in high wind areas (refer to Chapter 8). 

Roof overhangs are also considered in some green building standards and can be used to gain credits 

toward qualifying a home as a “green home.” For example, requirements similar to Table 4-1 are found in 

the ICC-700 National Green Building Standard (NGBS) where overhang widths are related to average 

annual rainfall rather than decay hazard index. But, the intent to encourage and reward durable 

construction is the same. 

Table 4–1: Recommended Minimum Overhang Width for 

One- and Two-Story Homes with Gutters13 

DECAY HAZARD INDEX Eave Overhang (Inches) Rake Overhang (Inches) 

Less than 35 N/A N/A

35 to 70 12 12

More than 70 24 or more 12 or more

 

13 Table adapted from Verrall, A.F., and Amburgey, T.L. 1978. Prevention and Control of Decay in Homes. Prepared 

by Southern Forest Experiment Station, Gulfport, MS for U.S. Department of Housing and Urban Development, 

Washington, DC. 
Durability by Design 25

Figure 4–1: Roof Overhangs 

Figure 4–2: Decay Hazard Index Map 

(Based on A Climate Index for Estimating Potential for Decay in 

Wood Structures Above Ground, Scheffer, 1971) 

4.2.3 Plan a Roof Configuration for Unobstructed Drainage 

In modern construction, adding complexity to roof plans is commonly done to improve curb appeal. But 

adding complexity to the drainage pattern of a roof can also create excessively concentrated or obstructed 

roof drainage patterns as shown in Figure 4–3. If not avoided or properly addressed, these conditions 
26 Durability by Design

often lead to moisture intrusion and durability problems. The following design actions should be 

considered: 

 Strive for easily drained roof geometries with minimal obstructions to roof water flow. Balance the 

desire for a roofline with curb appeal with the drainage performance. 

 Specify an adhered waterproofing membrane underlayment applied to roof valleys and adjacent 

vertical surfaces where flow concentrations occur as shown in Figure 4–3.

 Where concentrated flows discharge from the roof into gutters, use a gutter deflector or splash 

guard to prevent the water from “over-shooting” the gutter, and size gutters and downspouts 

accordingly (see Section 4.2.6). 

Figure 4–3: Typical Roof Drainage Problems to Avoid 

4.2.4 Design Roofing to Optimize Durability and Function 

Roof coverings provide a first line of defense against the elements. They also tend to be the most exposed 

component of a building’s exterior envelope. Therefore, roof coverings should be selected, detailed, and 

installed to provide durable resistance to water penetration. Wind, urban wild-land fire, and hail resistance 

are important roof system considerations in some regions (see Chapter 8). 
Durability by Design 27

The design considerations in this section are intended to enhance or help fulfill the objectives for a roof 

installation as found in the 2015 International Residential Code (IRC) which states: 

R903.1 General. Roof decks shall be covered with approved roof coverings secured to the building or 

structure ....Roof assemblies shall be designed and installed in accordance with this code and the approved 

manufacturer’s installation instructions such that the roof assembly shall serve to protect the building or 

structure. 

Building codes don’t address many of the details required for a complete and proper installation of the 

many available roofing products. Therefore, the statement regarding “in accordance with ... 

manufacturer’s installation instructions” should not be taken lightly! Roofing industry guidelines, as 

provided in Section 4.4, are also important resources to ensure a durable and effective roofing installation. 

Service Life—There are a variety of roofing materials with a wide range of estimated service life as shown 

in Table 4–2. Metal, concrete or clay tile, and slate roof coverings tend to provide the greatest durability as 

measured by estimated service life. But, they also represent the more expensive roof covering choices. 

Thus, more than three-quarters of all homes use composition roof shingles. The estimated service life 

varies significantly even within a given roof covering type. Differences in manufacturer warranties may be 

considered as one means of assessing expected service life. However, warranties and service life estimates 

must be taken with a grain of salt because of all the uncertainties which may affect actual installed 

performance. 
28 Durability by Design

Table 4–2: Roof Covering Durability Selection Data

Roof Covering Types Minimum

Roof Pitcha

Estimated Service 

Lifeb

(yrs.)

Composition Shingle 15 to 30

Single layer 15# felt underlay 4:12

Double layer underlay 2:12

Wood Shingle * 15 to 30

Metal (standing seam) * 20 to 50+

Concrete/Clay Tile 50+

Single layer 30# felt or roll roofing underlay 4:12

Double layer underlay 21⁄2:12

Slate * 50 to 100

Built-up Roof (low slope) * 12 to 30

Synthetic Membrane Roof (low slope) * 20+

* Code refers to manufacturer’s installation instructions

Table Notes: 

a. Minimum roof pitch based on Section 905 of the International Residential Code (IRC), International Code Council, Inc., 

Washington, DC, 2015. www.iccsafe.org 

b. Service life estimates are based on a combination of industry data and also the report: “Study of Life Expectancy of 

Home Components”, National Association of Home Builders and Bank of America, Washington DC, February 2007. 

Roof Pitch—As shown in Table 4–2, roof pitch is an important factor in the selection of a roof covering 

type. Steep-slope roof systems are designed for installation on slopes greater than 3:12 (14 degrees). 

Steep-slope roofs are water-shedding, not waterproof. Therefore, roof pitch is limited in accordance with 

Table 4–2 for various steep slope roofing products. To prevent water leaks, these roof systems rely on fast 

drainage, adequate overlapping of elements, and use of underlayment as a back-up layer of protection. 

While lower roof pitches are possible in some cases with doubling underlayment, low roof pitches are 

more prone to water intrusion and underlayments commonly used are not waterproof due to fastener 

penetrations or possible damage during roofing installation. Using enhanced roofing underlayment 

materials and practices (as discussed in Chapter 9) can help prevent water intrusion during extreme winddriven rain events. They also offer improved protection against wetting of building materials during 

construction when the building is temporarily “dried-in” with roof underlayment until installation of the 
Durability by Design 29

final roof covering. Examples of enhanced roof underlayments include mechanically fastened synthetic 

membranes, adhesively attached membranes, and specialty roof sheathing products with sealed joints. 

Low-slope roofing is more common in commercial building construction, but does find occasion for use in 

residential construction. Typical roofing material selections may include “hot-mopped” built-up roofs 

(BUR) with gravel ballast or synthetic membranes such as EPDM, PVC, and TPO which may be held in place 

by ballast, adhesion, or mechanical fastening. Low-slope systems are designed as waterproof roof systems, 

and use roof covering membranes designed for pitches of as low as 1⁄4:12. While low-slope roofs are 

commonly known as “flat roofs,” a dead flat roof surface is a serious design mistake since water will 

accumulate or pond on the roof and not drain. It is very important to use proper installation practices and 

skilled installers for these types of roofing systems because any defect in the installation, such as a faulty 

seam or joint, is destined to result in a leak which may go undetected until substantial damage has 

occurred. In addition, as with steep slope roofing, there are wind hazard-related design considerations 

associated with use of these roofing methods. 

Integrated Design and Construction: Roof Slope 

Considering several factors, a moderate roof pitch (e.g., in the range of 5:12 to 6:12) provides a favorable balance of 

pros and cons for water-shedding or steep-slope roof systems. For example, lower roof pitches will tend to decrease 

drainage efficiency, allow debris to accumulate, and increase wind uplift loads on the roof. In addition, as the roof 

ages or becomes damaged, leaks are likely to be more severe. Conversely, a steeper roof pitch will tend to increase 

the volume and rate of roof water discharge and increase the lateral wind loads on the building, requiring greater 

amounts of wall bracing to prevent collapse which may affect architectural considerations, such as placement and 

number of windows and doors. Furthermore, very steep slope roofs are more difficult and less safe to access for 

construction, maintenance, and replacement. If self-sealing asphalt composition shingles are installed on a very 

steep slope roof, they may not seal unless asphalt adhesive is applied using a “hand-tabbing” method (see 

manufacturer’s installation requirements for steep-slope roofs). However, designing an attic for usable space may 

necessitate use of a roof pitch greater than 6:12 which may be more favorable than considering an additional story. 

4.2.5 Design Roof Ventilation to Control Moisture and Ice Dams 

When designing and constructing a vented roof system, adequate ventilation is important for a number 

of reasons: 

 Condensation control 

 Temperature control of roof surface and attic space 

 Energy efficiency 

 Prevention of chronic ice damming at eaves 
30 Durability by Design

Ventilating unconditioned attic spaces beneath steep-slope roofs and roof cavities within cathedral 

roofs or below low-slope roofing systems (without adequate above deck insulation) is intended to 

prevent damaging levels of moisture in materials as a result of condensation or exposure to high 

humidity. Ventilation is also intended to reduce the temperature of the attic or space below the roof 

deck. This effect can reduce summertime cooling energy use and also prevent the formation of ice dams 

in the winter (addressed later in this publication). It also helps reduce the temperature of the roof deck 

and roofing material during hot periods, improving the durability of materials with volatile organic 

compounds (like composition asphalt shingles). In fact, some composition shingle warranties may 

require installation over a ventilated roof deck. Finally, roof ventilation can help remove indoor moisture 

which makes its way into the attic space as a result of air-leakage through the ceiling caused by an 

absence of or inadequate air barrier practices (refer to Section 4.3.2). This latter concern has been the 

cause of some amazing attic “rain storms” due to excessive condensation on the underside of roof decks 

in homes with high indoor humidity during the winter. 

Conditioned Attic Space–Unvented Roof Assemblies

Roof assemblies can be effectively designed without ventilation if requirements for unvented, conditioned 

roof spaces are carefully followed. This design strategy entails a number of integration issues for heat, air, 

and moisture control. Durability benefits of an unvented attic can include an insulated and air-sealed home 

for HVAC equipment and ducts; and reduced susceptibility to wind-driven rain, insect, or ember (wildfires) 

intrusion. However, due to the fairly limited application of this design in current new housing production, a 

discussion isn’t included in this section. Basic provisions for this practice can be found in Section 806–Roof 

Ventilation of the 2015 International Residential Code (www.iccsafe.org) and should be complemented with 

a building science perspective on the assembly.

Minimum Roof Ventilation—Attic spaces and roof cavities should be ventilated at least in accordance 

with minimum local building code requirements as represented in Table 4–3, and a greater amount of 

ventilation is advisable for reasons previously stated. In addition, for cathedral ceilings with long slender 

vent pathways between rafters, the minimum depths of the vent pathway (air pathway from inlet vent to 

outlet vent) should be 2” for roof slopes of 3:12 to 5:12 or 1.5” for roof pitches greater than 5:12 to 

minimize resistance to air flow. Typical building codes only require 1” vent pathway depth which may be 

inadequate for many applications. 

Sample roof ventilation configurations are shown in Figure 4–4 and apply to cases with reasonably 

balanced distribution of high and low vents and a recommended vertical separation of at least 3’ 

between high and low vents. However, if there is any imbalance in high and low vent amounts, it is 

better to place slightly more ventilation low than high in cold climates, to avoid creating low pressure in 

the roof space. Low pressure of this type can draw moist air from the house into the attic space. 
Durability by Design 31

Table 4–3: Minimum Roof Ventilation Requirements

Applicability Requirements 

Ventilation 

Amounta

Generally applicable. Vertical separation of inlet and outlet vents of no less than 3’ 

recommended. 

1:150

Vertical separation of inlet and outlet vents is at least 3’ with balanced inlet and 

outlet vent areasb, and a vapor retardercis installed on the warm side of the ceiling 

in cold climates. 

1:300

Source: Based on the 2015 IRC with additional recommendation of a minimum 3’ vent separation and requirement for balanced inlet 

area and vapor retarder, not just one or the other. 

Table Notes: 

a. Values are given as ratio of total unobstructed open area of inlet plus outlet vents (also known as “net free vent 

area”) to total horizontal projected area of the ventilated space (ceiling area). Therefore, vent size must be 

increased to account for obstructed vent area due to louvers and screens (refer to vent manufacturer technical 

data and net free area values per lineal foot of vent material). 

b. Inlet and outlet vent areas shall be considered balanced provided that at least 50 % and not more than 80 % of the 

required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated. 

(Exception: In cold climates, a minimum of 40 % and maximum of 50 % of the required ventilating area is 

recommended for the upper portion of the roof space).14 

c. The 2015 IRC requires a Class I (e.g., poly sheeting) or Class II (e.g., kraft paper) vapor retarder for the roof or ceiling

only in Climate Zones 6, 7, and 8 (see Figure 4–15 for U.S. Climate Zones). 

 

14 Lstiburek, J., “A Crash Course in Roof Venting”, Fine Homebuilding, August/September 2011, 

www.finehomebuilding.com 

Integrated Design and Construction: Coordinate Roof Ventilation Details 

Typical roof ventilation of homes relies on passive ventilation strategies (i.e., no fans or controls). Passive 

ventilation may be enhanced during wind events, but usually it is driven by the “weak” force of heated air 

rising in the attic–entered low and exiting high. These “weak” forces driving air exchange and movement 

sometimes leave stagnant areas if ventilation inlets and outlets are not well distributed (e.g. continuous 

vent openings at the ridge and along eaves). But, in some cases, continuous venting may be impeded by roof 

shape and also structural details, such as full-height blocking between the roof and walls to transfer seismic 

or wind forces through the building. In such cases, details for a roof ventilation plan and structural details 

should be coordinated. Also, such integrated design challenges speak to the importance of good air-leakage 

sealing of ceiling penetrations into the attic and indoor air relative humidity control to provide a “safety 

factor” against roof ventilation design trade-offs that are sometimes necessary due to competing design 

objectives. 
32 Durability by Design

Figure 4–4: Roof Ventilation Configurations 

Frequently, the required ventilation amounts are not properly calculated, not calculated at all, or not 

enforced. Again, this is a matter of proper design details and quality control during construction. Such 

mistakes coupled with other errors such as exhausting bathroom fans into an attic space and not airsealing the ceiling can cause major moisture durability problems (condensation) in roof systems. 

Example: How to determine the required vent amounts 

Consider the roof eave and ridge vent scenario shown in Figure 4–4 and assume that the roof ceiling area 

is 1,200 ft2 and that the eave and ridge vents are separated by more than 3’ vertically. Thus, Table 4–3 

requires a vent ratio of 1:300 or 1 ft2 net free vent area (NFVA) for every 300 ft2 of ceiling area. Consider 

also that this building is in a cold climate such that the exception in footnote ‘b’ of Table 4–3 applies. 
Durability by Design 33

The calculations follow: 

Required NFVA = 1,200 ft2 (ceiling area) x (1 ft2 NFVA / 300 ft2 ceiling area) 

= 1,200 / 300 = 4 ft2 NFVA 

Convert NFVA to inches: 4 ft2 x 144 in2 / ft2 = 576 in2 NFVA 

Consider the following split of vent area for the eaves (low) and ridge (high): 

Use 60% for eave vents = 0.60 x 576 in2 NFVA = 346 in2 NFVA 

(Divide by 2 to determine 183 in2 NFVA for the soffit on each side of the roof) 

 

Use 40% for ridge vent = 0.40 x 576 in2 NFVA = 230 in2 NFVA 

(50/50 split to 60/40 split of NFVA low/high is within an acceptable range) 

Determine the vent length required which depends on the vent product’s rated NFVA per foot length of 

vent (obtain this value from the vent manufacturer). Assume the product selected for both the eaves 

and ridge has a NFVA per foot value of 8 in2/foot length. 

Length of Eave (Soffit) Vent required on each side of the roof: 

 186 in2/ 8 in2/ft = 23’

Length of Ridge Vent required: 

 230 in2/ 8 in2/ft = 29’

The soffit vent should be continuous for the entire soffit and the ridge vent should also extend most of 

the roof length. Thus, the calculated lengths may need to be increased to accommodate the actual 

building roof dimensions to avoid creating stagnant areas. However, calculations as shown above should 

be redone to verify a balanced vent area or one that is slightly skewed with more eave (low) vent area 

than ridge (high) vent area if the building is in a cold climate. 

Ventilation to Prevent Ice Dams—The formation of ice dams at roof eaves is a common cause of roof 

water intrusion in cold climates where snow may accumulate on roofs during the winter. Elevated attic 

and roof temperatures during the winter cause snow on the roof to melt. Elevated attic temperatures 

may be caused by inadequate roof ventilation, poor ceiling insulation, indoor air leakage through the 

ceiling into the attic, leaky or uninsulated ductwork in the attic, heating equipment in the attic, or a 

combination of these factors. Consequently, snow melting on the upper portion of the roof will drain 

underneath the snow toward the colder roof eave where it may refreeze, creating an ice dam that 

causes the roof melt-water to pond and seep through water-shedding roof coverings as illustrated in 

Figure 4–5. A particularly severe ice dam is shown in Figure 4–6. In such cases, ice dams are not only a 

moisture durability concern, but also cause damage to roof eaves and guttering while presenting a lifesafety hazard (e.g., deaths have occurred from falling icicles or masses of ice). 
34 Durability by Design

Figure 4–5: Cross Section of an Eave Ice Dam

Army Corps Cold Regions Research and 

Engineering Laboratory as published in Disaster 

Safety Review, IBHS, Vol. 3, No.1, 2004.

Figure 4–6: Example of a Severe Eave Ice Dam 
Durability by Design 35

For enhanced protection against the formation of ice dams, use Table 4–4 to determine roof vent area 

ratios. The ventilation ratios in Table 4–4 are a function of the venting layout and ceiling (attic floor) 

insulation levels. These recommendations should be employed in areas with a ground snow load greater 

than 30 pounds per square foot (psf) and strongly considered in other areas where below freezing winter 

temperatures and roof snow accumulation are expected. The following additional practices also apply: 

 A balanced placement of high (outlet) and low (inlet) vents as shown in Figure 4–4 and Table 4–3 

above; 

 Use of air barrier practices in the ceiling to prevent warm, moist indoor air leakage into the attic 

space (refer to Section 4.3.2); 

 Adequate insulation and sealing of ductwork and HVAC equipment located in the attic space; and 

 Venting of all exhaust fans to outdoors (not the attic or attic eave). 

Where the roof ventilation recommendations in Table 4–4 exceed code-minimum roof ventilation 

requirements (have more net vent open area than is required with the result of the 1:300 calculation), 

additional benefits will also be realized in the control of water vapor diffusion and condensation in a roof 

system. Also, use of ice dam flashing in accordance with Figure 4–7 is still strongly recommended as a 

means of providing “back-up” protection.
36 Durability by Design

Table 4–4: Recommended Roof Ventilation Levels 

to Prevent Chronic Ice Damsa

(for climates with ground snow load ≥ 30 psf and other areas prone to ice dams)

R-value of Roof/Attic 

Insulation

Vent Ratiob

Vertical Separation

of Inlet (Eave/Cornice) and

Outlet (Ridge or Gable) Vents

3 ft 6 ft 9 ft 12 ft

Vented Attic Roofs

R 19 1:100 1:140 1:180 1:200

R 30 1:160 1:230 1:280 1:300

R 38 1:200 1:290 1:300 1:300

R 49 1:260 1:300 1:300 1:300

Vented Cathedral Roofs

R 19 1:100 1:140 1:250 1:250

R 30 1:160 1:230 1:250 1:250

R 38 1:200 1:250 1:250 1:250

R 49 1:250 1:250 1:250 1:250

Minimum Vent Depth for Air Passage in Cathedral Roofs

Roof Pitch Vent Depthc

3:12 to 5:12 2”

> 5:12 1 1⁄2”

Source: Crandell, J.H., “Ice Dams, Traditional and Improved Practices for Roof Ventilation and Prevention of Ice 

Dams,” Disaster Safety Review. Institute for Business and Home Safety, Tampa FL, 2004 

Table Notes: 

a. This table applies to roofs with a pitch of at least 3:12, an R-value of at least R 19, and a distance between inlets 

and outlets of no more than 40’. 

b. Values are given as a minimum ratio of total open or unobstructed area of inlet and outlet vents (also known as 

“net free area”) to total horizontal projected area of the ventilated space (ceiling area). Inlet and outlet areas shall 

be balanced to the maximum extent practicable. For example on a simple gable roof, one-half of the calculated 

vent area shall be at the ridge and one-fourth at each of the two eaves. 

c. Minimum vent pathway depth shall be maintained for entire ventilation air flow path from eaves to ridge or gable 

vents. 
Durability by Design 37

Figure 4–7: Eave Ice Dam Flashing

4.2.6 Properly Size Roof Drainage (Gutters and Downspouts) 

Properly designed roof gutters reduce the amount and frequency of roof run-off water that wets the 

above-grade walls or the foundation. A list of recommendations and a simplified design approach are 

presented below to help in the proper use of gutters. Further, an example problem is provided to illustrate 

the simplified design approach. 

 Ensure that gutters have a slight downward slope toward downspouts. 

 Downspouts that discharge directly to the ground surface should do so at least 2’ outward from 

the building. Splash blocks or plastic corrugated pipe are recommended to prevent erosion and to 

give further extension of discharge water away from the foundation, particularly for downspouts 

located at inside corners of building, if poor (such as expansive clay) soil conditions may exist, or 

where the ground surface slope away from the foundation is minimal. 
38 Durability by Design

 Where foundation backfill grade does not provide positive drainage away from the foundation, it 

should be corrected rather than simply relying on extension of downspout discharge. Should 

gutters become clogged causing water to spill over, the surface grade away from the foundation 

will be the only means of preventing potential water intrusion or damage to foundation walls. 

 Downspouts that discharge water below grade should do so into nonperforated corrugated or 

smooth plastic pipe. The pipe should be run underground to a suitable outfall. Do not connect the 

gutter drain pipe to the foundation drainage system; this practice will soak the foundation. 

 If local storm water management regulations require roof water infiltration on-site, do so at a 

maximum practicable distance from the building foundation; minimum separation distances 

required by local storm-water regulations may not be adequate. 

 Gutters and downspouts should be resistant to corrosion and abrasion from flowing water; 

material choices include aluminum (most popular), vinyl or plastic, copper, and coated metal 

(baked enamel or galvanized). 

 Use a gutter splash shield at inside corners (i.e., valleys) where fast moving water in a roof valley 

may “overshoot” the gutter.

 Gutters, downspouts, and splash blocks must be cleaned and properly maintained by the 

homeowner; where significant sources of roof and gutter debris exist (e.g., trees) a high-quality 

gutter guard system may be helpful in reducing the frequency of unclogging gutters. 

A typical gutter installation uses a 5” deep K-style gutter with 2” by 3” downspouts at one or both terminal 

ends of gutters. While this “field practice” may be adequate for many homes in typical climate conditions, 

the following simplified design approach is useful to determine situations where such designs may be 

inadequate. 

Only two steps are required to properly design a steep-slope roof drainage system using standard 

guttering products. Of course, this design method (drawn mostly from a 1999 industry reference15) 

assumes gutters are properly installed for positive drainage and that gutters are regularly cleaned to 

maintain a “clog-free” condition.

Step 1: Determine Design Rainfall Intensity 

The design rainfall intensity for roof drainage design is sometimes based on a 10-year return period and 5-

minute duration (Figure 4–8). However, other design return periods and durations may be used effectively 

or required by code (e.g., commercial building roof drainage). Conversion factors for other acceptable 

 

15 “All About Gutters” by Andy Engel. Fine Homebuilding Magazine, August/September 1999. 

www.finehomebuilding.com 
Durability by Design 39

design conditions are included in Figure 4–8. A standardized design criterion in U.S. model building codes 

for homes does not exist, so practical experience and judgment are important. 

Figure 4–8: Rainfall Intensity Map of the United States 

Step 2: Determine Roof Drainage System Spacing and Layout 

Based on a selected gutter size and type as well as the design rainfall intensity from Step 1, determine the 

maximum allowable plan (horizontal) area of the roof that the gutter can adequately serve from Table 4–5, 

including any adjustment required by the table notes. Based on this area and the roof geometry, 

downspout spacing and locations can be determined as shown in the example below. With use of the 

suggested minimum downspout sizes in Table 4–5, the gutter size will control the spacing of downspouts. 

It is also generally recommended that downspouts should serve no more than 50’ of gutter length. 
40 Durability by Design

Table 4–5: Maximum Allowable Tributary 

(Roof Slope ≤ 5:12)

Roof Plan Area (ft2)

Gutter

Design Rainfall Intensity (in/hr) Suggested 

Minimum 

Size and Type 3 4 5 6 7 8 9 10 Downspout 

Size

5” 1⁄2-round 775 581 465 387 332 291 258 232 3” dia.

6” 1⁄2-round 1272 954 763 636 545 477 424 382 3” or 4” dia.

4” K-style 763 572 458 382 327 286 254 229 2”x3”

5” K-style 1399 1050 840 700 600 525 466 420 2”x3” or 3”x4”

6” K-style 2279 1709 1367 1139 977 854 760 684 3”x4”

Table Notes: 

a. The tributary area served by gutter is defined by L x W. L is the length of the gutter to both sides of a downspout 

measured to termination of the gutter or to the high-point (drainage divide) between downspouts. W is the plan 

(horizontal) distance from the eave to the ridge of the roof area served. 

b. The values in the table assume gutters with a minimum slope to prevent ponding and reverse flow. For gutters 

sloped at 1/16” per foot or greater, the table values may be multiplied by 1.1. 

c. Allowable tributary roof plan areas (drainage areas) in the table are intended for roof slopes ≤ 5:12. For steeper 

roof pitches, multiply the tabulated areas by 0.85. 

Example: Sizing roof gutters and downspouts in two steps 

For the house located in Savannah, GA, as shown in Figure 4–9, the following example is provided to 

illustrate this best practice: 

Figure 4–9: Gutter Design Example 
Durability by Design 41

Step 1: Determine Design Rainfall Intensity 

From Figure 4–8, a design rainfall intensity of 7 in/hr is determined for the site. 

Step 2: Determine Roof Drainage System Spacing and Layout 

A 5” K-style gutter is selected from Table 4–5 with a maximum allowable roof tributary plan area of 600 ft2. 

Because the roof slope is 6:12, the allowable tributary roof area is 0.85 x 600 ft2 = 510 ft2. The actual 

horizontal (plan) roof area for the side shown is (14’x34’) + (14’x12’) = 644 ft2. 

The number of downspouts required is 644 ft2 / 510 ft2 = 1.3. The number of downspouts should always be 

rounded up, so two downspouts should be used, one at each end of the L-shaped gutter layout. The 

downspout size may be 2”x3” or 3”x4” as suggested in Table 4–5. Use of the larger size downspout, if 

architecturally acceptable, may help reduce the potential for clogging from debris. 

4.2.7 Specify a Climate-Appropriate Exterior Wall Covering Method 

Exterior wall covering materials and methods may look the same on the outside, but they may not perform 

the same even though they may be considered equivalent from a building code perspective. Exterior wall 

covering assemblies can include features behind the siding that either improve or diminish a wall’s ability 

to protect a building from rainwater intrusion and moisture accumulation. The importance of this 

consideration varies depending on climate. For example, some regions of the U.S. experience as much as 

40”/year of rainfall impinging on vertical surfaces such as walls of buildings! Thus, it is appropriate to 

consider that walls should perform like a vertical roof surface in deflecting and draining as much as 20 

gallons per square foot per year of rainwater away from a building. 

This section presents a simple procedure for selecting a durable and climate-appropriate method for 

constructing an exterior wall covering assembly—something that U.S. building codes do not address—to 

ensure risk-consistent performance in all climate conditions. Other important considerations include the 

proper use and integration of flashing methods (Section 4.2.8) with a variety of water-resistive barrier 

(WRB) options as addressed in the text box on page 46. Proper specification of wind-driven rain resistant 

window and door products is also important; this topic is addressed in Chapter 8 as it is especially 

important in areas subject to natural hazards such as tropical storms and hurricanes, although severe 

thunderstorms can also cause similar conditions but with a much shorter duration. 

There are essentially four methods for constructing a weather-resistant exterior wall envelope as 

illustrated and described in Table 4–6. Each of these methods of construction provides different levels for 

managing or resisting rainwater by relying, to differing degrees, on one or more of the following three 

principles: 
42 Durability by Design

Deflection—Use of cladding to deflect water away from protected building components including 

the underlying water-resistive barrier layer. 

Drainage—Use of a water-resistive barrier (WRB) integrated with other wall components and 

flashing to create a continuous drainage plane that directs water that leaks behind the cladding 

(not deflected by the cladding surface) to drain downward and out of the assembly. 

Drying—Use of a vented air-space behind siding to promote evaporation of water that is not 

immediately drained or which is absorbed. 

A fourth principle that always applies is related to the durability of the materials used for the various 

components of an exterior wall covering assembly as well as the protected structural material within the 

wall. Specification of building materials that are tolerant of moisture and resistant to degradation (rot or 

corrosion) will lessen the consequences of any moisture that defies the first three principles. Differences in 

the application of these principles are largely responsible for differences in the performance and durability 

of exterior walls. Other determining factors include severity of wind-driven rain climate, roof overhang 

shielding of the wall, and site wind exposure—all of which are addressed in this section. 
Durability by Design 43

Table 4–6: Exterior Wall Covering Assembly Methods

Drained Cavity Method—The drained cavity method relies 

on deflection, drainage, and drying to protect the wall from 

moisture damage. There are many possible variations. In 

general, a cavity exists to separate the cladding material 

from the surface of the underlying water-resistive barrier. 

The depth of the cavity, however, may vary. For example, 

vinyl siding may be placed directly on the WRB layer and still 

provide a cavity only restricted at points of contact (e.g., nail 

flanges). A minimum cavity depth of 3/8”is sometimes 

recommended, but often a depth of 3⁄4” or 1 1⁄2” is used 

based on the standard thickness of wood furring materials. 

Drained cavities increase the life of exterior finishes on 

wood surfaces and promote drying of wall assemblies after 

wetting episodes. For anchored masonry (brick) veneer, a 1” 

cavity depth is recommended to allow space for brick 

placement and mortar excesses. The drained cavity 

approach also can be applied to Portland cement stucco 

with use of a drainage mat or other appropriate means of 

creating a drainage cavity.

Face-sealed Method—This method relies exclusively on the 

ability of the outer surface of the wall and sealed joints 

around penetrations to deflect water and prevent it from 

penetrating the wall surface. If a defect in the wall surface 

or joint detailing (e.g., caulk) exists or occurs over time, then 

water will penetrate and potentially accumulate in the wall, 

causing damage to any moisture-sensitive materials within 

the assembly. One example of this type of system is known 

as conventional or barrier EIFS (exterior insulation finish 

system). However, current model building codes only allow 

the use of drainable EIFS (i.e., drained cavity) on residential 

wood-frame construction.
44 Durability by Design

Concealed Barrier Method—The concealed barrier method 

relies on porous cladding material adhered to or placed 

directly on an internal (concealed) water barrier or drainage 

plane. A common example is conventional stucco applied on 

two layers of Grade D building paper attached to a woodframe wall. This method also relies primarily on deflection 

of rainwater (like the face-sealed system) but also has 

limited capability to absorb moisture to later dry and to 

drain moisture through weeps (e.g., weep screed) at the 

base of the wall. However, there is no open drainage 

pathway to allow water to freely drain from the concealed 

moisture barrier. Also, if moisture is stored in the cladding 

from a recent rain event and the rain event is followed by 

sun exposure, the effect is to drive water vapor into the wall 

assembly, especially when the wall design approach uses a 

vapor permeable WRB material.

Rainscreen Method—A rainscreen is similar to the drained 

cavity method with some added features that reduce airpressure differential across the cladding system during 

wind-driven rain events; thus, water penetration through 

the cladding layer into the drainage cavity is further limited. 

At a minimum, this approach involves use of a rigid air 

barrier layer behind the cladding that is able to resist design 

wind pressures. Thus, wind pressure across the siding 

(which is vented or air-permeable and not air-tight) is 

reduced and is less likely to result in water being driven 

through the siding. Also, the cavity between the cladding 

and water/air barrier must be compartmentalized by use of 

air-tight blocking or furring at corners of the building (as a 

minimum practice) and sometimes more frequently for 

large facades with varying wind pressure regions. This 

feature prevents pressure differences on different surfaces 

of the building from “communicating” through a continuous 

cavity behind the cladding, which can cause unintended 

pressure differences across the cladding that drive 

rainwater through the cladding into the drainage cavity. 

Although the rainscreen method offers improved 

performance, the drained cavity method is usually 

considered a more practical alternative for typical 

homebuilding applications.
Durability by Design 45

Integrated Design and Construction: Water-resistive Barriers 

Cladding provides a primary surface for deflection and drainage of moisture. But, some amount of water still 

leaks behind essentially all claddings for various reasons. Therefore, a water-resistive barrier (WRB) provides 

a secondary (and final) drainage plane. It is intended to create a continuous water-resistive layer that drains 

water out from behind the cladding and prevents it from reaching sensitive materials deeper within the wall 

(e.g., wood sheathing and framing). 

Common WRB materials include various types of synthetic building wraps, 15# tarred felt, grade D building 

paper, certain insulating sheathing products, certain proprietary structural sheathing products, adhered 

membranes, and spray-applied coatings. The application of these materials must comply with the building 

code, follow the manufacturer’s installation instructions, and be properly integrated with approved flashing 

or sealing methods at penetrations and joints. 

Not all water-resistive barrier materials and assemblies are equal in performance, nor are they all required 

to meet a specific or consistent level of performance in a realistic water penetration assembly test, if 

required at all for some WRB materials. Therefore, it is recommended that WRB assembly test data be 

requested of the WRB manufacturer and, if available with acceptable results, follow the detailing 

requirements used in the test. 

Various WRB materials have different water vapor permeance (flow of moisture through a material) 

properties. Depending on climate and the degree of vapor permeability or impermeability, integration with 

the design and specification of other wall components for water vapor control may affect sheathing 

material selection, wall insulation amounts and locations, and optimal interior vapor retarder properties 

(refer to Section 4.3.3). 

The design process below can be used to assess and specify an appropriate method of construction for an 

exterior wall covering assembly (see Table 4–6) based on wind-driven rain climate, roof overhang 

protection, and wind exposure—the key factors governing the severity of rainwater loads experienced by 

walls. 

Step 1: Assess Site Climate Condition 

Climatic conditions are categorized on the basis of the potential for wetting of walls, especially wetting 

from wind-driven rain. The exposure categories are: 

 Severe—climates that cause frequent wetting due to wind-driven rain, such as coastal climates 

and areas prone to frequent thunderstorm events. 

 Moderate—climates that cause periodic exposure to wind-driven rain. 

 Low—climates that are relatively dry with little rainfall or wind-driven rain. 

The above classifications are intentionally subjective, as there are no clearly defined criteria in the United 

States for assessing wind-driven rain and its effects on building wall systems. However, a wind-driven rain 
46 Durability by Design

map is provided in Figure 4–10 as a means of classifying the severity of a local climate based on the above 

categories. 

Figure 4–10: Wind-driven Rain Map of the United States 

(Source: Underwood, S.J., A Multi-scale Climatology of Wind-driven Rain for the Contiguous United States 1971-1995, PhD 

Dissertation, University of Georgia, Athens, GA, 1999) 

Step 2: Assess Building Exposure 

The terrain surrounding a building impacts its exposure to wind-driven rain. The ratio of roof overhang 

width to the height of the protected wall below also alters the exposure of a given building wall to weather 

and wind-driven rain. Wide roof overhangs relative to wall height effectively reduce the exposure. 

Similarly, increased shielding of the site against wind tends to reduce the effects of climate. 

Table 4–7 may be used to determine a building’s exposure level, based on the climate condition 

determined in Step 1, the roof overhang ratio, and the wind exposure. The exposure level then leads to a 

basis for selecting an appropriate exterior wall covering assembly method in Step 3. The exposure levels in 

Table 4–7 can also be used on a smaller scale to get a sense of the exposure for particular faces of a 

building or even for specific envelope elements like a window. Understanding the exposure in this manner 
Durability by Design 47

can guide decisions on flashing details, potential use of greater overhangs, and areas to focus attention for 

construction quality control. 

Table 4–7: Building Exposure Levels

(H-high; M-moderate; L-low; N-negligible exposure)

Wind 

Exposurea

Overhang

Ratiob

(w/h)

Climate Severityc

(from Step 1)

Severe Moderate Low

No

Shielding

0 H H M

0.1 H M L

0.2 M L L

0.3 M L N

0.4 L L N

≥0.5 L N N

Shielded

0 H H M

0.1 H M L

0.2 M L N

0.3 L N N

0.4 L N N

≥0.5 N N N

Table Notes: 

a. The wind exposure conditions are explained as follows: 

 No Shielding (Open)—site receives no or little wind protection from surrounding buildings and natural 

obstructions to wind flow (e.g., open grassy field or waterfront exposure). 

 Shielded—site receives wind protection from surrounding dense development and/or closely spaced 

trees extending for a horizontal distance of at least 10 building heights from the building. 

b. Overhang ratio should account for both roof overhangs and overhangs from cantilevered floors. For 

a given wall, use the worst case (lowest) overhang ratio (w/h) where ‘w’ is the overhang width and 

‘h’ is the height of wall below the overhang or the distance from a component of interest (e.g., a 

window or door penetration) below the roof overhang, both in the same units of feet or inches. 

c. For buildings located at or near the top of topographic features such as ridges, bluffs, and 

escarpments, the indicated climate severity should be increased by one level or not more than ‘H’.

Step 3: Select an Exterior Wall Covering Assembly Method 

Based on the building exposure level determined in Step 2, use Table 4–8 to select an appropriate exterior 

wall covering assembly method based on relative performance expectations. Alternatively, other factors 

may be reconsidered in the building and site design to improve protection from rain, such as the use of 

larger overhangs to protect walls. 
48 Durability by Design

Table 4–8: Relative Performance of Weather-Resistant

Exterior Wall Methods a

Exposure

Level

(from Table 4–7)

FacesealedConcealed

Barrier

Drained

Cavity

Basic 

Rainscreen

High (H) NR NR Fair Good

Moderate (M) NR Fair Good Good

Low (L) Fair Good Good Good

Negligible (N) Good Good Good Good

Table Note: 

a. The ratings used to describe relative performance of the weather-resistant exterior wall systems are 

explained as follows: 

 Good—the system is likely to meet or exceed acceptable performance expectations and has a low 

risk of failure during the likely service life with a reasonable level of installation quality and 

maintenance. 

 Fair—the system is considered adequate, but may require careful attention to detailing, installation 

quality, and maintenance. The wall has a tolerable risk of failure during the likely service life. 

 NR—the system is not recommended for use on wood-frame home construction. 

Solid or mass walls, such as masonry and concrete wall systems without a separate exterior cladding, are 

not addressed in Table 4-6. These walls rely on deflection of rain as well as the ability to absorb moisture in 

a sufficiently thick and durable wall system. However, even these “mass” walls can become overwhelmed 

with moisture intake during extreme wind-driven rain episodes (e.g., hurricanes and tropical storms). 

Water-repellent surface treatments or coatings like latex paint may be applied to these walls to improve 

rain deflection and minimize absorption of moisture; however, such coatings should be semi-permeable to 

allow for drying towards the outside. Various water-repellant treatments are available for concrete and 

masonry, but they vary in cost, performance, and effective service life. Limited research indicates that 

polysiloxane-blended water repellents may provide the best water repellency and durability. 

Performance Evaluation of Water Repellents for Above-Grade Masonry, Research Highlights, Technical 

Series 00-118, Canada Mortgage and Housing Corporation, Ottawa Canada, www.cmhc-schl.gc.ca/Research. 

4.2.8 Flashing Matters: Do it Right! 

Water leaks in walls and roofs are commonly associated with flashing and detailing problems at 

penetrations and interfaces between wall and roof components. Despite the importance of flashing to the 

durability of buildings, building codes provide little practical guidance or defer to “manufacturer 

installation instructions” which may be incomplete or limiting in the choice of materials and methods used 

to construct a wall. 

This section focuses on key flashing concepts to provide a foundation for appropriate practices that can be 

extended to a variety of applications. In addition, typical building department inspections often do not 

address flashing adequately because it is usually concealed once exterior finishes are installed. It is 
Durability by Design 49

imperative that designers and builders consider this issue as a key element of construction plan detailing, 

construction trade coordination, and field quality control. Do not depend on caulk where flashing is 

feasible. Where caulking is unavoidable or necessary, refer to Section 4.2.9. 

Flashing materials and methods generally fall into one of the following three categories: 

Mechanical Flashing—Generally a rigid, preformed or formable, non-corrosive and water-resistant 

material (e.g., aluminum, copper, plastic, etc.) shaped to direct water away from joints or around 

components, mechanically fastened to a suitable substrate, and installed in an overlapping fashion 

to shed liquid water and prevent gravity-driven intrusion. Laps should be of a sufficient length to 

prevent liquid water intrusion due to capillary action or pressure differential. Mechanical flashings 

include preformed flashing components such as window and door pan flashings. 

Self-Adhering Flashing—Flexible, water-resistant facing materials coated completely or partially 

on at least one side with an adhesive material and which do not depend on mechanical fasteners 

for permanent attachment; also known as flashing tape. They are used to bridge gaps and joints 

between components such as window flanges, mechanical flashing, water-resistive barriers, and 

other components to provide a water-tight and durable seal such that liquid water is unable to 

penetrate by gravity, capillary action, or air pressure differential. 

Liquid-Applied Flashing—A type of flashing material that is a fluid or paste at the time of 

application and is applied by trowel, roller, spray, or other suitable method to provide a durable 

water-resistant coating or seal over compatible substrates and joints or interfaces between 

materials to prevent liquid water intrusion. 

The above flashing methods may be used individually or combined together for a variety of joint and 

interface details between various types of WRB materials and wall or roof components. In each 

application, installation and flashing details required by the WRB, flashing, or component (e.g., window or 

door) manufacturer should be consulted and component compatibility verified. In many cases, approved 

applications must use a very specific set of materials because they act as a system. 

The remainder of this section provides guidance and flashing concepts for a sample of applications with 

common materials. Regardless of the flashing method applied, the first step is to identify all the areas 

where flashing is needed. For example, refer to the house shown in Figure 4–11. As noted in Chapter 2 and 

shown in Figure 4–11, typical locations where flashing is very important (and often defective) include 

window and door heads, and step/kick out flashing at roof-wall intersections. While all flashing is 

important, these locations seem to be two of the most problematic and consequential. 
50 Durability by Design

Step flashing is illustrated in Figure 4–12. Step and kick out flashing should be used at roof-wall 

intersections, with the flashing extending at least 4” up the wall surface above the roof deck and 

integrated shingle-style with drainage plane above or sealed with self-adhering flashing tape approved 

for the application. 

Window flashing details should follow the window manufacturer’s installation instructions or provide an 

equivalent solution, as appropriate given the type of WRB approach being used. One example of window 

flashing sequence is shown in Figure 4-13, for a wall assembly which includes exterior foam insulation 

(which is becoming increasingly common based on energy codes). While the figure details the steps in this 

process, the key concept is integrating the WRB layer and window units with a compatible and approved 

self-adhering flashing tape to drain bulk water away from the window opening. This practice as well as 

other flashing practices should employ a drainage pan at the window sill to remove water that may 

penetrate the window unit itself (see Step 1 in Figure 4–13). 

Figure 4–14 illustrates a deck ledger flashing detail. This assembly often is created after the initial 

construction of a home, and is particularly important because it carries structural safety implications 

directly related to the long-term ability of the flashing to manage water. The flashing and fasteners used in 

treated wood materials must be compatible with wood preservative treatments for durable moisture 

protection (e.g., avoid aluminum or electroplate galvanized steel—see Section 8.2.2). 

For additional guidance on flashing methods and details, several additional resources with more extensive 

flashing details are noted in Section 4.4. Key flashing references listed in this section include Keeping Walls 

Dry (CMHC), Weather Resistive Barriers (U.S. DOE), and Guidance on Taped Insulating Sheathing Drainage 

Planes (U.S. DOE Building America program). Increasingly, useful flashing installation videos are easily 

found online as well. 
Durability by Design 51

Kick Out Flashing

Opening head/jamb/sill 

flashing

Horizontal change of materials 

flashing

Roof/wall junctions step 

flashing

Chimney flashing & counterflashing

Figure 4–11: Typical Locations Where Flashing Details Are Required 

Figure 4–12: Step and Kick Out Flashing 
52 Durability by Design

Figure 4–13: Window Flashing Sequence on a Wall With Exterior Foam Insulation 
Durability by Design 53

 

Figure 4–14: Deck Ledger Flashing Detail 

4.2.9 Properly Use Caulks and Sealants 

In the construction of exterior wall and roof coverings, there will be joints and seams that require or 

benefit from the appropriate specification, use, and maintenance of caulks or sealants. When possible, use 

flashing instead of caulking and use caulking only as a supplement to flashing that is properly integrated 

with the WRB installation (refer to Section 4.2.7 and 4.2.8). Whether caulking is used to cover up poor 

detailing or required as a necessary component in certain details, it can have a significant impact from a 

durability and liability standpoint.16

 

In the absence of guidelines for a specific application, the generic caulk recommendations in Table 4–9 

should be considered and the selected caulk manufacturer’s instructions carefully followed. Poor 

performing caulks or poorly installed caulking can fail in less than a year, creating an unreasonable 

expectation for maintenance and greater likelihood of moisture problems (see text box). A high quality 

caulk installation requires skill and appropriate ambient temperature, dry and clean surfaces, and an 

16 O’Connor, T.F., “The One Percent of Cost That Can Become 90 Percent of Trouble,” ASTM Standardization News, 

American Society of Testing and Materials, West Conshohocken, PA, June 2003. 
54 Durability by Design

adequate joint gap to allow the caulk to act elastically without pulling loose. Too thick of a bead restricts 

the flexibility of the caulk, which can result in failure of the joint. Instead, the use of backer rod really 

matters and can result in a better formed, more flexible bead that will adhere far better. For additional 

guidance, refer to appropriate resources in Section 4.4, such as ASTM C1193. 

Table 4–9: Caulk Selection Aida

Caulk Type Lifeb (yrs.) Relative 

Performance Comments

Oil-base 1–5 NR

Not recommended for durable exterior 

waterproof sealing

Latex (acrylic) 2–7 NR

Not recommended for durable exterior 

waterproof sealing

Butyls 5–10 Average No primer required

Acrylic (solvent 

release)

5–20 Average No primer required

Polysulfide 10–20 High Primer may be required for some surfaces

Silicone 10–25 High Primer may be required for some surfaces

Polyurethane 10–20 High Primer may be required for some surfaces

NR = not recommended 

Sources: CSA Standard A4440.4-2007, Annex C, Sealant Selection; Structures and Environmental Handbook, Eleventh 

Edition (Midwest Plan Service, 1983); Forgues, Y.E., “Properly Sealed Construction Joints,” Construction Practice, 

Institute for Research in Construction, National Research Council Canada, http://irc.nrc-cnrc.gc.ca 

Table Notes: 

a. This table is intended as a general guide. Manufacturer’s sealant application and installation 

recommendations should be consulted. 

b. Life expectancy estimates are based on ideal conditions with high quality installation. 

“Joints sealed with an elastomeric sealant usually fail from a combination of factors that can be summed 

up in six words—a lack of attention to detail. Too often, since the sealants are a small percentage of the 

work, they are perfunctorily specified, easily substituted, and haphazardly applied. Yet successful joints 

require meticulous design, precise sealant selection, and painstaking application.”

Why Sealant Joints Fail, Karen Warseck, AIA. 
Durability by Design 55

4.3 Recommended Practices for Water Vapor Management 

4.3.1 General 

With improved insulation requirements in modern energy codes, “green” codes, and above-code energy 

efficiency programs, envelope assemblies allow less thermal energy transfer resulting in energy savings. 

This benefit comes with a climate-dependent consequence: colder surface temperatures on materials to 

the interior side of an air-conditioned building in the summer, or exterior side in the winter. When these 

cooled surfaces are exposed to moisture-laden air leaks (from the exterior in the summer or from the 

interior in the winter) or excessive vapor diffusion through the assembly, they become more prone to 

condensation and moisture accumulation. Fortunately, this problem has durable solutions as discussed in 

this section. 

An effective air barrier installation (Section 4.3.2) and vapor diffusion control strategy (Section 4.3.3) have 

become increasingly important to the durability of modern energy efficient homes. Furthermore, new 

homes usually have a significant quantity of “built-in” or initial moisture embodied in materials like 

concrete and lumber which will create additional water vapor loads during the first year or so of a 

building’s operation as those products dry. Thus, reduction of “built-in” construction moisture has also 

increased in importance (Section 4.3.4). 
56 Durability by Design

Integrated Design and Construction: Water Vapor Control 

Uncontrolled and elevated indoor humidity levels have been identified as a key contributor where water 

vapor problems occur. Any attempt to control water vapor should start with attention to controlling indoor 

relative humidity levels and air-pressure differences across the building envelope. This integrated design 

topic is addressed in Chapter 5—HVAC and Plumbing. Appropriate mechanical system design (equipment 

sizing, ventilation, humidity control, etc.) provides a crucial “safety factor” against high indoor relative levels 

which can overwhelm reasonable attempts to cure the problem by use of air barriers and vapor retarders. 

As with the design of a structural system, the moisture control system of a home is only as strong as its 

weakest link. 

 Window condensation is often a sign of elevated indoor RH levels. 

4.3.2 Leverage Air Barriers to Manage Air Leakage and Moisture 

Air barriers are often considered purely as an efficiency measure to control energy losses associated 

with air-leakage to or from the building’s conditioned spaces. In reality, air barriers serve energy 

efficiency and moisture control functions. In fact, movement of moisture-laden air into well insulated 

building assemblies can cause much more condensation or moisture accumulation to occur within the 

assembly than might otherwise result from water vapor diffusion alone. Controlling water vapor 

diffusion with a vapor retarder is practically futile without also controlling air leakage with an air barrier. 

Thus, air barriers deserve special attention in design and construction from a moisture durability 

perspective. 

Going beyond code minimum practices can provide significant benefits in energy conservation and 

moisture control. For example, Table 4–10 provides important locations for air barriers intended to 

manage moisture and uphold the performance of the insulation based on the EPA’s ENERGY STAR 

Homes program. These may exceed minimum code requirements for only a single air barrier location on 

the interior or exterior side of an assembly. For example, in Climate Zones 4–8 an air barrier is required 

on the interior and exterior side of wall insulation for an exterior wall assembly. Similarly, air barriers are 

required at interior horizontal surfaces and exterior perimeter vertical surfaces of insulation for floors in 

all climate zones. Air impermeable insulation materials such as closed-cell spray foam can serve the 
Durability by Design 57

purpose of insulation as well as air barrier, and may be considered as an alternative to the 

recommendations in Table 4–10 which presumes the use of air-permeable cavity insulation. 

Table 4–10: Summary of EPA ENERGY STAR Air Barrier Requirements

Climate Zone

(Figure 4–15) Interior Air Barriera

Exterior Air Barriera

WALLS

1–3 Not required Required at exterior vertical 

4–8 surface of wall insulation Required at interior vertical 

surface of wall insulationb

CEILINGS

1–3 Required at interior or exterior horizontal surface of insulationc

4–8

Required at interior horizontal 

surface of insulationc Not required

FLOORS

1–8

Required at interior horizontal 

surface of insulation

Required at exterior (perimeter) 

vertical surface of insulation

Table Notes: 

a. Air barriers must be continuous and fully aligned with insulation. 

b. Also includes floor cavities which are part of the wall’s thermal plane, like a rim joist between upper and 

lower story walls. 

c. Also provide an air barrier at the exterior vertical surface of insulation near eaves using a wind baffle or 

equivalent. 

Figure 4–15: The International Energy Conservation Code Climate Zone Map 

Source: U.S. Department of Energy 
58 Durability by Design

Figure 4–16 shows one example of an air barrier installation on the interior side of an exterior wall and 

another on the exterior side. In both cases the air barrier is continuous with the ceiling at the roof level. 

Where an interior and exterior air barrier is installed, both of these practices are used as a means for 

improved insulation performance (energy efficiency) and air leakage control (water vapor control and 

energy efficiency). 

A practical interior air barrier installation involves detailing the interior gypsum board finish as an air 

barrier layer along with framing connections between walls. A typical exterior air barrier involves the 

use of a water-resistive barrier membrane (e.g., wrap) with taped joints and penetrations or an 

approved exterior panel type material, also with taped or sealed joints and penetrations. The two 

methods shown in Figure 4–16 are common examples. Whichever interior and/or exterior air barrier 

method and material is used must be coordinated with the vapor control strategy for the assembly 

(Section 4.3.3) and the rainwater management approach for exterior air barrier applications 

(Section 4.2).
Durability by Design 59

 

Figure 4–16: Examples of Exterior and Interior Continuous Air Barrier Strategies 

Source: Air Leakage Guide, U.S. Department of Energy, Building Technologies Program. 

Original Images from Building Science Corporation’s “Air Barriers” publication (Research Report 0403, 2004).

While there are a variety of air barrier materials worth considering and integrating into the overall 

moisture control strategy for a building envelope, these materials all must comply with building code 

and certain material or assembly testing requirements for air permeance. Of course, some materials like 

wood structural panels, gypsum board, concrete, and sheet aluminum or polyethylene, have obvious air 

barrier material characteristics. Others may depend on characteristics that are not easily observed by 

the naked eye. Thus, manufacturer data on air permeance (and also other properties such as vapor 

permeance) should be sought and used as a basis for design and material selection. 
60 Durability by Design

It is always necessary to use air barrier materials in a way that creates a continuous air barrier layer to 

prevent air leakage through a multitude of potential leakage points (Figure 4–17). Building codes and 

good practice include requirements to seal all joints and penetrations for visual confirmation or, 

alternatively, require air pressure testing of the building (e.g., “blower door” test) to verify performance 

of the final air barrier installation. This guide recommends use of a blower door test. It will measure the 

air change per hour (ACH) of a building to ensure energy code compliance and is a useful diagnostic 

and training tool (with the use of a smoke stick) to help locate and correct air leaks that may 

otherwise go undetected and cause potential moisture durability problems within assemblies. 

Figure 4–17: Air Leakage Points Requiring Special Attention for a Continuous Air Barrier Installation 

Source: Air Leakage Guide, U.S. Department of Energy, Building Technologies Program. 
Durability by Design 61

Integrated Design and Construction: Fuel-fired Heating Equipment and Air Barriers 

What is the conflict between a gas furnace or water heater and an air barrier? One wants air and the other 

holds it back. If an air-tight building is depressurized due to exhaust fan operations, wind, or the “stack 

effect” in the winter, then back-drafting of combustion equipment is more likely to occur and introduce 

combustion byproducts (e.g., water vapor and carbon monoxide) into the home. Because of these health 

and safety concerns, sealed combustion equipment with power drafting is recommended in Chapter 5. 

4.3.3 Water Vapor Diffusion Control: Do Your Homework...Before You Build 

Water vapor diffusion speaks to the movement of water in its gas or vapor form into and through 

materials. Thus, water vapor control involves selecting materials and assemblies with appropriate 

properties to manage, not eliminate, the effects of water vapor. 

The goals of water vapor control are: 

 To prevent excessive vapor movement into assemblies, 

 To allow vapor to move out, and 

 To control temperatures within the assembly to keep it in the vapor form (e.g., prevent 

condensation). When organic materials like wood are exposed to excessive water vapor or 

condensation, they will increase in moisture content to a point where damage may occur. 

Thus, water vapor control is a climate-dependent balancing act to control the accumulation of moisture to 

tolerable levels and ensure that the ability of an assembly to dry out exceeds its potential to get wet. In 

other words, moisture that accumulates to tolerable levels due to diffusion in one season must dry out by 

diffusion in the next season to avoid a long term cycling of moisture that results in a gradual year-to-year 

accumulation. In addition, to prevent mold growth moisture accumulation within organic materials (e.g., 

wood, paper-faced drywall) or high surface RH levels on these materials (e.g., greater than 80%) must be 

prevented from occurring over an extended period of time. 

To the extent that diffusion drying (moisture removal) exceeds diffusion wetting (moisture accumulation) 

over the course of a year, an assembly is considered to be more or less able to tolerate unintentional and 

uncertain amounts of moisture that may be introduced from other sources such as rainwater leaks 

(Section 4.2) or moisture-laden air leaks (Section 4.3.2). Building materials such as dimensional lumber, 

which have the ability to “store” moderate amounts of water vapor, can also serve as a “buffer.” They can 

absorb moisture during seasons with greater levels, and then release it when conditions are more 

conducive to drying. However, no reasonable approach to water vapor control is able to make up for leaks 

of rainwater or moist air due to poorly installed water-resistive barriers, flashing, and air barriers. 

Therefore, the recommendations for water vapor control in this section are predicated on a quality 
62 Durability by Design

execution of recommendations in Section 4.3.2 and Section 4.2. Similarly, excessive indoor relative 

humidity can overwhelm a reasonable vapor control strategy; control of indoor relative humidity is 

addressed in Chapter 5. 

As shown in Figure 4–18, adequate moisture-vapor control is much like the well-known fire prevention 

triangle. If any point of the triangle is not properly addressed, water vapor condensation and moisture 

accumulation problems are more likely to occur. A balanced design approach will consider all of these 

factors. 

Figure 4–18: Water Vapor Control “Triangle” and Two Strategies for a Cold Climate Application 

Regarding the “peak” of the triangle in Figure 4–18, an optimal range for indoor relative humidity is no 

more than 40% in the winter and no more than 60% in the summer. In particularly cold climates (e.g., 

Climate Zones 5 or greater), even lower indoor relative humidity levels during winter are recommended 

and may be needed as part of a reasonable water vapor control strategy. This recommendation is based 

on balancing an acceptable range of indoor relative humidity for occupant comfort (30% to 60% RH) with 
Durability by Design 63

the need to protect the building envelope from excessive water vapor flows during the winter heating and 

summer cooling seasons. Controlling indoor relative humidity is addressed in Chapter 5. In addition, 

construction moisture (Section 4.3.4) and foundation moisture (Chapter 3) can contribute significant 

amounts of water vapor to indoor air if not adequately addressed. 

The base of the triangle in Figure 4–18 involves the control of water vapor diffusion and internal surface 

temperatures. These relate to design of the building envelope assembly and require an integrated 

approach. There are three commonly accepted practices that, when done correctly, provide walls with 

a greater tendency to remove moisture by diffusion rather than allowing moisture to accumulate via 

moisture diffusion. 

One method controls moisture condensation or accumulation by reducing the entry of water vapor into 

the assembly (i.e., more emphasis is placed on vapor retarders). This strategy (Figure 4–18 lower left) also 

uses a vapor permeable wall exterior to allow outward drying during the winter. This assembly assumes 

vapor permeable cavity insulation. 

A second method (Figure 4–18 bottom center) incorporates air and vapor impermeable (low permeance) 

insulation materials in the cavity to control vapor flow through the assembly while still allowing materials 

on each face to dry to the interior or exterior, respectively. Assemblies using this strategy include SIPs 

panels and wood-framed assemblies with the cavity filled with air and vapor impermeable spray foam 

insulation. 

A third method (Figure 4–18 lower right) uses exterior insulation to keep the temperature of the assembly 

warmer. This temperature control approach prevents condensation from occurring on a surface within the 

assembly that might otherwise drop below the “dew point” temperature.

17 This assembly assumes vapor 

permeable cavity insulation. 

Finally, “vapor open” walls that allow water vapor diffusion wetting and drying in both directions may also 

be considered, but usually work best in dryer climates and those that are not very cold. Again, balance is 

the key. 

The principles behind the vapor control approaches described above are incorporated into “three rules” 

for water vapor control: 

 

17 The dew point temperature is most simply defined as the temperature at which dew (i.e., condensation) begins 

to form. For example, when air with a given water vapor content is exposed to a surface colder than the air, 

condensation will occur when the surface temperature is at the “dew point.” The more water vapor in the air, the 

more likely a surface will be cold enough to cause condensation to occur. 
64 Durability by Design

1. Maintain the envelope assembly’s ability to dry in at least one direction by not installing lowperm vapor retarders (e.g., vapor barrier) or other low-perm vapor retarding materials on 

both sides of an assembly. 

2. Do not put a low-perm vapor retarder (e.g., vapor barrier) or vapor retarding materials (e.g., 

vinyl wall paper) on the inside of an assembly in a hot/humid climate. 

3. Depending on climate zone and insulation strategy, seek to optimize the assembly’s ability to 

dry and limit the potential for wetting by vapor diffusion. 

The first two rules are pretty straightforward, yet deserve mention because failures to follow these two 

rules have resulted in significant moisture damage to buildings. The last rule, however, is a good bit trickier 

because it is a matter of optimization and there are many ways to achieve an optimal or at least 

acceptable assembly (and also many more ways to achieve one that is not). 

Tables 4–11 and 4–12 are offered as aids in applying the above three rules in most climate zones. Climate 

Zone 8 (subarctic) is excluded from this document due to its limited application in the United States and, in 

such severe conditions, the reader is encouraged to use a specially designed solution that meets or 

exceeds locally applicable code requirements. The provisions summarized in Tables 4–11 and 4–12 are 

based on a compilation of requirements in Part 9, Sections 9.25.4 and 9.25.5 of the 2010 National Building 

Code of Canada, Section 702.7 of the 2015 International Residential Code, Section 1405 of the 2015 

International Building Code, and various substantiating technical resources.18, 19, 20, 21, 22, 23, 24, 25, 26, 27

 Also 

included are a few modifications intended to enhance performance, offer guidance, or draw attention to 

considerations where the building code may be silent, incomplete or vague. However, it should be 

 

18 Lstiburek, J. (2011). Building Science Digest 106, Understanding Vapor Barriers, rev. April 2011. 

www.buildingscience.com 

19 Lstiburek, J. (2004). Research Report 410, Vapor Barriers and Wall Design, November 2004, 

www.buildingscience.com 

20 Straube, J. (2012). High Performance Enclosures: Design Guide for Institutional, Commercial and Industrial 

Buildings in Cold Climates, Building Science Press, www.buildingscience.com 

21 Straube, J. (2012). High Performance Enclosures: Design Guide for Institutional, Commercial and Industrial 

Buildings in Cold Climates, Building Science Press 

22 Karagiozis, A, Lstiburek, J., Desjarlais, A. (2007). Scientific Analysis of Vapor Retarder Recommendations for Wall 

Systems Constructed in North America. ASHRAE, Buildings X. 

23 Kumaran, M.K., and Haysom, J.C. (2000). Low-Permeance Materials in Building Envelopes, Construction 

Technology Update No. 41, National Research Council of Canada (revised March 2002) 

24 Kumaran, M.K. and Haysom, J.C. (2001). Avoiding Condensation with low-permeance materials, NRCC-44704, 

National Research Council Canada, Institute for Research in Construction 

25 Chown, G.A. and Mukhopadhyaya, P. (2005). NBC 9.25.1.2: The on-going development of building code 

requirements to address low air and vapour permeance materials, NRCC-47656, National Research Council Canada, 

Institute for Research in Construction 

26 Brown, W.C., Roppel, P., and Lawton, M. (2007). Developing a Design Protocol for Low Air and Vapour 

Permeance Insulating Sheathing in Cold Climates, Buildings X, ASHRAE 

27 NRC-Canada (1997). Air barrier Systems for Walls of Low-Rise Buildings: Performance and Assessment, ISBN 0-60-

16862, NRCC 40635, National Research Council of Canada 
Durability by Design 65

understood that these recommendations are based on various sources and assumptions, and do not 

preclude alternative solutions or exclusively define acceptable solutions. Therefore, the user is encouraged 

to use good judgment, seek professional assistance, verify the suitability of these requirements, and 

confirm compliance with the locally applicable building code and energy code. 

The application of Tables 4–11 and 4–12 requires an understanding of the following code definitions: 

VAPOR RETARDER CLASS. A measure of the ability of material or an assembly to limit the 

amount of water vapor that passes through that material or assembly. Vapor retarder class shall 

be defined using the desiccant (dry cup) method with Procedure A of ASTM E 96 as follows: 

Class I: 0.1 perm or less (e.g., sheet polyethylene, unperforated aluminum foil) 

Class II: 0.1 < perm ≤ 1.0 perm (e.g., kraft-faced fiberglass batts) 

Class III: 1.0 < perm ≤ 10 perm (e.g., latex or enamel paint appropriately rated and 

installed) 

VAPOR PERMEABLE. The property of having a water vapor permeance rating of 5 perms (2.9 x 

10-10 kg/Pa · s · m2) or greater, where tested in accordance with the desiccant method using 

Procedure A of ASTM E 96. For the purposes of this document, vapor permeability also is 

permitted to be assessed using the wet cup method (Procedure B) of ASTM E96. 

The vapor permeance of specific vapor retarder materials should be verified by checking product-specific 

test data in the process of applying the requirements of Tables 4–11 and 4–12. In addition, other wall 

material layers such as exterior sheathing material, building wrap, spray foam, and foam sheathing also 

should be verified. While generic data is available from various sources, such as the ASHRAE Handbook of 

Fundamentals, product-specific test data from the product manufacturer is usually the best source as 

properties can vary significantly even among like products. It also is important to consider the product 

manufacturer’s design data and technical resources which may contain alternative solutions than those 

included in Tables 4–11 and 4–12. 

While the vapor retarder class as defined above is based on vapor flow under relatively dry conditions, it 

also is important to know if the material’s properties change when tested under more humid conditions 

(i.e., perm rating per the wet cup or Method B of ASTM E 96). For example, uncoated wood sheathing 

products may be rated as a Class II vapor retarder under the “dry cup” condition, but can become vapor 

permeable (> 5 perm) under the “wet cup” condition. Similarly, some common vapor retarders, such as 

kraft paper, and other proprietary “smart vapor retarder” products, exhibit an ability to restrict vapor flow 

under low humidity conditions and permit vapor flow when drying conditions are needed. For example, 

the ASHRAE Handbook of Fundamentals reports that kraft paper has an ASTM E96 dry cup perm rating of 

about 0.3 perms (Class II vapor retarder) yet also has an ASTM E96 wet cup perm rating of about 1.8 perms 
66 Durability by Design

or greater (Class III vapor retarder)—a factor of 6 difference in vapor permeance due to difference in 

ambient air humidity. Thus, the selection of a smart vapor retarder can help optimize an envelope 

assembly to resist wetting during one period of the year and yet also promote drying during another 

season. 

Table 4–11: Water Vapor Control Methods and Requirements for Envelope Assembliesa

Climate Zone 

(Figure 4–15) Vapor Retarder Class and Location Requirements

1–2

A Class I or II vapor retarder should not be applied on the interior face of the 

envelope assembly. Use of a Class III vapor retarder on the interior face is permitted.

b

3 –7

Use one of the following methods depending on climate zone, insulation strategy, 

and vapor permeance of materials located on the exterior face of an envelope 

assembly:

A. Cavity Insulation Only (using air- or vapor permeable insulation): 

i. Use a Class I (CZ 5-7) or Class II (CZ 4-6) or Class III (CZ 3) vapor retarder on 

the interior side of the cavity insulation; it is recommended that vapor 

permeablecsheathing materials be used on the exterior side in CZ 5-7, or

ii. Use a Class III vapor retarder on the interior side of the cavity in CZ 4-6 

with vented claddingdand exterior sheathing as follows: wood structural 

panel (CZ 4-5), fiberboard (CZ 4-6), or gypsum (CZ 4-6).

B. Cavity Insulation + Continuous Insulation, or Spray Foame: 

Use a Class II or Class III vapor retarder on the interior face of the envelope 

assembly and meet or exceed the appropriate minimum insulation ratio in 

Table 4–12. Alternatively, use closed cell spray foam within the cavity in 

combination with or in lieu of exterior continuous insulation (refer to 

footnote c of Table 4–12). 

8 (subarctic) Design Recommended

Table Notes: 

a. The water vapor control methods and requirements of this table are best used under the following conditions: 

i. Indoor relative humidity not exceeding 40% (CZ 3-4), 35% (CZ 5-6), or 30% (CZ 7) during the coldest months of 

the winter heating season; lower indoor relative humidity will improve water vapor performance but may cause 

excessively dry conditions for human occupancy. 

ii. Use of a continuous air barrier system as described in Section 4.3.2 and quality installation methods to minimize 

uncontrolled air leakage into and through the assembly. 

iii. Use of a continuous water-resistive barrier system and flashing, together with a climate-appropriate exterior wall 

covering method as described in Sections 4.2.7 and 4.2.8, to minimize potential for rainwater intrusion. 

iv. Wall framing and other materials, such as wet-blown insulation, are dried to 15% or lower moisture content 

prior to installation of an interior vapor retarder (where required) and interior finishes (Section 4.3.4). 

b. A Class II vapor retarder that also qualifies as Class III using the wet cup method of ASTM E96 shall be permitted on the 

interior face (example: kraft-faced fiberglass batts). 

c. Vapor permeability shall be permitted to be assessed using the wet cup method (Procedure B) of ASTM E96. Also, other 

common recommendations include use of a 5:1 (outside to inside) permeance ratio. For example, if a 0.1 perm (Class 1) 

vapor retarder is used on the inside then minimum 0.5 perm is required on the outside. Similarly, if a 1 perm vapor 

retarder is used on the inside, then a minimum 5 perm is required on the outside. However, it is not recommended to 

extend this permeance ratio rule to the extreme of 0.05 perm inside (e.g., polyethylene sheet) with minimum 0.25 perm 

outside, because such a wall becomes questionable in its drying potential should it become wet due to causes other than 

vapor diffusion. 
Durability by Design 67

d. For the purposes of this table, vented cladding includes (1) vinyl lap or horizontal aluminum siding, (2) brick veneer with a 

clear airspace, or (3) other approved vented claddings with equivalent vent area. 

e. These provisions assume vapor impermeable continuous insulation, and thus also apply to continuous insulation which is 

more vapor permeable. In Climate Zones 5–7, where the exterior continuous insulation is a Class II vapor retarder material 

or has greater vapor permeance, use of a Class I interior vapor retarder on the interior may provide improved performance 

or allow reduction of minimum insulation ratios recommended in Table 4–12. 

Table 4–12: Minimum Insulation Ratios for Use With Class II and III Interior Vapor 

Retarders on Walls with Exterior Continuous Insulationa

Climate Zone

(Figure 4–15)

Maximum 

Heating Degree 

Days

(HDD65oF)

Class II Interior Vapor 

Retarderb

Class III Interior Vapor 

Retarderb, c

Min. 

Re/Ri

d

Examplese

(Ri + Re,ci)

Min. 

Re/Ri

d

Examplese

(Ri + Re,ci)

1–3 < 5,400 No limit No Limit No Limit No Limit

4 5,400 0.2 R13+R2.6ci

R20+R4ci 0.2 R13+R2.6ciR20+R4ci

5 (cold) 7,200 0.2 R13+R2.6ci

R20+R4ci 0.35 R13+R4.6ciR20+R7ci

6 (cold) 9,000 0.25 R13+R3.3ci

R20+R5ci 0.5 R13+R6.5ciR20+R10ci

7 (very cold) 12,600 0.35 R13+R6.5ci

R20+R7ci 0.7 R13+R9.1ciR20+R14ci

8 (subarctic) >12,600 Design Recommended

Table Notes: 

a. Refer to Note ‘a’ in Table 4–11 above for conditions of use. 

b. For the purpose of this table, a Class II vapor retarder includes kraft paper facer or other Class II vapor retarder materials 

that also qualify as Class III using the wet cup method of ASTM E96. Also, refer to footnote ‘e’ of Table 4–11. Also, a Class I 

vapor retarder may be permissible for continuous insulation materials that are not vapor impermeable—refer to 

manufacturer data. 

c. Spray foam with a maximum permeance of 1.5 perms at the installed thickness, applied to the interior cavity side of wood 

structural panels, fiberboard, insulating sheathing, or gypsum, is deemed to meet the continuous insulation requirement 

where the spray foam R-value meets or exceeds the specified continuous insulation R-value. 

d. Re = exterior continuous insulation (ci) component R-value; Ri = cavity insulation component R-value. 

e. Example insulation solutions are shown for combinations of cavity insulation (first value) plus continuous insulation 

(second value) that satisfy the required minimum insulation ratio. Other solutions of cavity insulation and continuous 

insulation, including continuous insulation only, with equal or greater insulation ratio are acceptable. Increasing the 

insulation ratio by increasing the continuous insulation R-value and/or decreasing the cavity insulation R-value will 

improve water vapor performance. These insulation ratios and their application address minimum requirements for water 

vapor control only; thermal performance for energy code compliance must be separately verified. 

Example: Using insulation ratios to check energy code solutions and alternative wall 

insulation strategies for adequate moisture durability 

Given: Assume the energy code requires R20+5 (2x6 wall with R20 cavity insulation and R5 continuous 

insulation). 

Find: What is the maximum (coldest) permissible climate zone for this wall with a Class II or Class III 

interior vapor retarder? 
68 Durability by Design

Solution: First, determine the insulation ratio, Re/Ri = 5/20 = 0.25. In accordance with Table 4–12, the 

maximum/coldest climate zone is 6 with a Class II interior vapor retarder and Climate Zone 4 with a Class III 

interior vapor retarder. While this example assembly may be permitted as a prescriptive solution in the 

energy code, the insulation ratio should be checked as demonstrated in this example. Consequently, the 

insulation locations and amounts may need to be adjusted to achieve moisture control while also still 

complying with the required energy code thermal performance. Some creative options are discussed next. 

For example, changing to a R13+R10ci insulation strategy using a 2x4 wall which is thermally equivalent 

will increase the insulation ratio to 10/13 = 0.77, providing much improved water vapor control or the 

ability to tolerate higher indoor RH conditions. 

Alternatively, closed-cell spray foam may be added to the cavity to achieve required thermal and 

moisture control performance (e.g., see Note c in Table 4–12). This is often efficiently done using a 

“flash and batt” approach (which may also be combined with exterior continuous insulation) as a means 

of economizing insulation cost while also creating an air barrier. 

Example: Getting More Creative with Insulation Ratios 

Given: Consider a wall assembly comprised of R15 high density batt insulation in a 2x4 wall, the use of 

exterior continuous insulation, and R2 insulating (foam backed) vinyl siding. 

Find: What would be the required R-value (and thickness) of the exterior continuous insulation to use 

this assembly in Climate Zone 6 with a Class III interior vapor retarder (latex paint on drywall)? 

Solution: In accordance with Table 4–12 a minimum Re/Ri ratio of 0.5 is required. Thus, the exterior 

continuous insulation amount must be at least R15 x 0.5 = R7.5. Because the insulating siding provides at 

least R2 of this exterior continuous insulation, the insulated sheathing only needs to make up the 

difference of 7.5R – R2 = R5.5. 

Thus, the following insulated sheathing options are possible: 

 1.5” of EPS foam sheathing (R6), 

 1” of XPS (R5) plus an insulated vinyl siding of R2.5 instead of R2, or

 1” of foil-faced polyisocyanurate foam sheathing (R6) 

While the above wall meets the moisture control objective, energy code compliance must also be 

checked. 
Durability by Design 69

For conditions, materials, or envelope strategies not addressed by the recommendations in Tables 4–11 

and 4–12, it is especially important to consult a building science professional to conduct a hygrothermal 

analysis of the proposed envelope assembly using material-specific properties and site-specific climate 

data—especially if a project involves risk factors such as anticipated high indoor RH levels or extreme 

temperature conditions. 
70 Durability by Design

Hygrothermal Analysis for Dummies? 

It is useful for builders and designers to have a sound conceptual and practical understanding of moisture 

and heat flow through assemblies. However, a conceptual knowledge may only help to identify and avoid 

the more obvious errors. When it comes to making decisions about acceptable material choices and 

assembly configurations, conceptual knowledge must be informed by the physics of heat and moisture 

transfer—otherwise known as “hygrothermal analysis” .... something that is more suited to math geeks like 

engineers. Even so, the use of hygrothermal analysis methods requires judgment as none of the methods 

are complete or absent of significant uncertainties. While static analysis methods (like a traditional Glaser 

or dew point analysis as described in ASTM C755, the ASHRAE Handbook of Fundamentals, and ASTM MNL 

18) are significantly incomplete because they lack certain moisture transfer mechanisms such as moisture 

storage and redistribution), the math is simple and it still represents an acceptable (often conservative) 

design practice. Static analysis methods can therefore yield useful results when used with good judgment 

and knowledge of the limitations. 

In recent years, the use of more sophisticated, dynamic hygrothermal modeling programs has become more 

popular and accessible. To varying degrees, these types of models provide a much more complete analysis 

of the physics involved, but at the price of many more input variables that are uncertain. One popular model 

is known as WUFI, with a limited-capability free version available from Oak Ridge National Lab (ORNL) 

(http://web.ornl.gov/sci/btc/apps/moisture/, 9/13/2014). ASHRAE Standard 160 was written to provide 

design guidance and criteria for the use of such dynamic hygrothermal analysis computer models, like 

WUFI. But, because of the uncertainties involved, some designers and researchers indicate that the results 

may be overly conservative, perhaps due to assumptions made for indoor relative humidity levels or 

cumulative error on a multitude of parameters, such as material property inputs, among others. Indeed, 

indoor relative humidity can vary significantly in reality (e.g., from 25% to 50% RH for similar homes even 

within the same climate zone). This speaks to the difficulty with executing any type of hygrothermal 

analyses with a high level of confidence in knowing the true probability of failure or success. 

Thus, a common use of these methods is to compare the predicted performance of an assembly with a 

known successful track record, to a proposed alternative assembly, to help guide decisions regarding its 

design and construction. Clearly, this exercise also requires judgment and experience. 

No matter how much effort is put into a hygrothermal analysis to design an envelope assembly, the 

resulting design may be pointless if other factors are not robustly addressed such as indoor relative humidity 

control (Chapter 5), air barrier and water-resistive barrier installation (Section 4.3.2, 4.2.7, and 4.2.8), and 

proper control of construction moisture (Section 4.3.4). 

Other Considerations in the Use of Foam Sheathing (Continuous Insulation): As with most sheathing 

materials, there are factors that must be considered other than moisture control or thermal performance. 

For example: 

 Where foam sheathing is not used as “oversheathing” (e.g., installed over a separate structural 

sheathing material), its wind pressure capability should be verified (refer to Section 316.8 of the 

2015 IRC and the manufacturer’s code compliance data). 

 Where vinyl siding is installed over foam sheathing alone (e.g., without a structural sheathing 

material), vinyl siding wind pressure ratings may require adjustment in higher wind loading 
Durability by Design 71

conditions (refer to Section R703.11.2 of the 2015 IRC). This has the benefit of encouraging the use 

of “above-minimum” vinyl siding products resulting in improved wind performance (durability) 

and also may result in installations with a smoother and straighter finished appearance. 

Alternatively, proprietary composite structural insulated sheathing products may be considered as a 

means of addressing the above considerations. 

Where thick foam sheathing is used, additional corner framing members or wider furring materials may be 

required to allow attachment of furring and/or cladding through thick foam sheathing at these locations. 

For additional guidance on these and other matters related to the use of foam sheathing refer to 

Additional Resources in Section 4.4. 

4.3.4 Construction Moisture Control—Dry It Out! 

The recommendations for vapor control in Section 4.3.3 provide solutions that allow walls to seasonally 

accumulate tolerable amounts of moisture and then release that moisture during seasonal periods 

conducive to drying. This is the way moisture control is supposed to work after a building is completed. 

But what happens if the building starts out wet? The answer to this question depends on a number of 

factors such as construction sequence, timing of completion, properties of materials, and the amount of 

moisture present in materials when assemblies are “closed in.” Typical problems associated with an 

inadequate moisture control strategy during construction are as follows: 

 Wet wood framing materials may shrink or bow excessively as they dry after construction. This 

can cause nail pops, squeaky floors, cracked drywall, bowed walls, and unlevel surfaces. 

Significant shrinkage after construction due to excessive moisture during construction can cause 

damage to plumbing, mechanical systems, and windows and sealants, compromising the first 

line of defense against moisture intrusion (Figure 4–19). 
72 Durability by Design

Figure 4–19: Deformed Window and Ruptured Caulking due to Shrinkage of Wood Framing 

This occurred as the building framing settled, while the brick veneer and foundation did not move. 

 Formation of mold within building assemblies and on construction materials. 

 Excessive indoor relative humidity during early operation of the building resulting in various 

moisture problems such as attic condensation, mold on interior surfaces, and window 

condensation. 

 Wetting of insulation materials and degradation of moisture-sensitive material properties due to 

premature installation prior to completion of house dry-in (roofing, windows, water-resistive 

barrier, and flashing installed) (Figure 4–20). 

 Use of wet-blown insulations (e.g., some types of fiberglass or cellulose) and not allowing 

sufficient drying time prior to close-in of the assembly, or drying only minimally to industry 

recommended 25% maximum moisture content prior to close-in. 
Durability by Design 73

Figure 4–20: Water Streaming From Cavity Insulation as it is “Wrung Out”

The insulation was installed prior to the complete installation of the water-resistive barrier and flashing on exterior walls. 

This caused rainwater infiltration during construction and is an example of poor construction sequencing. 

The generation or release of moisture from construction materials and practices can be substantial. For 

example, a 4” thick concrete slab covering 1,000 square feet can release as much as 1 ton of water vapor 

over the course of its curing process (several months to a year or more). Wood materials that are enclosed 

in construction when still too moist can release as much as 2 tons of water vapor as it dries over the course 

of many months after the building is completed. The common practice of using fuel-fired heaters to “dry” 

buildings can create as much as 1 gallon of water (in the form of water vapor) for each gallon of fuel 

burned (water is a by-product of the combustion of fossil fuels). When moisture is not controlled during 

construction, the potential for moisture-related problems during the first year of building operation is

significantly increased. 
74 Durability by Design

Fortunately, problems with construction moisture or “built-in” moisture can be minimized with the 

following actions: 

 When supplemental drying is desired to accelerate construction, avoid the use of fuel-fired 

heaters. Instead, use ventilation when conditions are correct, use heaters that do not generate 

moisture (e.g., electric or forced air with combustion products vented to the outdoors), and after 

the building is enclosed and dried-in, use dehumidifiers. 

 Follow a construction sequence that achieves a “dried-in” status as soon as possible, and prior to 

installation of moisture sensitive materials other than necessary for structure and dry-in. 

 Store construction materials elevated above ground and with water-shedding cover (e.g., tarp or 

under roof) that is adequately vented to avoid trapping moisture. Refer to Figure 4–21 for an 

example of what not to do! Materials can also be scheduled for “just-in-time” delivery to avoid 

exposure during jobsite storage. 

 Use a moisture meter to ensure materials are adequately dry when received and especially prior 

to enclosure within a building assembly (e.g., installation of insulation, vapor retarders, drywall, 

flooring, etc.). For wood framing, a moisture content of 15% or less should be attained prior to 

enclosure for shrinkage control purposes (e.g., prevention of drywall nail pops). For installation of 

wood floors, a much lower moisture content is advisable as well as equilibration of the moisture 

content of floor framing, sheathing, and wood flooring. 

Figure 4–21: Example of Unacceptable Construction Material Storage 
Durability by Design 75

4.4 Additional Resources 

Air Barriers—Airtight Drywall Approach for All Climates, BSC Information Sheet 401, U.S. Department of 

Energy, Building America. 

Asphalt Roofing Manufacturers Association, www.asphaltroofing.org

ASTM 2112-07 Standard Practice for Installation of Exterior Windows, Doors, and Skylights. American 

Society of Testing and Materials, www.astm.org

ASTM C1193-13 Standard Guide for Use of Joint Sealants, American Society of Testing and Materials, 

www.astm.org

ASTM E241-09 Standard Guide for Limiting Water-Induced Damage to Buildings, American Society for 

Testing and Materials, www.astm.org

Canadian Woodframe House Construction, Canadian Mortgage and Housing Corporation, 2013. 

www.cmhc-schl.gc.ca

Foam sheathing (continuous insulation) application guidance: 

 http://fsc.americanchemistry.com

 www.buildingscience.com

 http://www.drjengineering.org/client/1/foam-sheathing-committee-fsc

Grin, A., and Lstiburek, J. (2012) Guidance on Taped Insulating Sheathing Drainage Planes: Final Report, 

Building America Report 1301, December 2012. https://basc.pnnl.gov/resources/guidance-tapedinsulating-sheathing-drainage-planes

Healthy and Affordable Housing: Practical Recommendations for Building, Renovating and Maintaining 

Housing, U.S. Department of Housing and Urban Development and U.S. Department of Energy, 

Washington, DC. 2004. www.huduser.org 

International Building Code (IRC), International Code Council, Inc. Washington, DC. 2015, www.iccsafe.org 

International Energy Conservation Code (IECC), International Code Council, Inc., Washington, DC. 2015, 

www.iccsafe.org 

Kerr, D., Keeping Walls Dry, Canadian Mortgage and Housing Corporation, November 2004, www.cmhcschl.gc.ca

Lstiburek, J. (2011). “A Crash Course in Roof Venting”, Fine Homebuilding, The Taunton Press, Inc., 

www.finehomebuilding.com
76 Durability by Design

Lstiburek, J.W., Water Management Guide, Building Science Corporation. 2004. 

Moisture Control in Buildings (ASTM Manual 18), American Society of Testing and Materials, West 

Conshohocken PA, 2009, www.astm.org

Moisture Control Guidance for Building Design, Construction and Maintenance, U.S. Environmental 

Protection Agency, December 2013 www.epa.gov/iaq/moisture

Moisture Resistant Homes, U.S. Department of Housing and Urban Development, Washington, DC. 2006. 

www.huduser.org

National Building Code of Canada, 2010. http://www.nrccnrc.gc.ca/eng/publications/codes_centre/2010_national_building_code.html

National Roofing Contractors Association, www.nrca.net 

Water Penetration Resistance of Windows—Study of Codes, Standards, Testing, and Certification, 

Research Highlights, Technical Series 03-125, CHMC-SCHL, November 2003, www.cmhc-schl.gc.ca

Weather-Resistive Barriers, How to Select and Install Housewrap and Other Types of Weather-resistive 

Barriers, Technology Fact Sheet, U.S. Department of Energy, October 2000. 

http://web.ornl.gov/sci/roofs+walls/insulation/fact%20sheets/weather%20resistive.pdf
Durability by Design 77

CHAPTER 5—HVAC AND PLUMBING 

5.1 General 

The issue of durability may not immediately come to mind with respect to a home’s heating, ventilating, 

and air conditioning (HVAC) system, but a poorly designed and installed system can create humidity 

problems that damage the structure as well as home furnishings. Such a system can also create 

unbalanced pressure zones in a building, which essentially push or pull air (and the water vapor it carries) 

through gaps in the building envelope that may result in condensation. Moreover, an improperly designed 

system can shorten the life of the equipment due to frequent short cycling, restricted air flow that causes 

the blower motor to work harder and less efficiently, or excessive dirt and dust accumulation. 

In recent years, the International Residential Code (IRC) and the International Energy Conservation Code 

(IECC) have incorporated more provisions that pertain to best HVAC practices. Yet, these are not 

comprehensive enough when it comes to the broad durability implications of HVAC—nor are they 

consistently adopted, implemented, and enforced. So while some of the recommendations in this section 

reiterate important basic code minimum requirements, others are beyond-code practices that will help to 

assure efficient HVAC operation as well as durability of the system and the home itself. 

Although recommendations are called out individually, efficient and durability-enhancing HVAC must be 

implemented as a system. All components of the HVAC system work together as a system so that the 

equipment, ductwork, registers, and controls must be designed and installed in an integrated approach. 

For instance, a new home will often have a properly sized central heating/cooling system (good), along 

with a duct system without any sizing or design (bad) and an undersized, poorly installed exhaust fan to 

“ventilate” the master bathroom (common). The result: a phone call to the builder from an unhappy 

homeowner complaining about high humidity and window condensation in the master bedroom. Besides 

being a comfort problem, over a longer term, this high humidity and condensation can lead to mold 

growth, degradation of drywall, and similar durability issues. 

So we cannot design a good HVAC system using an a la carte approach. Efficient, durability-enhancing 

HVAC as a system is the goal. And this “system” goes beyond heating, cooling, and ventilation as HVAC is 

so interconnected with other building systems (see Durability Web graphic Figure 1–1). Foremost among 

the connections are HVAC—building envelope interactions, and the implications flow both ways. The 

envelope’s thermal and air sealing characteristics dictate the HVAC size. At the same time, the HVAC 

system can drive air/moisture movement into building cavities and cause envelope damage. While these 

are just two examples, the key message is that enhanced durability of the envelope involves looking at 

HVAC interactions, and vice versa. 
78 Durability by Design

5.2 Recommended Practices—HVAC 

5.2.1 Properly Size Cooling Equipment and Delivery System 

Bigger is not always better! In the case of space cooling, equipment that is larger than the building load 

requires actually does harm. Oversized equipment can lead to moisture problems since the air conditioner 

may not run long enough to adequately dehumidify indoor air during summer cooling months. This can 

lead to mold and condensation on surfaces and possibly in 

wall cavities, not to mention the comfort issues this creates 

from cool but clammy air. 

 Use proper methods such as the Air Conditioning 

Contractors’ Association (ACCA) Manual J to 

determine the design heating/cooling loads. 

Equally important, use simple QA checks to assure 

what looks like “right sizing” isn’t really “wrong 

sizing”.

o Load sizing has been required by code for some time. It’s been made simpler and faster 

via load sizing software too. So having a load calculation for a home is a bare minimum 

at this point—now we need to make sure it’s actually accurate! As the example in 

Figure 5–1 shows, seemingly reasonable—but inaccurate—values for just six basic 

inputs to a load calculation can cause under- or over-sizing errors approaching 25%. The 

combined impact of the six inaccurate inputs is actually a bit less than the total of their 

individual impacts, because the inputs interact with each other when they’re modeled 

together. In real-world examples, more egregious errors throw off design loads by a 

factor of 2 or more. 

o For this reason, builders and their contractors should employ the ENERGY STAR Homes 

HVAC checklists. The HVAC Contractor checklist includes written documentation of key 

design variables including outdoor design temperature and corresponding location; 

home orientation; predominant window Solar Heat Gain Coefficient (SHGC); etc. 

o For added assurance of a good load calculation, the HVAC Rater checklist requires a 

rater to double-check that the outdoor design temperature is appropriate and accurate; 

that the home orientation matches the actual home; that the predominant window 

SHGC is within 0.1 of the installed windows; etc. 

o Builders and their trade partners can take advantage of these resources regardless of 

participation in the ENERGY STAR Homes program. Links to both checklists are found in 

Section 5.4. 

Sealed Combustion and Durability? 

Sealed combustion HVAC equipment does not really 

improve the durability of the home, but it does 

improve energy efficiency. It also contributes to 

assuring good indoor air quality because the 

equipment is far less susceptible to backdrafting 

due to fan assisted exhaust. 
Durability by Design 79

Figure 5–1: Impacts of Incorrect Inputs for a Loading Sizing Calculation 

Inaccurate values for just these six load calc inputs results in equipment sizing errors of -29% or +23% 

o Multiple software packages for calculating building heating and cooling loads are 

available. Several residentially oriented packages can also be used to model energy 

performance as well as code compliance. A link to DOE’s software directory can be 

found at the end of this chapter. 

 Use ACCA Manual S for selecting proper 

equipment sizes. 

 Central air conditioning equipment 

should be within 95–115% of design 

cooling load or the next nominal size 

 Central warm air furnaces should be 

within 100–140% of design heating load 

or the next nominal size, unless cooling 

load dictates larger equipment 

 Central boilers should be within 

100–115% of design heating load or the next nominal size 

It is equally important that ductwork is also properly sized, designed, and of course, installed per design. 

 Design and size the duct system in accordance with ACCA Manual D and blower performance data 

from manufacturer equipment specs. 

 Airflow at each register to be within 20% or 25 cfm of design airflow, whichever is greater. Provide 

balancing dampers at each register. Measure air flow at each register and balance accordingly. 

Variable Speed Equipment

Variable speed heating and cooling equipment is 

not as susceptible to moisture and performance 

issues that result from oversizing because they 

can typically provide both good efficiency and 

moisture removal (in cooling mode) under part 

load conditions. When using variable speed 

equipment be sure to check the manufacturer’s 

and ACCA’s sizing guidance, which will usually 

be more flexible.
80 Durability by Design

Ductwork that is not properly sized will result in a blower that is starved for air and/or very high or low 

flow rates. Excessively high or low flow rates can set up pressure imbalances that induce infiltration or 

exfiltration through the building envelope. Such 

flow rates also put undue wear and tear on the 

compressor and shorten the life of the equipment. 

A link is provided at the end of this chapter that 

serves as a good resource for proper air 

distribution system design. 

Rules of thumb for sizing should never be used 

for sizing equipment or ductwork. Why? Homes 

just aren’t that simple and uniform in today’s 

industry. Even a single builder will often have 

various shell options (e.g. windows) and floor 

plan options that can significantly affect the 

loads. So, 400 ft2/ton, or 800 ft2/ton, or any 

other generic value basically becomes more of a 

guess than an estimate.

Figure 5–2: Conditioned Attic 

5.2.2 Keep Ducts in Conditioned Space 

 Design the HVAC (as well as affected framing and building assemblies) to locate all ducts within 

conditioned space. If all ducts cannot be located within conditioned space, ensure that most of 

them are located within this space. 

Keeping all supply and return ductwork within conditioned space assures that any air leakage that does 

occur won’t be lost to the outdoors. This design strategy can also help avoid durability issues such as the 

formation of condensation on the outside of cooling ducts located in humid spaces like an attic or crawl 

space. With repeated exposure to moisture, nearby framing or drywall can be damaged and harbor mold 

growth. 
Durability by Design 81

On the space heating side, supply ducts located in the attic can contribute to the formation of ice dams—

especially if the ductwork is leaky as well. In addition to impacting the durability of a home, ducts in 

unconditioned space combined with poor duct sealing can compromise indoor air quality. Contaminants 

can be pulled into return ductwork and subsequently distributed throughout the home. 

5.2.3 Seal Ductwork 

Leaky ducts can lead to a host of problems 

such as indoor air that is too dry or too 

humid, condensation, and buildup of dust 

and dirt that is then redistributed through 

the equipment and throughout the home. 

Leaky ducts can also create pressure 

imbalances in the home because equal 

amounts of supply and return air aren’t 

being delivered. With significant supply 

duct leaks to unconditioned spaces, the 

home is depressurized, which drives 

infiltration of outdoor air. The return ducts 

are pulling more air back to the equipment than is being supplied. The opposite happens when there is 

substantial leakage on the return duct side and portions of the home become pressurized. Under these 

conditions, conditioned indoor air is forced through small cracks or crevices that may exist in even the 

tightest building envelope. In either case, when air is traveling through building cavities, it is likely to carry 

moisture. When this air finds a cold surface, chronic condensation can occur. 

Integrative Design and Construction 

Maintaining all ductwork within the conditioned space is usually a collaborative effort during both the 

design and construction stages. 

For instance, if bulkheads are to be used, this element must be integrated into the floor plan design, and it 

must be determined how the cavity will be air sealed from the attic–plus which trade partner will perform 

this work (e.g., framer, drywall contractor). 

Any change to the duct location triggers these types of coordination issues. Builders and their trade partners 

fare best with pre-construction design coordination. 

Strategies for Keeping Ducts 

Within Conditioned Space

1. Create a conditioned attic, basement, or crawl space.

2. Use bulkheads to conceal ducts dropped below a ceiling.

3. Insulate ducts with R-8 insulation, encapsulate with 1.5” 

closed cell foam insulation, and bury under at least 2” 

blown insulation.

4. Use high throw registers located on interior partitions.

5. Use jump ducts for returns to minimize the amount of 

ductwork 

6. See link at end of section for more details.
82 Durability by Design

Figure 5–3: Example of Duct Joint Which Should be Air-Sealed 

The HVAC contractor thought his job was finished, yet multiple joints such as this were left unsealed. 

 Seal ductwork with mastic or approved foil tapes (UL 181-A compliant). Seal ducts at the following 

locations: 

 All transverse and longitudinal connections and seams, 

 Ductwork connections at equipment, 

 Connections between ductwork and register boots, 

 Connection between register boot and subfloor, ceiling, or wall, 

 All holes in heating and cooling equipment, and 

 Gasketed access panel and filter slot. 

 Strive to achieve total duct leakage less than or equal to 4 cfm per 100 sf of conditioned floor area 

when tested at a pressure differential of 25 Pa. 

5.2.4 Select Good Air Filters and Pave the Way for Effective Replacements 

 Select a filter with a MERV (Minimum Efficiency Reporting Value) of at least 6—which will trap 

particles as small as 1 micron. 

Don’t skimp on the filter you select or specify for your forced air heating and cooling equipment. The very 

inexpensive fiberglass filters—a.k.a. “rock-stoppers”—are only effective in preventing large particles from 

being distributed on equipment coils. Higher efficiency pleated and electrostatic filters will stop much 

smaller particles and thus, not only protect the equipment but also help to improve indoor air quality. 

 Post key homeowner information to help ensure effective filter replacements. 

In addition to selecting an effective filter, it is essential to replace filters when they become dirty because 

buildup will restrict air flow and can damage the equipment. In furnaces, inadequate air flow can 

ultimately cause the heat exchanger to become too hot—potentially resulting in temperature limit 
Durability by Design 83

switches to turn off the system or even cracking the exchanger. In air conditioning units, restricted air flow 

causes the opposite problems—coils which are too cold—causing ice buildup on the cooling coils. This in 

turn can lead to equipment damage and water leaks as all of the ice melts. 

While the builder or HVAC contractor will install the initial filter, it will be the homeowner who continues 

this maintenance task. To avoid problems down the road and help ensure good performance, 

builders/contractors should clearly post this information at or near the filter location: 

 Filter dimensions 

 Initial filter resistance (in inches Water Column) at a specified cfm airflow 

Believe it or not, providing this information instead of only the MERV rating gives the homeowner a better 

chance of maintaining a well-functioning filter and HVAC system. This is due to the fact that for a given 

MERV rating, different filters can have much different pressure drops depending on the velocity of the air 

flow and the thickness and surface area of the filter. If a builder/contractor wishes to provide a 

recommended MERV rating for replacement filters, it should be clear that the selected filter must also 

adhere to the pressure drop criteria as well. Also, be sure that filter replacement is clearly stated in the 

homeowner’s maintenance expectations. 

5.2.5 Install Local Exhaust Ventilation 

The 2012 International Residential Code requires exhaust ventilation to the outdoors for kitchens and 

bathrooms under the following conditions: 

a. When there is no window in the bathroom 

b. Above ranges in the kitchen unless: 

a) Allowed by the manufacturer AND 

b) Natural or mechanical ventilation is provided.

Despite these code requirements, which have actually been in place for several years in earlier editions of 

the IRC, we often see recirculating range hoods where they are not technically allowed (and also not a 

good idea) and bath fan ducts “dead-ended” in the attic. Even though a window may exist, during many 

times of the year they will simply not be opened due to comfort, allergy, noise, or security issues. 

Recommended practices to assure effective removal of moisture and energy efficient performance include 

the following: 

 Always provide mechanical exhaust ventilation to the outdoors in bathrooms, kitchens, and other 

locations prone to moisture, odors, or other air contaminant sources (e.g., a garage shop area). 
84 Durability by Design

 Follow these ASHRAE 62.2 guidelines (following) and/or manufacturers’ recommendations when 

sizing kitchen and bath exhaust. It’s easier to throttle down the installed flow with a damper—but 

difficult/impossible to boost the flow to an undersized system. 

 Measure and adjust actual flow rates to verify they meet design intent. 

Table 5–1: Exhaust Airflow Rates For Local Ventilation

Area To Be Exhausted Continuous Exhaust 

Rate

Demand-Controlled 

Exhaust Rate

Bathrooms/Toilet 

Rooms

> 20 cfm (averaged over

each hour)

> 50 cfm

Kitchens  Vented range hood—min. 100 cfm capacity or

 Any other type of exhaust fan (including 

downdraft)—min. 300 cfm capacity or

 Exhaust fan in enclosed kitchen with min. 

capacity 5 ACH based on kitchen volume. An 

“enclosed kitchen” whose permanent 

openings to adjacent spaces ≤ 60 ft2

Source: ASHRAE 62.2-2013 (with Addendum C)

 Use ENERGY STAR labeled ventilation equipment whenever available. Besides being more energy 

efficient, these fans have maximum allowable sound levels of 3 sones or less for bath fans and 2 

sones or less for range hoods (500 cfm max). This is important. Why? Noisy fans don’t get used. 

 Control the bath exhaust fan on a timer or humidistat to assure that 1) the fan runs long enough to 

adequately remove moisture and 2) the fan is not forgotten and left on too long, consuming more 

energy than is necessary. 

Range hoods present a special case for consideration. With the increasing frequency of high capacity, 

“commercial grade” range installations, concern arises regarding the very high exhaust rates that 

manufacturers recommend for the accompanying hood. With the substantial depressurization that is likely 

to occur as several hundred cfm (or more) is drawn out of the home, there is the potential for backdrafting 

of combustion equipment that is not sealed—for instance an atmospherically drafted gas water heater. 

Installing Local Exhaust Which Actually Works

Simply providing a fan and running a duct to the outside is not enough to assure that the local exhaust system 

performs as expected and provides the intended result—especially for bath exhaust where warm humid air is 

being removed. Field-tested exhaust flows are very often not even 1⁄2 of the fan’s rated flow. Duct runs should 

have minimal bends, avoid kinks like the one shown in Figure 5-5, and be sized correctly according to Table 5–4.
Durability by Design 85

The 2012 IRC requires a source of makeup air equal in volume for range hoods in excess of 400 cfm. Such 

outdoor air intakes must be dampered and must be automatically controlled to operate simultaneously 

with the exhaust hood. 

 Vent range hoods directly to the outdoors to remove odors 

and particulates from the home, rather than simply trapping 

or recirculating them. 

 Follow code or manufacturer’s recommendations for 

makeup air for range hoods that exceed 200 cfm, whichever 

is more stringent. This will reduce the possibility of 

backdrafting combustion contaminants and help assure a safe level of indoor air quality. 

5.2.6 Install Whole-House Mechanical Ventilation 

As energy codes and above-code programs drive homes to become increasingly tighter, it’s critical to 

install some type of “intentional” whole-house mechanical ventilation. Not only is the turnover of indoor 

air beneficial to help remove indoor contaminants and allergens, but it also helps maintain desirable 

indoor humidity levels in some climates. 

In designing, installing, and commissioning a whole-house ventilation system, follow these key provisions 

drawn from the ASHRAE 62.2 standard: 

 Select a fan based on its cfm—as rated for 0.25” w.c. static pressure—to assure adequate 

airflow and allow for somewhat longer duct runs. Common practice is to pick a fan based on its 

cfm at a lower pressure resistance like 0.1” w.c. If the duct layout and other components offer 

more resistance than this level, the fan doesn’t move enough air.

 Make sure duct diameter is at least 4”—preferably 6”. Again, sizing the system for more flow and 

not less provides some flexibility. If the installed system moves too much air, it can be throttled 

down to a low flow relatively easily (but the reverse isn’t true). ASHRAE 62.2 recognizes that 

supply, exhaust, or balanced systems can all serve as the whole-house ventilation system. 

 Size, install, and commission a system with the capacity to deliver the ventilation rates shown 

below in Table 5–2 (ASHRAE Standard 62.2 Table 4.1a). In reality, builders are well served to 

educate homeowners on what the system does, how it works, and why it is important. The home’s 

“fresh air system” is just like the air-conditioning—the homeowner may not use it all of the time, 

but he/she should know what it does and when/how to use it. Homeowner know-how is especially 

critical in more severe climates with extreme temperature or humidity levels. 

ENERGY STAR Range Hood 

Specifications:

Maximum Capacity: 500 cfm

Minimum Efficacy: 2.8 cfm/Watt

Maximum Sone Rating: 2 sones
86 Durability by Design

Table 5–2: Minimum Whole-House, Continuous Mechanical Ventilation Rates

(cfm)

Floor Area (ft2) Number of Bedrooms

0–1 2–3 4–5 6–7 > 7

< 1500 30 45 60 75 90

1501–3000 45 60 75 90 105

3001–4500 60 75 90 105 120

4501–6000 75 90 105 120 135

6001–7500 90 105 120 135 150

> 7500 105 120 135 150 165

Source: ASHRAE Standard 62.2

 For intermittent operation of the Whole-House Mechanical Ventilation system, maintain a 

maximum duty cycle of four hours, meaning that the fan operates at least 1 hour in every 4-hour 

segment. For intermittently operating systems, the flow rate (from Table 5–2) must be multiplied by 

the appropriate factor from Table 5–3 below to account for periods of fan off time.

Table 5–3: Intermittent Whole-House Mechanical Ventilation Rate Factors

Run-Time Percentage in Each

4-Hour Segment 25% 33% 50% 66% 75% 100%

Factor 4 3 2 1.5 1.3 1.0

Source: ASHRAE Standard 62.2

Whole house ventilation can be delivered via the supplyside, central fan integrated system shown in blue and/or 

with exhaust fans set to run on a regular basis. 

Building America Solution Center/Building Science Corp.

Figure 5–4: Whole-House Ventilation System Types 

Whole-house ventilation can be delivered via the supply-side, central fan integrated system shown in blue and/or with 

exhaust fans set to run on a regular basis. 
Durability by Design 87

The preceding recommendations are admittedly only a sampling of good whole-house mechanical 

ventilation system design, and readers are encouraged to review the Additional Resources links at the 

conclusion of this section for more in-depth guidance. 

5.2.7 Design and Install Ventilation Ducts Properly 

As with providing mechanical ventilation in general, the durability issue at stake relative to ventilation 

duct work is moisture. Elevated moisture levels and indoor humidity not only damage interior finishes, 

but can also lead to mold and rot of structural members. The best ventilation fans and controls simply 

cannot do their job of managing moisture if the ducts are improperly designed and installed (see 

examples in Figure 5–5). 

Good and bad duct examples. Good: straight, supported exhaust duct venting to outdoors. 

Bad: kinked duct with very restricted flow and compromised performance. 

Building America Solution Center

Figure 5–5: Right and Wrong Exhaust Duct Installations 

 Insulate ductwork that runs through cold spaces or exterior wall cavities. In addition, slope 

ductwork down and away from the fan housing as much as possible so that any condensation will 

flow out towards the exterior. 

 Use straight duct runs. Avoid sharp bends everywhere, but avoid all bends within 2’ of fan to 

minimize static pressure and fan noise. 
88 Durability by Design

 Adequately support ductwork but ensure that it is not pinched. 

 Follow ASHRAE 62.2 guidance in sizing ductwork for local exhaust ventilation and whole-house 

ventilation according to the rated fan capacity and the length of the run, as shown in Table 5–4. 

 Verify flow rates with flow hood measurements. 

DC Motors—Belt and Suspenders Approach for Getting Good Ventilation Air Flow Rates

Several manufacturers are offering exhaust fans with DC motors. Not only are these fans more efficient 

than typical AC induction motors, but many models are also capable of automatically maintaining their 

programmed cfm rate even when working against high static pressure levels in the duct work.

Table 5–4: Prescriptive Duct Sizing Chart

Duct 

Diameter 

(inches)

Maximum Length in Feet

Fan Rating in cfm Certified @ 0.25 in. w.c.

Flex Duct (cfm) Smooth Duct (cfm)

50 80 100 125 150 200 250 300 50 80 100 125 150 200 250 300

3

n/

a

n/a n/a n/a n/a n/a n/a n/a 5 n/a n/a n/a n/a n/a n/a n/a

4 56 4 n/a n/a n/a n/a n/a n/a 114 31 10 n/a n/a n/a n/a n/a

5 nl 81 42 16 2 n/a n/a n/a nl 152 91 51 28 4 n/a n/a

6 nl nl 158 91 55 18 1 n/a nl nl nl 168 112 53 25 9

7 nl nl nl nl 161 78 40 19 nl nl nl nl nl 148 88 54

8 and 

above nl nl nl nl nl 189 111 69 nl nl nl nl nl nl 198 133

nl = no limit

n/a = not allowed: No use of this duct diameter will meet ASHRAE Standard 62.2 requirements.

This table assumes no elbows. Deduct 15’ (5 m) of allowable duct length for each elbow.

Source: ASHRAE 62.2

5.2.8 Provide Supplemental Dehumidification or Humidity Controls in Humid Climates 

In climates that have frequent or extended periods of high humidity, even a properly sized air conditioning 

system may not operate often enough to adequately maintain acceptable indoor humidity levels. As 

discussed throughout this section, high humidity can lead to the formation of mold on surfaces inside the 

home as well as condensation on building surfaces. Plus, occupant comfort is diminished with high or very 

low humidity even if the air temperature is comfortable. 
Durability by Design 89

 

Figure 5–6: International Energy Conservation Code Climate Zones Map 

Source: U.S. Department of Energy 

 Provide supplemental dehumidification or humidity-based controls that can maintain indoor 

relative humidity at or below 60%, for Climate Zone 1 and those portions of 2A and 3A below the 

white “warm humid” line in Figure 5–6. Also consider these systems for homes in Zones 3A, 4A, 

and 5A which have high performance building envelopes where central cooling will be less 

frequent. 
90 Durability by Design

Why? Many parts of the country see at least some 

muggy weather during portions of the year. 

Basements often may be quite humid either during 

and after construction or during summer months 

when the temperature remains low relative to the 

outdoors thereby elevating relative humidity (RH). 

A maximum 60% RH level balances durability in the 

home, occupant comfort, and operating costs 

during these humid periods of the year. During 

cold winter months lower limits apply (see text box 

to the right).

There are several types of dehumidification 

systems available: 

1. Portable dehumidifiers requiring manual 

emptying of condensate buckets unless 

plumbed to a drain, 

2. Whole-house dehumidifiers that 

supplement the moisture removal 

capacity of the air conditioning system 

and utilize the central ductwork, 

3. Ventilating dehumidifiers that draw air 

into the home, remove moisture, and 

then deliver this air to the return side of 

the central air handling unit, and 

4. Variable speed central air conditioning 

equipment that includes a 

dehumidification mode to operate based 

on indoor humidity levels (regardless of 

whether there is a call for cooling. 

The cost of each type of system increases as its 

functionality becomes more sophisticated, and as 

with ventilation systems homeowners need some 

basic education in using the systems correctly. 

Controlling Indoor Humidity and Energy Use—It’s a 

Balancing Act

Based on research by the U.S. Department of 

Energy’s Building America program, the cost of 

dehumidifying below 60% RH gets expensive. In fact, 

decreasing the setting of a dehumidifier from 60% to 

50% can increase energy consumption by a factor of 

5! So while indoor RH control is vital in moist 

climates to manage moisture and comfort, builders 

and their homeowners also need to be aware that 

additional drying can ramp up energy bills.

Integrative Design and Construction—Humidity 

Control and a Dry Envelope 

Chapter 4 of this guide provides details on the 

application of vapor retarders and also minimum 

ratios for exterior and cavity wall insulation levels, 

when exterior (continuous) insulation is used. The 

goal of vapor retarders or a combination of vapor 

retarders and insulation ratios is to manage water 

vapor by limiting its access to cold surfaces internal 

to the wall or to keep the temperature of those 

surfaces above the dew point. In other words – to 

prevent condensation and moisture accumulation 

problems in the wall! 

A lot of design assumptions go into the underlying 

analysis – notably the indoor relative humidity (RH) 

levels. The higher the indoor RH level, the more likely 

it is for envelope moisture problems to occur with 

any water vapor management strategy. Systems are 

needed which can manage and maintain target RH 

levels: generally < 40% for winter (or lower in 

Climate Zone 5 and colder), and < 60% for summer. 

Systems such as supplemental dehumidifiers, HRVs, 

and even carefully programmed and controlled 

humidifiers – should be part of the HVAC 

design...and are also critical to the envelope design. 

See Section 4.3 Water Vapor Control. 

Finally, home owner education is a major element of 

indoor RH control. For example, bath fan usage and 

humidifier use and set point are both major factors 

in controlling indoor humidity. 
Durability by Design 91

5.3 Recommended Practices—Plumbing 

A home’s plumbing system is supposed to ensure that water gets to and from the places we need it, at the 

required temperatures. The very nature of plumbing—moving water around a house—demands that these 

systems be viewed with an eye towards durability. Accordingly, a few basic recommended practices 

follow. 

5.3.1 Select More Durable Piping Materials for Supply and Waste Lines 

 Select durable piping materials such as Type L copper, cross-linked polyethylene, or braided 

stainless steel for the following types of applications: 

o Hard water: For instance, where the water is hard (e.g., has a high mineral content) a 

heavier gauge copper—the heavier Type L versus Type M copper—is recommended. 

CPVC, cross-linked polyethylene (PEX), or polypropylene materials are good alternatives as 

well. 

o Supply risers: Supply risers for sinks, lavatories, and toilets are usually chrome or crosslinked polyethylene. An upgrade to these materials which is more durable and less likely 

to leak is braided stainless steel flexible piping. Depending on the installation, it is usually 

easier to install as well. 

o Appliance supply lines: Braided steel piping is also suitable for ice maker, refrigerator 

water dispenser, and dishwasher supply lines as well as washing machine hoses. Leaks in 

refrigerator and dishwasher lines often go unnoticed for long periods of time because 

they may not be immediately visible. Such leaks can result in thousands of dollars in 

damage to floors, drywall, and even structural members. Increasingly, washing machines 

are located on first or second floors for convenience. Burst hoses or drainage leaks can 

cause significant damage due to the volume of water. High quality hoses and catch pans 

are a wise safeguard that can minimize damage.

5.3.2 Protect Plumbing Pipes From Freezing

This, of course, is a no-brainer, but sometimes inadvertent location of supply or even drain lines can 

result in frozen pipes during cold weather. Keep all supply lines within the conditioned space and within 

interior wall or ceiling cavities. In very cold climates, even pipes located on the warm side of fiberglass 

insulation in an exterior wall can be subject to freezing. 

 If you must run the piping in an exterior wall cavity, use a rigid board or spray foam behind 

the pipe to minimize infiltration and maximize the R-value separating plumbing from 

outdoors. Rigid and spray products offering R6 – R7 per inch are readily available. 
92 Durability by Design

Example of plumbing lines well protected from freezing because of high R-value, 

low air-infiltration spray foam insulation between the pipes and outdoors. 

Figure 5–7: Spray Foam Insulation Protecting Pipes Installed in an Exterior Wall 

Do the same if you must run supply lines in the band joist area or floor cavity of an uninsulated crawl space 

or basement. It is best to insulate crawl spaces and not vent them to the outdoors. 

5.3.3 Use Durable Shower Design Details and Materials 

Whenever possible, use seamless tub and shower units to eliminate seams and reduce reliance on 

sealants. Where two-, three-, or four-piece units are necessary, (a bath remodel, for instance), use 

manufacturer-recommended sealing methods. 

Tile is a frequent choice around tubs and showers. Unfortunately, this is not the preferred material from a 

durability standpoint: grout can crack and leak, it is difficult to clean, and it may contribute to mold 

growth. If tile is used, be sure to use a cementitious, glass-mat, or fiber-reinforced backer board meeting 

2012 IRC R702.4.2 behind it—never paper-faced drywall or “greenboard.” Additional membrane products 

for walls and joints, along with gaskets to go around piping, are available and may be considered. 

 
Durability by Design 93

5.4 Additional Resources

ENERGY STAR Version 3.1 Rev 07 HVAC System Quality Installation Contractor Checklist—This checklist 

identifies best practices for sizing and designing space heating and cooling systems for new homes. Specific 

field verification measures are also described. 

ENERGY STAR Version 3.1 Rev 07 HVAC System Quality Installation Rater Checklist. This checklist includes 

several “checks” an energy rater or builder can use to verify several important inputs in a load calculation 

are appropriate. 

HVAC Quality Installation Specification—Residential and Commercial Heating, Ventilating, and Air 

Conditioning (HVAC) Applications—Air Conditioning Contractors of America. This industry specification 

outlines design, installation, and testing procedures to assure proper performance of residential and 

commercial HVAC systems. Although the focus is new systems, recommendations are applicable to 

existing systems to the extent possible. 

The U.S. DOE Building Energy Software Tools Directory provides an extensive listing of public and private 

energy-related modeling software. The directory can be sorted by subject, with the subject “Load 

Calculation” returning numerous tools which include load calculation functionality. 

Air Distribution System Design, from the Southface Energy Institute, provides a concise summary of 

terminology and best practices for designing the air distribution system in residential buildings. 

The Building America Solution Center provides extensive technical information on residential building 

systems and can be navigated by checklist (ENERGY STAR, DOE Zero Energy Ready) or by building system. 

See basc.pnnl.gov search for “ducts in conditioned space” under the Building Systems navigator, for 

instance. 

 
94 Durability by Design

CHAPTER 6—SUNLIGHT 

6.1 General 

Sunlight is made up of visible light and non-visible radiation such as ultraviolet (UV) and infrared (IR). 

Depending on the color and surface characteristics of an object, various wavelengths of solar radiation 

may be absorbed, reflected, and emitted (i.e., “released”). The more light absorbed and the less heat 

capacity (i.e., thermal mass), the greater the object’s ability to be heated by sunlight. For example, a dark 

driveway becomes much hotter on a sunny day than a light colored concrete sidewalk. Thus, the sun 

produces two significant effects that attack materials and shorten their life expectancy: 

1. Chemical reaction (i.e., breakdown) from ultraviolet radiation and heat 

2. Physical reaction (i.e., expansion and contraction) from daily temperature cycles caused by objects 

absorbing and emitting heat gained from sunlight 

The chemical and physical reactions caused by sunlight can cause colors to fade and materials to 

become brittle, warp, or crack. Deterioration can happen relatively quickly (a year or less) or over longer 

periods of time depending on the characteristics of a material and its chemical composition. In some 

cases, materials like plastics that are vulnerable to UV 

degradation can be made resistant by adding UV 

inhibitors to the chemical formulation. A prime example 

is vinyl siding. As an alternative approach, materials can 

be protected from sunlight by matter of design (e.g., 

providing shading or using reflective coatings). 

UV light is not always bad. For example, there are 

portable UV light wands available for use to sterilize 

surfaces or materials, UV is used to disinfect water and 

wastewater, and older style blueprints were created 

with UV light and special paper that “developed” into 

the blue line drawing. 

However, almost everyone has witnessed or 

experienced the painful effects of UV radiation on skin, 

which causes sunburn. Consider that the exterior of a 

house is like its skin. Therefore, the proper selection of materials determines to what degree the 

building exterior will be able to withstand the damaging effects of UV radiation similar to the sun 

protection factor (SPF) for skin. The amount of solar radiation also varies by geography (Figure 6–1), 

with the number of cloudless days strongly affecting the dose of UV radiation over the lifetime of a 

product. As you see in Figure 6–1, even though Florida is a very warm and humid state by Climate Zone 

Integrative Design and Construction—

Designing With the Sun 

Sunlight isn’t just a durability concern—

and in many cases it can be harnessed in 

very beneficial ways to provide 

wintertime heating and added light to 

the indoors. 

From an integrated design standpoint, 

passive solar heating and daylighting are 

design tactics which can increase 

building efficiency and homeowner 

satisfaction—designers just need to also 

account for managing this sunlight. 

Example issues are avoiding overheating 

solar-exposed spaces and using materials 

which can withstand the sunlight 

exposure. 
Durability by Design 95

characteristics, its solar radiation is significantly impacted by the amount of cloud cover, as opposed to 

the Southwest, where sunshine is abundant and clouds are few. 

National Renewable Energy Laboratory

Yearly Average Solar Irradiation

Figure 6–1: Yearly Average Solar Irradiation Map 

The following section presents a few measures that can help to manage the effects of solar radiation on 

building materials and systems. For homes, some of the primary problems associated with solar radiation 

are color fading, premature asphalt roof shingle failure, and vinyl siding warping. Excessive exposure to 

sunlight will also cause caulk joints to fail quickly if the proper caulk is not used. In addition, when shining 

through windows, sunlight can cause interior colors to fade and possibly unwanted heat gain at certain 

times of the year. 

6.2 Recommended Practices 

6.2.1 Size Overhangs to Provide Specific Amounts of Shading 

As with rain on the building envelope, properly sized roof overhangs can minimize the exposure to solar 

radiation and, hence, minimize radiation-related problems. The width of a roof overhang that will protect 

walls from excessive solar exposure in the summer while allowing heat gain through windows from winter 

sunshine depends on where the building is located relative to the equator. The sun is higher overhead in 

the summer than in the winter. In addition, for any day of the year, at the more northern higher latitudes 

the sun is lower in the sky than at lower latitudes. Therefore, buildings situated farther south receive 

greater protection from the summer sun by roof overhangs, as shown in Figure 6–2. 
96 Durability by Design

Figure 6–2: Building Latitude and its Impact on Overhang Effectiveness 

The solar angle factors of Table 6–1 can be used to help determine overhang width to achieve the desired 

shading effect on south-facing surfaces. An example calculation which follows shows how the solar angle 

factor is used. 

Table 6–1: Solar Angle Factors (SAF)a

Date

Latitude (degrees North)

24 32 40 48

Key West, FL Charleston, SC New York, NY Victoria, B.C., Can.

To prevent winter 

shading:

Dec 21 (winter solstice) 1.5 2.0 3.0 5.4

Jan 21 and Nov 21 1.2 1.7 2.4 3.8

Feb 21 and Oct 21 0.8 1.0 1.4 1.9

Mar 21 and Sept 21 0.4 0.6 0.8 1.1

To produce summer 

shading

Apr 21 and Aug 21 0.2 0.4 0.5 0.7

May 21 and Jul 21 0.1 0.2 0.4 0.5

June 21 (summer solstice) 0.0 0.1 0.3 0.5

Source: Structures and Environment Handbook, 11th Edition, Midwest Plan Service, Iowa State University, Ames, Iowa, 1983.

a

Factors apply for times between 9:00AM and 3:00PM for winter shading, and at noon for summer shading. Direct south 

facing orientation is assumed.

Latitudes for a sample of other U.S. Cities: Atlanta: 33, Chicago: 41, Denver: 39, Los Angeles: 34.
Durability by Design 97

6.2.2 Specific Light or Reflective Exterior Finishes in Hot Climates 

As a second line of defense against damage from solar radiation, light colored materials and finishes can be 

selected. White is excellent and aluminum reflective-type coatings are even better. Light colors can reduce 

summertime cooling load and should reduce energy bills—especially in cooling-dominated climates—by 

lowering the solar heat gain into the building. 

If properly accounted for in cooling load calculations, lighter colored roofing may allow for the use of 

smaller capacity air conditioning 

units. For example, REM/Rate 

modeling software has a simple 

“Roof Properties” set of inputs 

including the color of the roof 

covering. Users of this software 

or similar programs can very 

quickly model and observe the 

changes which result to the 

cooling load from switching from dark to light or reflective roof coverings. 

The effect of building exterior color on solar heat gain is illustrated in Figure 6–3. It is very important, 

however, to keep light colored finishes like roofs relatively clean to take full advantage of their reflectivity. 

 EXAMPLE: DETERMINE ROOF OVERHANG WIDTH TO PROTECT WALL 

AGAINST SUMMER SUN 

Find the overhang length (OL) to shade 6’ of wall below the roof overhang for June through July. The home is 

located at latitude of 32 degrees North. It is desired to provide shade for 6 ‘of wall below the overhang at 

mid-day (i.e., to bottom edge of windows). 

 Solar Angle Factor (SAF) = 0.2 (for July 21) from Table 6-1

 Wall distance below overhang to shade (WD) = 6’

 OL = SAF x WD = (0.2)(6’) = 1.2’

Use a 16” (1.33’) overhang which will provide roughly 6’8” of shading below the overhang. 

Determine degree of shading in the winter (using Feb 21) as follows: 

WD = OL/SAF = 1.33’/ 1.0 = 1.33’ or 1”. 

The selected overhang width will provide no more than about 16” of shading to the wall during the main 

winter months of November through February. However, some shading to the top few inches of windows 

will occur in the early and late winter months when maximum solar heat gain may be desirable. But, in this 

case, the overhang width should not be decreased in the interest of maintaining weather protection of the 

wall.

Integrative Design and Construction—Overhangs Affect Sun, 

Wind and Rain 

As the shading calculation example illustrates, overhangs can be strategically 

designed to provide shading of windows. Designers should keep in mind that 

overhangs also can provide protection to exterior walls from rain (Section 

4.2.2) and also increase wind uplift forces on the roof assembly (Section 9.2). 

The final strategy for how overhangs are used in a design is largely a function 

of the project location. 
98 Durability by Design

Similarly dark colored finishes can be chosen when more solar absorption is a wanted effect —especially in 

climates where heating is the dominant load. 

Figure 6–3: Effect of Surface Coloration on Solar Heat Gain 

6.2.3 Protect Reservoir Claddings From Rain Exposure and/or Specify Exterior Wall Materials to Limit 

Inward Vapor Diffusion 

A design topic which links moisture management and sunlight is exterior “reservoir” wall materials. 

Reservoir cladding materials can absorb bulk water from rain which hits their surfaces and is absorbed. 

Cladding systems made of masonry, stucco, fiber cement, and wood can all absorb some amount of water. 

When these surfaces are wetted (“charged” with water) and subsequently exposed to high levels of 

sunlight, the cladding materials experience much higher temperatures and vapor pressures. The elevated 

vapor pressures drive water vapor towards the exterior but also into wall assemblies, especially if the 

building interior is at a much lower vapor pressure (i.e., it contains much less humidity due to air 

conditioning). 

From a design standpoint, overhangs can be used to limit exterior wall exposure to rainfall and water 

absorption for these reservoir claddings. If we take away the water source, this problem is managed. See 

Section 4.2.2 for design guidance on sizing overhangs for moisture protection. 

If overhangs are not feasible for aesthetic reasons, a need to reduce wind uplift design requirements, or 

other reasons, then the hygrothermal performance of the wall system must be more carefully scrutinized. 

In fact, even with the use of overhangs, use of reservoir cladding systems warrants more careful 

examination of the wall’s treatment of water vapor. Design recommendations include the following. 

 If water vapor will enter the wall, make sure it can pass all the way through. In other words, if 

the wall sheathing, WRB, and exterior insulation (if present) collectively form a highly permeable 

assembly, then water vapor will be able to migrate into the wall. With this knowledge, we need to 
Durability by Design 99

make sure the vapor can dry to the interior, so interior polyethylene vapor retarders, vinyl wall 

paper, and other very low perm materials should not be used on the wall interior. 

 Forming the outer layer of the wall with low or moderate vapor permeance materials can limit 

inward vapor migration from the wetted cladding. The low/moderate vapor permeance layer 

might be the exterior insulation and/or the WRB. This layer will limit the diffusion of water vapor 

even when vapor pressure at the cladding is elevated. However, it’s critical to note that adding less 

vapor permeable materials towards the outside of the wall will also limit the ability for interior

moisture to dry towards the exterior (as in winter conditions). Thus, this recommendation needs 

to be directly integrated with the design guidance on insulation levels and perm ratings found in 

Section 4.3. 

 Walls at “high risk” from reservoir cladding / solar-driven moisture problems should be 

examined more closely with hygrothermal modeling (see discussion in Section 4.3). Risk factors 

include little/no wall protection from rainfall; high moisture storage capacity (e.g., masonry); and 

high solar exposure. Projects in mixed heating/cooling climates are also more challenging because 

the dominant vapor drive goes in both directions for significant portions of the year. 

In addition to this guidance, the Additional Resources includes a reference from Building Science 

Corporation on reservoir cladding moisture issues. 

6.2.4 Specify UV Protective Glazing Materials in High Solar Regions 

To prevent fading damage to the interior furnishings and to reduce the need for air conditioning, there are 

glazing options for windows and doors that block UV radiation. There are myriad different types of glazing 

and films that can be added or specified for windows providing differing properties for desired 

performance. At a minimum, the energy code’s values for window transmittance (U-value) and the solar 

heat gain coefficient (SHGC) offer some UV protection. If more UV blocking is necessary, films adhered to 

the glass will provide this additional UV and/or SHGC protection. 

6.2.5 Specify UV Resistant Materials 

Some materials are naturally UV-resistant, while others require the addition of UV inhibitors in the makeup of the material. For example, concrete or clay tile roofing and Portland cement stucco or brick siding 

are naturally resistant to UV radiation and are also resistant to temperature effects compared to other 

exterior building materials. On the other hand, plastics are prone to “dry rot” (embrittlement from 

excessive UV exposure) unless UV inhibitors are provided. Plastics are also prone to significant expansion 

and contraction from temperature swings. Vinyl siding is a perfect example of how this technology has 

advanced. Years ago vinyl siding was prone to UV degradation, but is now highly resistant to both cracking 

and fading. 
100 Durability by Design

Be sure that UV inhibitors are used in materials that require protection. Many low budget components, 

such as some plastic gable end vents, may also lack UV resistance. All other factors being equal, choose the 

material with the best UV resistance if exposure to the sun is a concern. 

6.2.6 Design Landscaping to Provide Shading in High Solar Regions 

Trees planted near a home along the southern and western exposure provide shading when most needed 

during the day. Deciduous trees, such as maple or oak, should be used so that winter sun can reach the 

building. With appropriate planning, trees can also serve as a wind break to minimize the effects of winddriven rain. Trees should be planted far enough away from a house to prevent possible damage from limbs 

or roots, as well as clogging gutters. (And in fact, trees deserve consideration as a natural hazard in high 

wind areas. see Chapter 9). Bear in mind that the greatest amount of solar radiation is generally received 

between 9 AM and 3 PM. However, shading of only the late day sun (i.e., after 3 PM) is often a preferred 

and more practical solution for many sites. 
Durability by Design 101

EXAMPLE: DETERMINE LOCATION OF SHADE TREE TO PROTECT AGAINST SUMMER SUN

Use the following equation and the solar angle factors (SAF) from Table 6-1 to determine the appropriate 

location of a maple tree (mature height of 60’) southward of a building wall (8’ height) that is to be shaded 

during summer months. The building latitude is 400 North.

D = SAF * (ho – hs)

Where:

D = distance between object obstructing the sun at highest point and item to be shaded

ho = height of the object obstructing the sun

hs = height of object to be shaded

SAF = solar angle factor (from Table 6-1)

The following values are given:

ho = 60’

hs = 8’

SAF = 0.4 (from Table 6-1 for May 21 or July 21)

Substituting in the equation above,

D = (0.4)*(60 ft – 8 ft) = 20.8’

Therefore, the center of the maple tree should be located about 21’southward from the wall or windows to 

be shaded. Not that the shading at the first day of summer (June 21) will be slightly less due to the higher 

solar angle than assumed above. In addition, the tree should not overhang the building at its mature age. 

Thus, a distance smaller than about 20’ is not recommended and the distance should be increased for trees 

that are larger at maturity.

6.3 Additional Resources 

BSC Information Sheet 305: Reservoir Claddings for All Climates. Building Science Corporation, May 

2014. This information sheet is a concise, four-page discussion of the underlying drivers of reservoir 

cladding moisture issues with design strategies to manage this issue. 
102 Durability by Design

CHAPTER 7—INSECTS

7.1 General 

Insects are not just a nuisance; some are also a serious threat to building durability. The following types of 

insects are known to damage wooden materials in homes and in other structures: 

 Termites 

 Carpenter ants 

 Wood-boring beetles 

 Carpenter bees 

While all of the above insects can pose a threat to wood-framed homes, termites are the most prevalent 

and damaging insect. Therefore, most of this chapter addresses issues and practices related to the control 

and prevention of termite infestation. Some of the practices for repelling termites, such as eliminating 

hidden areas that termites can travel through undetected, are also relevant to carpenter ants. Carpenter 

ants and wood-boring beetles, like termites, can be treated chemically with insecticides. Carpenter bees 

can be deterred by plugging entrance holes that commonly occur on wood siding and soffits. 

There are about 57 species of termites in the United States that can be placed into two groups: 

subterranean (ground inhabiting) and non-subterranean (wood inhabiting). Subterranean termites are the 

most common and are responsible for most termite damage to wood structures. Therefore, this chapter 

focuses on subterranean termites. If non-subterranean 

termites are present, special measures may be necessary 

to eliminate them. Fortunately, non-subterranean 

termites live in much smaller colonies and are much 

slower acting than subterranean termites. 

One variety of the subterranean termite group is the 

Formosan termite, an Asian termite introduced to the 

United States following WWII. The Formosan termite is 

different from the native subterranean termite in that it 

has a much greater colony size and thus damages wood 

at a much faster rate. Estimates state that a colony of 

Formosan termites will consume nearly 1,000 pounds of 

wood per year, whereas other termite varieties will only eat a few pounds annually. Formosan termites are 

also more likely to survive in a building with minimal ground contact, even though they require a constant 

Integrative Design and Construction: 

Manage Moisture to Manage 

Insect Risks 

Moisture is an essential ingredient for a 

successful colony of wood destroying 

insect to take hold. Understanding the 

concepts within Chapter 4 of this guide is 

a critical component is deterring possible 

infestation. Much like mold, if the 

moisture is not present, most likely the 

insects will not be either—get the water 

out and away from the home. 
Durability by Design 103

source of water like other subterranean termites. Formosan termites are expanding in range, and are 

currently found in the Gulf Coast states and southern states along the Atlantic coast. 

A termite hazard or probability map, shown in Figure7–1, is frequently used by building code authorities, 

designers, and builders to determine when certain termite prevention or control methods should be 

used. The building codes may vary in delineation of the termite probability zones based on local 

conditions. The inclusion and degree of termite control and prevention used in a building depends on 

the risk of termite infestation, as well as local experience. Talk to neighbors to determine where and 

when historical infestations have occurred. 

In summary, termites like to eat wood and they don’t care if it’s in your home. In areas subject to 

termite infestation, at least one of the practices listed in Section 7.2 should be used. 

Figure 7–1: Termite Probability (Hazard) Map 

Source: HUD Durability by Design (2002); original source unknown 
104 Durability by Design

7.2 Recommended Practices 

There are basically three techniques for controlling or 

preventing termite damage: 

 Chemical soil treatment or baits, 

 Termite shields, and 

 Use of termite resistant building materials. 

These measures can be combined for enhanced 

protection certainly, and in higher risk areas this 

redundancy can be very important as treatments and 

termite shields are unlikely to be 100% effective in all 

cases. 

7.2.1 Choose Appropriate Chemical Treatment Types 

Chemical treatments for termite control come in a variety 

of forms. Generally, chemical treatments for termites 

include soil termiticides, termite baits, and treated wood 

products. This section will only discuss soil and bait 

termiticides. 

Chemical soil treatments are designed to form a protective barrier around a structure to prevent termites 

from contacting or penetrating the building. Soil treatments are the most common form of termite control 

used by the building community. Commercially, there are an assortment of soil treatments that, by various 

biological means, kill termites or repel them. Termiticides are generally preferred over repellents. 

Termite baits are encapsulated termiticides designed to lure insects to the bait, and then—once eaten—

kill the termites. The poisons are designed to act slowly so as to not repel the insect and to facilitate the 

consumption and transport of the poison to the nest. Other termites ingest the termiticide from the 

insects that feed directly on the baits through secretions emitted by the original feeders. 

Application 

Chemical soil treatments are generally applied to the soil around the foundation of a home to act as a 

shield against termites. The treatments are performed prior to pouring the slab or foundation, shortly after 

foundations and slabs are poured, and at periodic intervals for the life of the structure. Directions vary 

according to the chemical used, but these locations are of special concern for chemical application: 

Rules of Thumb 

 Prevent subterranean termite access 

by diverting water away with properly 

functioning downspouts, gutters and 

splash blocks.

 Store firewood at least 20’ away from 

the home.

 Keep mulch at least 15” from the 

foundation

 Indoors, homeowners should reduce 

humidity through proper ventilation 

of crawl spaces, attics and basements 

to avoid attracting subterranean 

termite swarms.

Source: www.pestworld.org/pestguide/termites/subterranean-termites/
Durability by Design 105

 Soil along foundations and crawl spaces; 

 Areas of soil disturbance such as bath traps which may create areas of lower resistance to 

movement; 

 Soil under appurtenances such as attached slabs and porches; 

 Soil in inaccessible or concealed spaces; and 

 Soil in proximity to slab or foundation penetrations due to plumbing, wiring, etc. 

Termiticides are applied by one or more of the following methods: trenching around a foundation and 

flooding the trench with a sprayer; inserting a rod at periodic intervals around a foundation and injecting 

the chemical in the soil; and drilling holes in masonry slab or foundations and injecting the chemicals into 

the soil through the holes. Factors such as access to targeted areas, presence of landscaping, and the 

chemical employed dictate the treatment option used by the pesticide applicator. A certified pest control 

operator (PCO) is required for application of most termiticides. 

Performance of termiticides varies considerably with climate, soil type, structure design, and homeowner 

practice. Locations with frequent precipitation, impermeable or very permeable soils, or great soil 

disruption from landscaping activities will require frequent reapplication in order to maintain termiteresistive properties. 

Termite baits are applied to the ground at intervals around the home as prescribed by the product label. 

Some bait systems employ only a cellulose bait that requires frequent monitoring. Once termite activity is 

detected, a poison is inserted into the bait housing. Other bait systems contain both termite lure and 

poison in one formulation. The key to satisfactory performance in a bait system still is proper monitoring 

and placement. Do-it-yourself termite bait kits are available to the general public, but the temptation is to 

purchase too few and monitor the baits infrequently, thus severely hampering their effectiveness. This 

requires much diligence on the part of the homeowner. Many pest control operators offer bait systems 

which better assure proper bait placement and monitoring. 

Reapplication and Inspection Services 

Chemical termiticides have a limited life because of leaching or chemical degradation. In addition, 

homeowner activities such as disruptive landscaping tend to limit the effectiveness of chemical 

treatments. Therefore, many homeowners opt to employ a termite service offered by pest control 

operations (PCO). 

Typically, a contract with a PCO involves an initial treatment of the structure with a chemical termiticide 

or bait system, followed by an annual inspection of the structure with periodic retreatment performed 

when required. PCO warranties may offer free retreatment if infestation is detected along with some 

coverage of repair costs, depending on the specific plan. The benefits of an inspection and treatment 

service include periodic inspection of a home by knowledgeable technicians and quick remedial action 
106 Durability by Design

when infestation is detected. A client can be better assured of a competent applicator if the PCO is a 

member of the National Pest Management Association (NPMA). NPMA promulgates the standards that 

constitute proper treatment of buildings.

7.2.2 Specify a Termite Shield System 

A termite shield is placed between a masonry foundation and wood framing to prevent termites from 

gaining access to the wood framing components. Termite shields (Figure 7–2) must be of termite-resistant 

materials such as metal or concrete. Some termites are able to chew through plastics and thin metals. 

Also, any seams in a termite shield must be soldered or otherwise sealed. 

Since termite shields require a high degree of care in installation, they are best used in combination with 

soil treatment. They should always be used when there are potential hidden pathways. Construction 

types known to create hidden pathways for termites include slab-on-grade (except monolithic slabs of 

good construction), masonry construction, and brick veneer construction. 

Hidden pathways allow termites access to wood materials through pathways that cannot be detected 

during periodic inspection. When there are no hidden pathways in construction, subterranean termites 

can be easily detected by the presence of shelter tubes or tunnels that are made of mud to protect them 

from light and keep them moist. Because termite shields are difficult to install on slab-on-grade 

construction or split-level construction, other methods of termite protection (e.g., soil treatment) are 

generally preferred for these types of foundations. It is also noteworthy that termites can gain hidden 

access through cracks as small as 1/32” wide. Therefore, if concrete is used as a barrier to termites, it 

should include welded wire fabric or sufficient reinforcement to control cracking. Examples of concrete as 

a termite barrier are illustrated in Figure 7–3. 
Durability by Design 107

Figure 7–2: Use of Termite Shields 

Figure 7–3: Use of Concrete as a Termite Barrier 
108 Durability by Design

7.2.3 Use Termite Resistant Building Materials for Added Protection 

Wood can be protected against termite damage by use of preservative treated wood (e.g., ACQ, CA-B, CAC or Borate). CCA was used in residential construction up until 2004 when it had been voluntarily 

discontinued. Using treated lumber to frame a home can add hundreds or few thousands of dollars to the 

price of a typical home. Such a drastic measure, however, is only used in particularly severe termite hazard 

areas like Hawaii. 

As an alternative, preservative-treated wood may be used in isolated locations such as foundation sills and 

floor framing directly above the foundation. This practice is particularly appropriate for crawl space 

construction and for basement construction when ceilings are finished such that these elements are not 

easily inspected for infestation. Alternatively, naturally decay-resistant wood (e.g., heartwood of redwood 

and eastern red cedar) may be used, but at even greater expense than preservative treated lumber. For 

this reason, materials such as galvanized cold-formed steel may be a cost-effective alternative and are 

frequently used in Hawaii to complement or compete with the use of preservative treated wood. Concrete 

and masonry building materials are favored alternatives in areas such as Florida. 

 
Durability by Design 109

CHAPTER 8—DECAY AND CORROSION 

8.1 General 

At a moisture content of greater than 25 %, wood is subject to fungal attack or decay. Decay will be rapid 

when the temperature is in the range of 70 to 850F. The potential for wood decay when exposed to the 

outdoors, therefore, varies in accordance with climate (refer to Decay Hazard Map, Figure 4–2). However, 

wood exposed to excessive moisture within a building assembly in any climate, particularly one with a low 

drying potential (see Chapter 4), will grow mold and rot. 

There are essentially three options for preventing decay of wood which may be exposed to the elements: 

 Protect (or separate) wood from moisture 

 Use naturally decay-resistant wood 

 Use preservative treated wood 

Of equal concern is the corrosion resistance of fasteners that must hold wood joints firmly together. 

Corrosion of insufficiently protected metal fasteners can eventually lead to complete failure of key 

fasteners which support assemblies like deck joists. 

Finally, it is important to consider cost-effective alternatives to wood that offer potential durability and 

maintenance benefits. In combination with measures presented earlier in this guide, particularly in 

Chapter 4, recommendations in the following section should address the major concerns regarding 

durability of wood construction. 

8.2 Recommended Practices 

8.2.1 Design to Maintain Minimum 8” Clearances to Protect Wood from Ground Moisture 

One of the oldest and most reliable practices to prevent wood and other moisture sensitive materials from 

decay is to separate the wood from the constant uptake of moisture from the ground. Decay conditions 

can occur when wood is in direct ground contact or when moisture wicks through other materials such as 

concrete or masonry. Some well-known details for separation of wood from ground moisture are shown in 

Figure 8–1. 

Most building codes require a minimum of 6” clearance between untreated wood and the exterior grade. 

A minimum of 8” clearance between untreated wood and the exterior ground level is recommended in 

this guide. In other conditions shown in Figure 8–1, ground clearance recommendations vary (e.g., floor 

beams in crawl spaces are required to have a minimum 12” clearance to interior grade as much for reason 

of access as for moisture protection). 
110 Durability by Design

Figure 8–1: Details to Separate Wood from Ground Moisture 

8.2.2 Match the Treatment Level of Preserved Wood and Fastening Systems to the Application and 

Exposure 

There are often situations where wood cannot be adequately separated from ground moisture or 

protected from exterior moisture sources. In these situations, either naturally decay-resistant wood or 

preservative-treated wood must be used. In some cases, wood alternatives may be considered, such as 
Durability by Design 111

plastic porch posts with metal pipe inserts, plastic decking, concrete posts or piers, or plastic lumber 

composites. 

For preservative treated wood, the American Wood Protection Association (AWPA) maintains the Use 

Category System (UCS), first introduced in 1999 and used to define a range of usage categories, or 

biodeterioration hazard levels, for treated wood products. The UCS covers a wide range of conditions 

spanning residential to marine construction, with most residential applications generally found in use 

categories 2 through 4. Table 8–1 below summarizes typical residential Use Categories and Applications, 

drawn from specification recommendations for Southern Pine.28

Table 8–1: Common Use Categories for Preservative Treated Wood in Residential Applicationsa

Use Category Typical End Use Applications 

(not all inclusive)

Commonly Used Preservative 

Systems

UC2 Above Ground, Interior: 

Damp

Sawn interior framing, flooring, 

furring strips, millwork/trim, sill 

plates, roof trusses, subflooring

Borates, Carbon based, Copper 

azoles and quats

UC3A Above Ground, Exterior: 

Protected

Sawn painted/coated fascia, 

fence pickets, gazebo material, 

millwork/trim, siding

Carbon based, Borates, Copper 

azoles and quats

UC3B Above Ground, Exterior: 

Exposed

Sawn decking, deck joists and 

beams (not subject to frequent 

wetting); deck cross-bracing, 

railing components, stair 

stepping, unpainted fascia, 

fencing, gazebo material, 

millwork/trim, siding, floor 

trusses, 

Copper azoles and quats, Carbon 

based

Glulam beams for decks, 

gazebos, etc.

Copper Naphthenate, Oxine 

Copper

UC4A Ground Contact / 

Freshwater: General Use

Sawn deck, fence and generaluse posts; deck joists and beams 

(ground contact) and stair 

stringers; gazebo supports; 

Copper azoles and quats

a. Note that additional applications and preservative systems may apply in residential settings. This table is intended to relate 

common residential design scenarios for preservative treated wood, and is based on Southern Pine. For other wood 

species, treatments and applications should be confirmed with the material supplier. 

The table above serves as a quick reference, with conditions 3B and 4A being the most common for 

pressure-treated Southern Pine lumber. More in-depth information on the Use Categories, preservative 

retention minimums by product/end use, and specification details are available from AWPA Standard U1 

and the Southern Pine Council (see Additional Resources section at the end of the chapter). 

 

28 Pressure-Treated Southern Pine, Standards, Specifications, Applications. Southern Pine Council, 2014. 
112 Durability by Design

When a residential design includes preservative treated wood, there are several basic recommendations 

to help assure good durability performance. 

 Specify Ground Contact (UC4A) pressure-treated lumber for applications which more closely fit this 

condition, as opposed to Above Ground (UC3B). Typical applications to specify UC4A instead of 

UC3B are: 

o deck joists and deck beams in close proximity to the ground 

o deck decking, joists, and beams subject to frequent wetting from moisture sources like hot 

tubs 

o stair stringers with ground contact or in close proximity to the ground 

 Verify that treated lumber delivered on site matches the specification and the Use Category. For 

example, if deck joists close to the ground are specified to be UC4A, field verify that this is the Use 

Category for the lumber, as listed on the lumber’s plastic end tag or the ink stamp.

 Select fasteners and connectors with the appropriate corrosion resistance given the type of 

preservative treated wood they’re being used with. Corrosion resistance of fasteners and 

connectors used with treated wood products is not a new issue, but has gained increased 

emphasis since the phase-out of Chromated Copper Arsenate (CCA) in residential applications in 

2004. The alternative treatments have generally been shown to be more corrosive. The building 

application should define the Use Category, and this information is used to select the preservative 

treated wood system. Based on the preservative treatment being used, the wood supplier should 

be able to provide: the chemical retention level, whether or not ammonia was used, and 

ultimately a connector/coating recommendation. If the wood supplier cannot provide a 

connector/coating recommendation, the preservative treatment information can be 

crossreferenced with the fastener/component supplier’s recommendations. For example, Simpson 

Strong-Tie offers a Technical Bulletin on Preservative Treated Wood which includes recommended 

coatings as a function of the preservative type and the application’s exposure (see Additional 

Resources). 

Beyond following this selection and specification process, several other rules of thumb apply for 

connectors/fasteners used with preservative treated wood: 

o Hot-dip galvanized or stainless steel fasteners/connectors are recommended for use with 

preservative treated wood. Electroplated, electro-galvanized, and mechanically galvanized 

coatings should not be considered to be hot-dip galvanized. 

o The thicker the galvanized coating, the longer the expected service life of the fastener, 

connector, anchor, or other hardware. 
Durability by Design 113

o Hot-dip galvanized fasteners (like nails) should be used with hot-dip galvanized connectors 

(like joist hangers). The same is true for stainless steel. Hot-dip galvanized components 

and stainless components are dissimilar metals and should not be used with each other. 

o To be more conservative in aggressive environments, specify stainless components. Note 

that cost and product availability are both significant considerations in the use of stainless 

components. 

8.3 Additional Resources 

American Wood Protection Association Standard U1-14, Use Category System: User Specification for 

Treated Wood. http://www.awpa.com/standards/U1excerpt.pdf 

Pressure-Treated Southern Pine, Standards, Specifications, Applications. Southern Pine Council, 2014. 

Preservative Treated Wood Technical Bulletin, Simpson Strong-Tie, 

http://www.strongtie.com/ftp/bulletins/t-ptwood08-r.pdf
114 Durability by Design

CHAPTER 9—NATURAL HAZARDS 

9.1 General 

There is perhaps no area of the United States that is 

spared from natural hazards. Coastal areas experience 

hurricanes and flooding; “tornado alley” spans the central 

part of the country from the Gulf Coast to North Dakota; 

earthquake-prone areas are most notably in California, but 

significant faults also exist in Hawaii, Alaska, South 

Carolina, and portions of Missouri, Arkansas, and 

Tennessee; wildfire risk is highest in the West, Southeast, 

and upper Great Lakes, but structures have been lost 

across the country due to wildfires; and the frequency of 

severe thunderstorms and hail is high or very high in at 

least half of the country! 

Although we typically think of durability as “standing-thetest-of-time,” a durable home must also be able to 

withstand risks in the form of disasters that may last for 

only minutes or hours. This section of the guide therefore 

provides guidance regarding design and construction 

methods that will improve a home’s ability to withstand 

the forces exerted upon it by natural disasters. 

Aside from the dialogue about the exact causes of extreme 

weather events, there is little debate that extreme 

weather events are becoming more frequent. The Institute 

for Business and Home Safety, which has tracked the occurrence of disasters and consequent costs to 

insurance companies since 1950, has shown an increase in the quantity and severity of events over just the 

past 23 years. Based on industry risk analysis, for example, the number of states experiencing insurance 

claims valued at over $2 billion rose from only 7 states between 1990 and 1999, to 24 states between 

2010 and 2013.29

 

Local building codes—typically derived from model national codes—establish the minimum level of 

disaster mitigation that is legally allowed from the standpoint of public safety and general welfare. 

However, although such measures may be good for the whole, they may not be optimal for any one 

 

29 Verisk Analytics. The Financial Effects of Natural Disasters—online tool for mapping disasters by type and by 

timeframe. http://www.verisk.com/property-claim-services/natural-disasters.html, last accessed August 2014. 

The chance of homeowners experiencing a 

natural disaster is far greater than winning 

the lottery. Should they be inclined to buy a 

lottery ticket to offset some of the cost of 

building their new home, they are taking a 

large risk with a small part of their income 

for an extremely small chance of winning a 

big sum of money. 

Viewed in this context, doesn’t it make more 

sense for home buyers also to consider 

investing a modest sum of money (rolled 

into the mortgage) to better their odds of 

their home surviving a future extreme 

weather event (probably more likely than 

the lottery) and thus, enjoy the benefit of a 

more resilient home for years to come?

This chapter is based on the expectation 

that the answer to this question is “YES.”
Durability by Design 115

individual’s circumstances. The building code also maps hazard areas by region when in fact the degree of 

risk may vary based on site-specific characteristics. For instance, homes located in very open terrain may 

face greater threat from high winds. Some homeowners may want additional protection from potential 

damage—especially if the cost of hazard insurance is quite high. Furthermore, it almost always is less 

expensive to include measures when building a new home rather than go back and try to retrofit later. In 

short, there are many strong reasons to consider additional, cost-effective disaster mitigation measures for 

new homes. 

The following sections of this chapter outline the major natural hazards and a subset of recommended 

mitigation measures for each. Note that these measures are only a sample of recommended measures 

that generally offer a 

significant improvement for 

a modest cost. Additional 

resources for disaster 

mitigation are then listed at 

the end of the chapter. 

Safer Stronger Homes – HUD 1993, Crandell 2005

Figure 9–1: Examples of Roof and Building Damage From High Winds 

9.2 Recommended Practices—High Wind Regions 

High wind regions cover a large portion of the United States While we associate the Gulf and Atlantic 

coasts with hurricanes and the Midwest with tornadoes, severe thunderstorms can happen anywhere. 

Microbursts and “derechoes” have become familiar terms and cause very significant damage in localized 

areas. Roof damage is one of the most frequent types of damage and particularly serious because it is 

often coupled with significant water damage.

9.2.1 Enhance Roof and Wall Sheathing Connections: Use Deformed Nails or Screws for Enhanced Roof and 

Wall Sheathing Connection. 

Studies show as much as a doubling of withdrawal capacity, if not more, when deformed shank fasteners 

are used instead of smooth shank (assuming same size shank and adequate head size). Screws provide 

even greater holding power. Either deformed nails or screws are therefore very effective strategies in any 

area subject to high winds, with the added costs typically less than $250. Of course installation quality is 

still critical, as overdriving nails into sheathing can significantly reduce capacity and nails missing framing 

members, whether ring shank or not, will offer no uplift resistance. However, not all ring shank nails are 
116 Durability by Design

equal; as a minimum recommendation, use ring shank nail specifications as 

found in the ICC 600 Standard for Residential Construction in High Wind 

Regions, Section 309.3 (www.iccsafe.org). 

A key benefit of the improved sheathing pull-off strength, beyond occupant 

safety and limiting potential wind-debris impacts to neighboring homes, is that 

loss of even one roof sheathing panel in a major wind-driven rain event causes 

disproportionate damage to a home. Just one missing sheathing panel can allow 

a home to become inundated with water, which compounds the insurance loss 

in addition to displacing the residents. 

Insurance Institute for 

Business and Home Safety

Figure 9–2: Ring Shank Nail 

9.2.2 Select a Roof Shape and Slope to Minimize Potential for Wind Damage 

Hip roofs and gable roofs which have a pitch between 5:12 and 6:12 have been shown to perform 

exceptionally well during severe wind events (Figure 9–3). Moderately pitched hip roofs provide a more 

“streamlined” building compared to other roof geometries regardless of which direction the wind blows. 

Moderate roof slopes aren’t as prone to uplift that can pull off entire roofs and they also limit the lateral 

force that can cause building collapse. 

An optimal roof slope can reduce lateral wind load by as much as a factor of three and reduce uplift loads 

by 50% or more! 

U.S. HUD

Figure 9–3: Roof Pitches and Their Implications for Wind Forces 
Durability by Design 117

9.2.3 Use Wind Resistant Roofing and Enhanced Underlayment in High Wind Areas as Required by the IRC

Standard asphalt composition roof shingles, the most common roof covering for residential buildings, vary 

in their capability to resist wind loads. Using better performing shingles can help prevent roofing damage 

leading to potential moisture intrusion and more serious consequences to building contents. In hurricaneprone and high wind areas, the IRC requires that shingles must be rated to meet hurricane level wind 

speeds according to ASTM Standard D 7158 classes G or H or ASTM D 3161 class F. Shingles meeting ASTM 

D 7158 Class H are rated for wind speeds as high as a Category 4 

hurricane (or very severe thunderstorm downburst). Selecting 

higher performing shingles is advisable in all wind regions for 

durable roof construction. 

Using a shingle rating greater than that required by code, if 

available, will also provide some relative benefit in limiting 

damage. Further, designers and builders should also be careful to 

cross-reference shingle ratings with the locally applicable, codemandated wind speeds. As a matter of practical importance, 

following the shingle manufacturer’s installation instructions for 

high wind conditions and also ensuring a good adhesive tab seal 

between shingle courses are crucial to durable asphalt shingle 

roofs. Recent insurance industry research indicates that “SBS 

polymer modified shingles” are most likely to provide a durable 

roof installation. 

Figure 9–4: Examples of Roof Covering Loss 

Underlayment and Flashing—The IRC 

requires a minimum of a single layer of 15# 

felt lapped 2” for composition shingle 

roofing on slopes ≥ 4:12; a double layer of 

underlayment is required for shallower 

pitches. Flashing is required in valleys, at 

sidewalls, and wherever there is a roof 

penetration such as chimneys and vents. In 

high wind areas, a 4” overlap, reduced 

fastener spacing (12” grid pattern with 6”oc 

at lap joints), and plastic or metal cap nails 

are required. 

Figure 9–5: Sheathing Seams Covered With Bituminous Tape
118 Durability by Design

Because non-adhered membrane-type underlayment (e.g., 15# felt) isn’t intended to be an exposed 

water-resistant barrier under high wind conditions, it does not provide a durable back-up should roof 

shingles become damaged and torn off the roof from wind. Therefore, in high wind regions it’s good 

practice to seal the roof deck below. There are several options including taping of the seams of the 

sheathing with a bituminous tape (Figure 9–5). For even further protection in the event of shingle damage 

or loss, the entire roof can be covered with a self-adhering membrane. As another alternative, a recent 

test at the IBHS Research Center on full-scale roofs demonstrated that two layers of ASTM D226 Type II or 

ASTM D4869 Type IV underlayment can provide excellent water intrusion resistance without requiring 

application of tape or a self-adhering membrane. 

9.2.4 Select and Locate Trees to Avoid Damage to Property 

While trees can be very useful in protecting materials and finishes from harsh sunlight (Chapter 6) and 

reducing cooling loads in the summer (Chapter 5), and even sheltering buildings from high winds—there 

are considerations to address when planting trees around a home (or situating a new home among 

existing trees) in high-wind regions. Some basic guidance can be gleaned from looking at those species, 

characteristics, and conditions which have demonstrated a successful survival rate under previous severe 

storms and hurricanes. 

1. The following species have performed well during Florida hurricanes over a 10-year period 

between 1995 and 2005: live oak, sand live oak, and sabal palm. Crape myrtle and bald cypress 

also have withstood major storms. 

2. Select trees that attain a maximum height of 30’ or less, and display open branching patterns. 

Such trees present less surface area for the wind to blow against and allow the wind to pass 

through the canopy rather than exert pressure that can topple it. 

3. Plant trees in groups rather than individually, but allow enough spacing between them for a 

strong root system to develop. Recommended distance between trees is approximately the 

expected diameter of the canopy at maturity. 

4. Avoid trees that have shallow root systems such as Bradford pears. Likewise, avoid species that 

become brittle and break or split easily when mature. Examples include poplars and aspens. 

5. Selective pruning of branches in tree canopies by a knowledgeable arborist can thin the crown 

and reduce wind loads on the tree. 

9.3 Recommended Practices—Flood Prone Regions 

Flooding can be the most serious consequence of a hurricane as was recently the case for the Gulf Coast 

(Katrina) and New York/New Jersey (Sandy). Moreover, flooding and severe damage can occur quite a 

distance from where a hurricane or tropical storm initially makes landfall, as was seen in Pennsylvania, 

Upstate New York, and Vermont with storms Irene and Lee. However, hurricanes are certainly not the only 

cause of flooding. In many communities, flooding can be a frequent occurrence when rapid winter snow 

melt combines with substantial rain events. Thus, buildings in coastal regions as well as low lying areas 
Durability by Design 119

adjacent to rivers and streams can be susceptible to significant property damage. Given that upstream 

development and land-use changes can alter flood elevations at a given site, it is recommended to err to 

the conservative when it comes to the risk of flooding. 

9.3.1 Raise the First Floor Level at Least 1’ Above the Base Flood Elevation (BFE) in A Zones and at Least 3’

above the BFE in V Zones

FEMA has classified different parts of the country according to their risk of flooding. V Zones are those 

subject to the greatest risk and are typically those properties located directly on the coast. As such, they 

are exposed to the largest waves during a hurricane storm surge and thus, have the highest recommended 

“freeboard” or elevation above the 100 year Base Flood Elevation. A Zones are the second highest risk area 

and are typically those properties located near a lake, river, stream, or other body of water. 

Raising the first floor level at least 1’ above BFE in A Zones and at least 3’ above BFE in V Zones greatly 

enhances the flood resistance of a home. This measure will keep wood and finish materials high enough to 

prevent or limit water damage and the subsequent problems this causes. In V Zones, coastal A Zones 

where surge can be a problem or in riverine areas where flood waters may be moving swiftly, open 

foundations and extra elevation are critical to allow a clear area for flow through. Figure 9–6 illustrates 

these concepts. Further, flood maps for all regions of the country are available at 

https://msc.fema.gov/portal. 

 

FEMA: http://www.region2coastal.com/coastal-mapping-basics

Figure 9–6: Diagram of Flood Zones and Base Flood Elevation 
120 Durability by Design

9.3.2 Install a Sump Pump in Basements. 

Sump pumps can remove incoming water due to severe 

storms or other heavy sources of exterior water near the 

home’s foundation. Adding the sump pump is crucial but 

the system is made even more effective by the following 

measures. 

 Diverting water at least 10’ away from the 

foundation. Too many sumps simply discharge the 

expelled water right back against the foundation! 

See Figure 9–7. 

 Provide battery backup so that the sump will 

operate during a power outage. 

 Ensuring that the foundation’s perimeter drain 

system feeding into the sump pit is installed 

properly to prevent clogging over time (e.g., gravel 

with filter fabric as shown in Figure 3–4). 

 Air sealing the cover of the sump pit to avoid 

indoor air quality problems from radon, odors, etc. 

Integrative Design and 

Construction: Foundation Design in 

Flood-Prone Regions 

Elevated foundations in flood prone 

regions have the critically important job of 

protecting the home from flood waters, 

whether in the form of rising water or 

flowing water. These same 

foundation/floor systems also need to 

manage heat loss/gain as well as chronic 

moisture from outdoor humidity. 

In this case the integration of these 

functions clearly starts with the flood 

resistance of the structure. From this point, 

the builder and designer should identify the 

best ways to insulate the home’s 

foundation and prevent cooled surfaces 

from developing chronic condensation. See 

Chapter 4 for a discussion of unvented 

crawl space foundations. 

Figure 9–7: Sump Pump Discharge Right Next to Foundation 
Durability by Design 121

9.4 Recommended Practices—Seismic Regions 

While the most severe and frequent earthquakes have occurred in California and portions of Alaska and 

Hawaii, damaging earthquake activity has been experienced in many other parts of the United States. We 

have seen the major amounts of damage that can occur in a very short period of time. Although rare, very 

strong quakes can give rise to tsunamis that cause their own damage and fatalities. Just as for high wind 

and flood prone regions, the International Residential Code (IRC) maps seismic risk regions of the United 

States and prescribes minimum construction and design requirements for those buildings constructed in 

high risk areas. However, there are some modestly priced measures which builders and designers can 

apply to provide added seismic protection to meet or exceed code. 

9.4.1 Reinforce Connections Between Building Components and Assemblies to More Reliably Transfer 

Loads Along a Continuous Path to the Foundation. 

This measure not only provides protection in earthquake regions, but also in areas prone to high wind 

events. Ensuring that all assemblies from the roof to the foundation are securely connected to one another 

is simply good building practice and in most areas is simply required by code. Figure 9–8 shows some of 

most important connections. 

To improve a home’s durability in seismic zones of moderate or greater risk, design for the next higher risk 

category for seismic (or wind) requirements as specified in the 2012 IRC. This will involve strengthening the 

load path connections beyond the code-minimum levels, and will also afford increased wind resistance for 

the home. 
122 Durability by Design

Figure 9–8: Continuous Load Path Diagram 

Source: Insurance Institute for Business and Home Safety 

9.4.2 Secure Water Heaters, Space Conditioning Equipment, and Other Heavy Items 

Preventing heavy objects in your home from toppling or falling not only protects them from damage but 

also may protect occupants from injury. Some items like water heaters and water tanks would not only be 

damaged themselves but also might cause additional damage due to water or possible fire. Some of the 

items to consider include refrigerators, freestanding ranges or wood stoves, computers, televisions, and 

shelving. In basements or unfinished spaces, large equipment can be strapped to the wall and/or bolted to 

the floor. In finished areas, freestanding shelving and bookcases can be securely screwed to the wall. It is 

also recommended to put heavier or breakable items on lower shelves. 

9.5 Recommended Practices—Wind-Driven Rain Areas 

Almost all parts of the country experience heavy rain storms accompanied by high winds. This can drive 

rain behind siding and around windows and doors where it can become trapped in wall cavities and lead 

to moisture damage and mildew and mold. To view the intensity of wind-driven rain levels around the 

United States, see Figure 4–10. 
Durability by Design 123

9.5.1 Use Enhanced Water-Resistive Barrier (WRB) Materials and Best Flashing Practices to Keep Water 

Out of Walls and Buildings. 

Although a WRB and flashing details are required by the building codes, certain practices can increase 

their effectiveness in extreme wind-driven rain events. Under abnormal conditions, such as a hurricane 

or severe wind-driven rain event, quality of installation becomes particularly important because the 

consequence of any defects are magnified. Also, one can enhance WRB installation and flashing details 

following recommendations and resources cited in Chapter 4. For example, overlapping of joints can be 

increased (i.e., 4” instead of 2” lap) and joints sealed. Also, WRB materials can be more carefully 

selected by requesting full-scale assembly water-penetration test data from the manufacturer (not all 

WRB materials or assemblies will pass such testing). 

9.5.2 Understand, Verify, and Specify Appropriate Wind Ratings for Windows 

Standardized wind-driven rainwater penetration tests are conducted with wind pressures that are only 15 

to 20 % of the design wind pressure. A code-minimum fenestration product is based on a test wind 

pressure that corresponds to about a 35 mph wind speed—equivalent to a common thunderstorm gust. 

Consequently, such doors and windows are likely to leak during significant wind events, even when they 

are properly installed and flashed. 

There are several important implications from this “disconnect” between water penetration tests and 

design wind pressures: 

1. Good window flashing practices (Chapter 4) aren’t just “extra credit.” They provide a necessary 

belt and suspenders type of approach to limiting (or mitigating) water infiltration through and 

around windows. Of particular importance to address this issue is the use of pan flashing to 

remove water that penetrates the window unit itself, not necessarily the flashing of it. 

2. At a minimum, builders and designers should verify that glazing in windows and doors meets 

requirements for the locally applicable wind pressure loading (design wind pressure). Window 

labeling and certification should indicate the appropriate wind pressure rating. 

9.6 Recommended Practices—Hail Prone Areas 

Like wind-driven rain, hail is most likely to occur in areas susceptible to severe thunderstorms. It is 

probably the roof covering material that will suffer the most from the impact of hail. Hail can cause 

damage that significantly reduces the life of roofing shingles and roof causes leaks to develop. 

9.6.1 Install Impact Resistant Roof Coverings Having a Class 4 Rating When Tested to UL 2218 or FM 4473. 

Most manufacturers have a line of shingles that have been tested to withstand significant hail events. The 

test standards typically cited are UL 2218 and/or FM 4473. UL 2218 is primarily intended for flexible roof 

covering materials while FM 4473 tests the impact resistance of rigid materials such as wood shakes, slate, 

and metal roofing. There are four classes of impact resistance with Class 4 having the highest performance. 

Recent IBHS research has shown that the most durable and hail resistant Class 4 asphalt shingles are those 
124 Durability by Design

made using polymer modified asphalt. In some states such as Texas, insurance premium discounts are 

available for roof coverings meeting this standard, which helps make these products more cost-effective. 

This category of roof covering material is also usually more wind-resistant as well. 

9.7 Recommended Practices—Wildfire Regions 

In recent years, property damage and loss of life due to wildfires has been significant and on the 

increase, primarily due to more people living in wildland-urban interface regions. The U.S. Forest Service 

reported that over 30% of the country’s housing units are located in these high risk areas. Over the past 

ten years especially, there has been a steady increase in wildfires exceeding 50,000 acres.30 Although the 

Wildland-Urban Interface Institute developed a map of the severity of fire risk across the country, 

serious fires have broken out in what was once considered low or moderate risk areas. As a result, the 

National Institute of Standards and Technology is developing something akin to the use of the Richter 

scale for earthquakes—but for wildfire—to better predict and characterize the risk and severity and 

more communities are adopting Wildland-Urban Interface Code provisions. 

9.7.1 Use Exterior Roof and Wall Claddings That are Non-combustible or Have a Minimum of a 1-hour Fire 

Rating. 

The majority of roof shingles manufactured today have a Class A fire rating—providing the greatest fire 

resistance in a fiberglass shingle. Metal roofs are non-combustible and provide an excellent option. Tile 

roofs are also non-combustible but care must be taken to ensure that gaps under tiles at eaves of barrel 

vault roofs are plugged with bird stops. 

Siding or cladding materials having a 1-hour fire rating include 

natural and synthetic stone, brick, concrete, stucco, metal, and 

fiber cement. 

9.7.2 Use Attic Venting Specifically Designed to Resist the Entry 

of Embers or Eliminate Attic Venting With an Unvented Attic. 

Do Not Install Any Vents or Openings in Foundation Wall. 

These measures prevent fire from entering combustible, 

unconditioned building spaces where it can quickly spread 

throughout the home. For example, eave venting of the attic 

can utilize boxed eaves with strip soffit vents located along the 

outer edge of the overhang. The strip vents are covered with 

non-combustible, corrosion resistant mesh having openings no 

larger than 1⁄4”. 

 

30 Wildfire, Wildlands, and People: Understanding and Preparing for Wildfire in the Wildland-Urban Interface, U.S. 

Forest Service, 2013. 

Resiliency and Disaster Resistance 

In the aftermath of recent disasters and 

prolonged power outages, housing 

resiliency has deservedly attracted 

significant attention. While many aspects 

of resiliency are beyond the scope of this 

guide (e.g. on-site power 

generation/storage, local food supply)—

designing a home to withstand natural 

hazards is fundamental to resiliency. Thus, 

the brief collection of best practices 

presented in this chapter also serve to 

make homes more resilient. 
Durability by Design 125

Other products or systems specifically designed to resist fire or ember entry will incorporate a finer mesh 

secondary screen (i.e., one that is set back in the vent device), and other design features on the exterior 

side. In wildfire prone regions, check with the local fire or building department for a list of approved 

products. 

An unvented attic design, where there are no attic vent openings to the outside, is another option (see 

Section 4.2.5 for additional discussion).

For foundation walls the same logic applies, and unvented foundation spaces like crawl spaces are 

recommended. The International Residential Code has recognized unvented crawl spaces for several years 

and includes provisions on this type of design. 

9.8 Additional Resources 

The following resources offer additional guidance and best practices regarding design and construction 

strategies that will offer superior protection from natural disasters and severe weather events. While 

nothing provides 100% assurance, incorporating measures beyond minimum code requirements will 

increase the likelihood that your homes will withstand significant events with minimized damage. Some of 

the resources cited below address multiple types of hazards; some are geared specifically to a particular 

hazard. 

The FORTIFIED HomeTM Program and the FORTIFIED for Safer LivingTM Program were developed by the 

Insurance Institute for Business and Home Safety (IBHS) to strengthen new and existing homes against 

various hazards that they may experience. In addition to specifying and describing the best practices for 

different hazard areas, it also provides a list of the most serious risks by state. Based upon location, the 

programs identify a set of mandatory measures that must be implemented in order to receive the 

designation. A trained and certified FORTIFIEDTM Evaluator provides third party verification that your 

project has been designed and constructed to meet the hazard mitigation standards appropriate for your 

area. The programs go a step further than the building codes via a more conservative mapping of regions 

that are prone to high winds, severe winter weather, thunderstorms, and hail. IBHS also has a number of 

regional guides for enhancing wildfire resistance that are available on the website. 

 FORTIFIED Program Overview 

 Wildlife Home Assesment and Checklist

This Homeland Security site provides a library of case studies and best practices by type of hazard. Use the 

Advanced Search tab to identify mitigation strategies for the hazard(s) you are most concerned about. 

https://www.llis.dhs.gov/bestpracticeslist

The Building Science Corporation has a wealth of practical information regarding best building practices, 

building science, and building for resiliency. This document is one of several that are useful when trying to 
126 Durability by Design

identify construction measures and details that will mitigate damage due to natural disasters. 

http://www.buildingscience.com/documents/digests/bsd-111-flood-and-hurricane-resistant-buildings

The Federal Emergency Management Association is typically one of the first federal government agency 

responders when natural disasters occur. Being a central player in recovery efforts, they have intimate 

familiarity with the types of damage that occur and best mitigation strategies to enable buildings to 

withstand natural hazards. The documents below pertain to best construction practices in coastal and high 

wind areas and areas prone to flooding and/or earthquakes. 

 FEMA Coastal Construction Manual

 FEMA Homebuilder’s Guide to Earthquake Resistant Design and Construction 

 FEMA Wind Retrofit Guide for Residential Buildings

 Protecting Your Home from Wind Damage

The National Institute for Standards and Testing is another government organization that provides 

numerous resources related to hazard mitigation. The above report covers flood resistant construction, 

but the site also has resources addressing other hazards as well. 

http://www.wbdg.org/resources/env_flood.php

This article provides a good overview of construction materials, methods, and details that made the 

difference between a home that survived a California wildfire and those that did not. 

http://www.finehomebuilding.com/how-to/articles/fire-resistant-details.aspx

This links to the Wildland-Urban Interface Standard, one of the body of international codes. It outlines 

codified measures for buildings located in regions susceptible to wildfire. 

http://www.codepublishing.com/wa/wenatchee/html/Wenatchee03/Wenatchee0336.html

"Wind and Trees: Lessons Learned from Hurricanes." University of Florida, The Institute of Food and 

Agricultural Sciences, September 2007. This document explains key issues and strategies for urban forest 

management, to mitigate the damage related to trees in extreme wind events. 

http://edis.ifas.ufl.edu/pdffiles/FR/FR17300.pdf 

 
Durability by Design 127

APPENDIX A—BUILDER/DESIGNER DURABILITY CHECKLIST 

The checklists on the following pages serve as a refresher for a number of key durability issues which 

must be addressed. One checklist covers the design phase (or pre-construction), while the other 

addresses the construction phase. The design phase checklist is organized by the durability driver and 

the construction checklist is organized primarily by common construction sequencing, and also by 

durability driver. The lists can be modified as appropriate. 

Homeowner-related durability resources are also critical and are highlighted in the last section of this 

checklist. Further, references for developing homeowner education materials on home maintenance are 

found in Section 2.2. 
128 Durability by Design

Designer’s and Builder’s Durability Checklist

Design Phase

Durability Driver

Responsible 

Party

(D = Designer; 

B= Builder)

Ground and Surface Water—Chapter 3

□ Have gutters been sized and specified? D

□ Have downspout size, location, and outlet point been detailed? D

□ Does site have adequate slope to remove roof run-off? D

□ Has adequate foundation backfill material been specified? D

□ Is grade compaction specifically included in construction documents / contracts? B

□ Are ground clearances between framing, siding, and ground properly maintained? B

□ Is foundation drain specified with proper aggregate and filter fabric? D

□ Are drainpipes located below the top surface of the basement slab? B

□

Is the foundation drainage system properly installed to provide positive flow of 

foundation water away from the building? B

□ Is foundation drain outlet specified— either through daylighting or sump pump? D

□ Is ground vapor barrier specified to be placed directly below the concrete slab? D

□ Is foundation wall damp proofing or waterproofing specified as required? D

□

Have foundation wall insulation materials been specified to limit air leakage and 

allow inward drying? D

Rain and Water Vapor—Chapter 4

□

Have adequate roof overhangs been specified, considering rain protection, wind, 

and shading? D

□ Does the roof have adequate slope for the roofing material being used? D

□ Has valley flashing been adequately detailed? D

□ Has step/kickoff flashing been specified and detailed? D

□ Have all roofing penetrations been adequately flashed and detailed? B

□ Has roof drip edge been specified? D

□ Has attic vent location and design been specified? D

□ Has a secondary drainage plane been specified? D

□ Has eave ice flashing been specified, if required? D

□ Are the drainage plane and flashings at windows and doors properly detailed? D

□ Have window head, jamb, and sill flashing details been specified? D

□ Have door head flashing details been specified? D

□ Has siding corner detail been specified? D
Durability by Design 129

□

Has air barrier detailing been specified, taking into account if the barrier is 

interior, exterior, or both? D

□

Has the thermal envelope design been reviewed to ensure sufficient water vapor 

management and the ability to dry? D

HVAC & Plumbing—Chapter 5

□

Do HVAC plans and specs include a deliberate strategy for indoor RH control, 

including accurate load and equipment sizing, accurate duct design, and specified 

exhaust ventilation?

D

□

Has whole-house ventilation been specified as necessary? If so, has the outside 

air flow been included in the load/equipment sizing? D

□ Has supplemental dehumidification been considered and specified as necessary? D

□ Have details been specified for plumbing located in exterior walls? D

Sunlight—Chapter 6

□ Has shading of the building been considered and planned? D

□

If reservoir cladding is used on exterior walls, have they been detailed to limit rain 

exposure and/or walls designed to manage inward vapor diffusion? D

□ Have UV resistant materials been specified for susceptible exterior components? D

Insects—Chapter 7

□ Are termite protection measures specified? D

Decay & Corrosion—Chapter 8

□

Is the minimum 8” clearance (or greater) to protect wood from ground moisture

clearly specified and integrated into plans? D

□

Is treated lumber adequately specified (and field verified) given the exposure and 

the application? D

Natural Hazards—Chapter 9

□

Have the location-specific natural hazards been evaluated, with above minimumcode details included in the design to enhance long-term durability and disaster 

resistance? 

B & D
130 Durability by Design

Communications & Education

□

Is the builder “taking credit” for their enhanced durability efforts in the form of 

third party labeling, at-a-glance communications products, etc.? See text box in 

Section 2.1.

□

Is the builder providing home buyers with clear maintenance-related educational 

materials and checklists? See Section 2.2 for resources.
Durability by Design 131

Designer’s and Builder’s Durability Checklist

Construction Phase

Sitework

Durability Driver Ch. Reference

□ Verify site has adequate slope to remove roof run-off. 3

□ Verify shading of the building has been considered and planned. 6

□ Proved termite protection measures when appropriate. 7

□

□

Foundation

Durability Driver Ch. Reference

□ Verify adequate foundation backfill material is provided. 3

□ Provide grade compaction as specified in construction documents. 3

□ Provide adequate ground clearances between framing/siding and soil. 3

□ Provide specified foundation drain with proper aggregate and filter fabric. 3

□ Verify drainpipes are located below the top surface of the basement slab. 3

□ Properly installed foundation drainage system to provide positive flow of 

foundation water away from the building. 3

□ Verify foundation drain outlet goes either through daylighting or sump pump. 3

□ Verify ground vapor barrier is placed directly below the concrete slab. 3

□ Verify foundation wall damp proofing or waterproofing is as specified. 3

□ Verify foundation wall insulation materials has installed as specified to limit air 

leakage and allow inward drying. 3

□ Provide minimum 8” clearance (or greater) to protect wood from ground 

moisture.

8

□

□

Framing

Durability Driver Ch. Reference

□ Verify attic vent locations as specified. 4

□ Verify secondary drainage plane is installed as specified. 4

□ Verify eave ice flashing in installed as specified. 4

□ Verify the drainage plane and flashings at windows and doors are installed as 

specified.

4

□ Verify window head, jamb, and sill flashing details are installed as specified. 4

□ Verify air barrier construction is installed as specified. 4

□ Verify building shading has been installed as specified. 6

□ Verify treated lumber is installed as specified. 8

□

□

Roof

Durability Driver Ch. Reference

□ Verify adequate roof overhangs are installed as specified. 4

□ Verify the roof has adequate slope for the roofing material being used. 4

□ Verify valley flashing has been installed as specified. 4
132 Durability by Design

□ Provide step/kickoff flashing as detailed. 4

□ Verify all roofing penetrations been adequately flashed. 4

□ Provide roof drip edge as specified. 4

□ Verify door head flashing details are installed as specified. 4

□

□

Rough-in

Durability Driver Ch. Reference

□ Verify siding corner detail is installed as specified. 4

□ Verify thermal envelope installation to ensure sufficient water vapor management 

and the ability to dry as specified. 4

□ Verify equipment for indoor RH control, equipment sizes, installation of duct 

design, and specified exhaust ventilation are as specified. 5

□ Verify whole-house ventilation installation if specified. If so, verify the outside air 

flow volume is as specified. 5

□ Verify supplemental dehumidification is installed as specified. 5

□ Verify construction details for plumbing located in exterior walls. 5

□

□

Finishes

Durability Driver Ch. Reference

□ When reservoir cladding is used on exterior walls, verify it is detailed to limit rain 

exposure and/or walls designed to manage inward vapor diffusion. 6

□ Provide UV resistant materials for susceptible exterior components as specified. 6

□

□

Landscaping

Durability Driver Ch. Reference

□ Provide gutters if sized and specified. 3

□ Verify downspout sizes, locations, and outlet point(s) as specified. 3

□

□

Miscellaneous

Durability Driver Ch. Reference

□ Insure the efforts for enhanced durability are recognized in form of third party 

labeling. See text box in Section 2.1. 2

□ Provide home buyers with clear maintenance-related educational materials and 

checklists. See Section 2.2 for resources. 2

□ Verify location-specific natural hazards been evaluated and above minimum-code 

details are included in the design to enhance long-term durability and disaster 

resistance.

9

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□
Durability

byDesign

U.S. Department of Housing and Urban Development | Offi ce of Policy Development and Research

A Professional’s Guide to Durable Home Design

U.S. Department of Housing and Urban Development

Office of Policy Development and Research

Washington, DC 20410-6000

October 2015