Moisture Management 

for High R-Value Walls

R. Lepage, C. Schumacher, and 

A. Lukachko

Building Science Corporation

November 2013
NOTICE

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iii

Moisture Management for High R-Value Walls

Prepared for:

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

R. Lepage, C. Schumacher, and A. Lukachko

Building Science Corporation

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40337-04

November 2013
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

1 Introduction........................................................................................................................................... 1

1.1 Problem Statement...............................................................................................................1

1.1.1 Definition and Classification of High R-Value Walls.............................................2

1.1.2 Sources of Moisture for High R-Value Walls..........................................................4

1.2 Past Building America Research .........................................................................................5

1.3 Research Questions..............................................................................................................7

2 Modeling Methods ................................................................................................................................ 8

2.1 Technical Approach.............................................................................................................8

2.1.1 Selection of High-R Walls.......................................................................................8

2.1.2 Hygrothermal Simulations.....................................................................................19

2.1.3 Metrics for Analysis...............................................................................................30

2.1.4 Climates, Locations, and Boundary Conditions ....................................................32

2.1.5 Building Enclosure Selection.................................................................................33

2.1.6 Simulation of Moisture Loads ...............................................................................33

3 Results and Discussion ..................................................................................................................... 36

3.1 Limits of Vapor Diffusion and Air Leakage......................................................................36

3.2 Indoor Relative Humidity and Air Leakage.......................................................................37

3.3 Interior Vapor Barriers.......................................................................................................38

3.4 Wall Performance ..............................................................................................................38

3.4.1 Advance Framed Walls With Exterior Insulation..................................................39

3.4.2 Hybrid Advance Framed Walls With 2 in. of Exterior Insulation, 2 in. of ccSPF, 

and 3.5 in. of Batt Insulation..................................................................................43

3.4.3 Advance Framed Walls With 7.25 in. of ccSPF Cavity Insulation .......................46

3.4.4 Double Stud Walls With CFI and Polyethylene Vapor Barrier.............................49

3.4.5 Hybrid-Insulated Double Stud Walls With ccSPF and CFI...................................54

3.4.6 Double Stud Wall With ocSPF Cavity Insulation..................................................58

3.4.7 Truss Wall With CFI Insulation and Polyethylene Vapor Barrier.........................62

3.4.8 Structural Insulated Panels.....................................................................................65

3.4.9 Concrete Block Wall With 3.5 in. of Exterior Insulated Finishing System...........68

3.5 Wall Summary ...................................................................................................................70

3.6 Cost Considerations...........................................................................................................71

4 Recommendations and Conclusions ............................................................................................... 74

4.1 Future Considerations........................................................................................................76

References................................................................................................................................................. 77

Appendix A: Representative Detail Drawings........................................................................................ 80

Appendix B: Inputs for Hygrothermal Simulations ............................................................................. 101
vi

List of Figures

Figure 1. Common approaches to high R-value enclosures.................................................................. 4

Figure 2. Wall 1: Exterior insulation on advance framed wall.............................................................. 11

Figure 3. Wall 2: Exterior insulation on hybrid advance framed wall.................................................. 12

Figure 4. Wall 3: Advance framed wall with ccSPF cavity insulation.................................................. 13

Figure 5. Wall 4: Double stud wall with fully installed cellulose fiber insulation (CFI)...................... 14

Figure 6. Wall 5: Hybrid-insulated double stud wall with 2-in. of ccSPF and CFI fill......................... 15

Figure 7. Wall 6: Double stud wall with 9.5 in. of ocSPF....................................................................... 16

Figure 8. Wall 7: Truss wall with 9.5 in. of CFI fill.................................................................................. 17

Figure 9. Wall 8: SIP wall.......................................................................................................................... 18

Figure 10. Wall 9: CMU wall with EIFS .................................................................................................... 19

Figure 11. Wall section 1: Advance framed wall with 4 in. of exterior insulation............................... 21

Figure 12. Wall section 2: Advance framed wall with 2 in. of exterior insulation, 2 in. of ccSPF, and 

3.5 in. of batt insulation ..................................................................................................................... 22

Figure 13. Wall section 3: Advance framed wall with 7.25 in. ccSPF cavity insulation..................... 23

Figure 14. Wall section 4: Double stud wall with 9.5 in. of CFI ............................................................ 24

Figure 15. Wall section 5: Double stud wall with 2 in. of ccSPF and 7.5 in. of CFI ............................ 25

Figure 16. Wall section 6: Double stud wall with 9.5 in. of ocSPF....................................................... 26

Figure 17. Wall section 7: Truss wall with 9.5 in. of CFI ....................................................................... 27

Figure 18. Wall section 8a: SIP wall with 11.25 in. of EPS.................................................................... 28

Figure 19. Wall section 8b: SIP joint ....................................................................................................... 29

Figure 20. Peak daily MCs for advance framed wall insulated walls in modeled locations.............. 40

Figure 21. Advance framed wall insulated walls construction moisture drying ................................ 41

Figure 22. Advance framed wall insulated walls peak MC with air leakage condensation ............... 42

Figure 23. Bulk water leak in advance framed walls with exterior insulation..................................... 43

Figure 24. Peak daily MC for hybrid advance framed wall insulated walls in modeled cities .......... 44

Figure 25. Hybrid advance framed wall insulated walls construction moisture drying .................... 45

Figure 26. Peak daily MC for hybrid advance framed wall insulated walls with air leakage in 

modeled cities..................................................................................................................................... 46

Figure 27. Peak daily MCs for advance framed wall insulated walls with ccSPF cavity insulation in 

modeled cities..................................................................................................................................... 47

Figure 28. Advance framed wall with ccSPF cavity insulation construction moisture drying ......... 48

Figure 29. Peak daily MCs for advanced frame wall insulated walls with ccSPF cavity insulation 

and air leakage in modeled cities ..................................................................................................... 49

Figure 30. Peak MCs for double stud walls in modeled cities.............................................................. 50

Figure 31. Double stud walls construction moisture drying ................................................................ 51

Figure 32. Peak MCs for double stud walls in modeled cities, with and without air leakage ........... 52

Figure 33. Bulk water leak in double stud walls with exterior insulation............................................ 53

Figure 34. Cellulose MC for bulk water leak in Chicago ....................................................................... 54

Figure 35. Peak MCs for hybrid double stud walls with ccSPF in modeled cities after 3 years....... 55

Figure 36. Hybrid-insulated double stud walls construction moisture drying................................... 56

Figure 37: Peak MC for double stud walls with ccSPF in modeled cities, with and without air 

leakage................................................................................................................................................. 57

Figure 38. Bulk water leak in hybrid double stud walls ........................................................................ 58

Figure 39. Peak MC for double stud walls with ocSPF in modeled cities ........................................... 59

Figure 40. Annual peak daily MCs for 40% and 20% indoor RH in Chicago ....................................... 59

Figure 41. Construction moisture in double stud walls with ocSPF ................................................... 60

Figure 42. Peak MCs for double stud walls with ocSPF in modeled cities, with and without air 

leakage................................................................................................................................................. 61

Figure 43. Bulk water leak in double stud walls with ocSPF................................................................ 62

Figure 44. Peak MCs for truss walls in modeled cities ......................................................................... 63

Figure 45. Truss walls construction moisture drying ........................................................................... 63

Figure 46. Peak MCs for truss walls with in modeled cities, with and without air leakage............... 64

Figure 47. Peak MCs for SIPs walls in modeled cities .......................................................................... 65
vii

Figure 48. SIP walls construction moisture drying ............................................................................... 66

Figure 49. Peak MCs for SIP walls in modeled cities, with and without air leakage.......................... 67

Figure 50. Yearly peak daily RHs at the GWB surface for CMU walls— baseline, air leakage, and air 

leakage and vinyl wallpaper .............................................................................................................. 69

Figure 51. CMU wall with 3.5 in. of EIFS construction moisture drying.............................................. 70

Unless otherwise noted, all figures were created by BSC.

List of Tables

Table 1. Current Recommended "Whole-Wall" Minimum R-Valuea Including Thermal Bridging ....... 3

Table 2. Wall Comparison Chart (Straube and Smegal, 2009a)............................................................ 10

Table 3. Biodegradative Organisms and Mechanisms.......................................................................... 30

Table 4. Simulated Cities and Heating Degree Day and Cooling Degree Day Data ........................... 32

Table 5. Summer and Winter Indoor Conditions.................................................................................... 32

Table 6. Boundary Conditions ................................................................................................................. 33

Table 7. Air Leakage Rate Conversions.................................................................................................. 34

Table 8. Peak Daily MCs for Exterior Insulated Wall Assembly, With Class I and III Vapor Barriers37

Table 9. Peak Daily MCs for Cavity Insulated Wall Assembly, With Class I and III Vapor Barriers.. 37

Table 10. Exterior Insulation Requirements and Alternates for the Use of Class III Vapor Retarders39

Table 11. Insulation Ratio for a Given Interior RH and Outdoor Temperature ................................... 40

Table 12. Advance Framed Wall With Exterior Insulation Drying Times To Reach 20% MC ............ 42

Table 13. Hybrid Advance Framed Wall Drying Times To Reach 20% MC.......................................... 45

Table 14. Advance Framed Wall With ccSPF Cavity Insulation Drying Times To Reach 20% MC ... 48

Table 15. Double Stud Walls Drying Times To Reach 20% MC............................................................ 51

Table 16. Hybrid-Insulated Double Stud Wall Drying Times To Reach 20% MC ................................ 56

Table 17. Double Stud Wall With ocSPF Drying Times To Reach 20% MC......................................... 60

Table 18. Truss Wall Drying Times To Reach 20% MC ......................................................................... 64

Table 19. Hybrid Advanced Frame Wall Drying Times To Reach 20% MC.......................................... 66

Table 20. Moisture Durability Risk for Proposed Wall Assemblies in Select Climate Zones, 

Including Air Leakage ........................................................................................................................ 71

Table 21. Total and Area Cost for Advance Framed High-R Walls1 ..................................................... 72

Table 22. Total and Area Cost for Double Stud High-R Walls1............................................................. 73

Unless otherwise noted, all tables were created by BSC.
viii

Definitions

ACH Air changes per hour

BSC Building Science Corporation

ccSPF Closed cell spray polyurethane foam

CFI Cellulose fiber insulation

CMU Concrete masonry unit

EIFS Exterior insulation and finish system

EPS Expanded polystyrene

GWB Gypsum wall board

IRC International Residential Code

MC Moisture content

o.c. On center

ocSPF Open cell spray polyurethane foam

OSB Oriented strand board

PIC Polyisocyanurate

RH Relative humidity

SIP Structural insulated panel

WRB Water resistive barrier

WUFI Wärme und Feuchte instationär

XPS Extruded polystyrene
ix

Executive Summary

In recent years, rising energy costs, energy security concerns, and social change have generated 

increased demand for better thermal performance. Building standards and construction codes 

have required higher minimum R-values. High-R assemblies, however, can be more susceptible 

to moisture problems. Different design considerations, construction techniques, and strategies are 

needed to ensure long-term service and durability of these assemblies.

The following report explains the moisture-related concerns for high R-value wall assemblies 

and discusses past Building America research work that informs this study. Hygrothermal 

simulations were prepared for several common approaches to high R-value wall construction in

six U.S. cities (Houston, Atlanta, Seattle, St. Louis, Chicago, and International Falls) 

representing a range of climate zones (2, 3, 4C, 4, 5A, and 7, respectively). The simulations are 

informed by experience gained from past research in this area and validated by field 

measurement and forensic experience.

The modeling program was developed to assess the moisture durability of the wall assemblies 

based on three primary sources of moisture: construction moisture, air leakage condensation, and 

bulk water leakage. The peak annual moisture content of the wood-based exterior sheathing was 

used to comparatively analyze the response to the moisture loads for each of the walls in each 

given city. Walls that experienced sheathing moisture contents between 20% and 28% were 

identified as risky, whereas those exceeding 28% were identified as very high risk.

All of the wall assemblies perform well under idealized conditions. However, only the walls with 

exterior insulation, or cavity insulation that provides a hygrothermal function similar to exterior 

insulation, perform adequately when exposed to moisture loads. Walls with only cavity 

insulation are particularly susceptible to air leakage condensation. None of the walls performed 

well when a precipitation-based bulk water leak was introduced to the backside of the sheathing, 

emphasizing the importance of proper flashing details.

This report is intended for designers and builders who are concerned about best practices for 

moisture management in high R-value walls, and for researchers who may need to assess other 

high R-value assemblies.
1

1 Introduction

1.1 Problem Statement

Rising costs of energy, concerns relating to climate change, and demands for increased comfort 

have led to the desire for increased insulation levels in many new buildings. However, increasing 

the insulation used in new construction may lead to increased problems in managing moisture. 

Depending on the insulation strategy, new construction techniques and strategies may need to be 

employed to ensure that external and internal moisture sources are properly handled, such that 

moisture-sensitive materials are protected and maintained at safe levels.

Reducing the heat flow across an enclosure (by increasing insulation levels) may decrease its 

durability relative to standard construction, depending on how that heat flow reduction is 

achieved. High R-value walls are no different. By adding insulation inside of wood sheathing or 

cladding, the moisture content (MC) of the sheathing or cladding will rise in cold weather, the 

risk of condensation increases significantly, and outward drying potential is reduced. Adding 

insulation also increases the risk of condensation in the summertime only if cooling is present, 

whether by natural heating, ventilation, and air conditioning (HVAC) systems or natural cooling, 

but in this circumstance on the exterior side of the interior finish, especially if the interior finish 

is vapor impermeable (vapor barriers, cabinets, mirrors, etc.). In short, pressure to increase 

energy efficiency has a potential “systems effect” on the moisture-related performance of new 

and existing housing, and this impact must be understood to mitigate unexpected and unintended 

performance and durability problems.

The risk of moisture damage depends on a number of factors, including climate (seasonal 

changes, orientation, exposure) and interior conditions (temperature, relative humidity [RH], 

pressurization), as well as particulars of the wall assembly such as cladding type, the presence or 

absence of a ventilation and drainage gap behind the cladding, the sheathing material, the type 

and location of insulation material, the vapor permeance of various layers (including vapor 

control layers and finishes), and the sensitivity of the assembly to workmanship errors, 

movement over time, and environmental changes. The range of factors involved makes 

understanding and predicting moisture-related performance a complicated activity.

A significant amount of laboratory and field research has been conducted to better understand the 

moisture performance of materials, subassemblies, and enclosure systems. A significant amount 

of research is still underway; however, research is increasingly conducted by the private sector 

and is not immediately made available to those who are designing and building. At the same 

time, insulation and airtightness standards continue to become more stringent while the number 

of available building materials and systems continues to increase. Designers and builders, faced 

with greater demands and more options, are now seeking more information and guidance from 

manufacturers, consultants, and standards organizations; however, real physical testing, analysis,

and reporting have not kept up with the industry demand for information and guidance.

For interim guidance, the fundamental physics of moisture properties and motion in building 

components and systems, complemented with empirical evidence and observations, can be

applied to infer the moisture-related problems in existing and proposed buildings. Sophisticated 

hygrothermal simulation tools have been developed. When the limitations of the simulations are 
2

understood, and the simulation results are calibrated against field and laboratory measurements,

these tools can extend our ability to make recommendations.

This paper builds on past research work and building science theory and uses hygrothermal 

simulations to examine the moisture sensitivity of select high-R wall systems to boundary 

conditions and design decisions. Section 1 provides the background for the study and presents 

the research questions addressed by the work. Section 2 explains how the range of potential 

factors was limited to significant cases and describes the approach to the hygrothermal 

simulations. Section 3 presents the results of the study and Section 4 provides recommendations,

including climate-specific guidance and drawings to describe appropriate construction 

assemblies. 

1.1.1 Definition and Classification of High R-Value Walls

The term high R-value enclosure attempts to bring together what is known about delivering 

exceptionally good control of heat flow through walls, roofs, windows, and foundations. High Rvalue enclosures are more than just assemblies with an increased amount of insulation. These 

enclosures are systems that are airtight, have little thermal bridging, manage solar heat gain, 

ensure human comfort, are buildable at production scale, and provide moisture control to ensure 

durability and health expectations are met.

There are no widely accepted definitions of the terms high R and high R-value, but they are

usually understood as providing higher thermal control than the building code mandates. For the 

purposes of this report, a high-R enclosure provides an effective R-value that meets or exceeds 

those listed in Table 1, but also meets high standards for buildability, durability, health, and 

comfort.

R-value is commonly used in reference to the thermal resistance of insulation products. 

However, this metric does not account for the impacts of thermal bridging, air leakage, 

installation quality, and thermal mass—i.e., it does not account for many of the factors that affect 

thermal performance in real-world structures. It is this multitude of factors that work together to 

deliver good thermal control. Oak Ridge National Laboratory has proposed “whole-wall Rvalue,” which is the R-value for the whole opaque wall including the thermal performance of not 

only the “clear wall” area, but also all typical envelope interface details. Although this does not 

account for all of the impacts listed above, it is a better indicator of performance.

In 2009, Building Science Corporation (BSC) was tasked with preparing a Building America 

white paper on high thermal performance enclosures (Straube, 2010). This paper defined 

performance requirements, reviewed past and current research, and outlined the research gaps in 

this area, including the need to demonstrate and document methods to achieve high levels of 

thermal performance and airtightness. Table 1 below provides BSC’s recommended “wholewall” minimum R-value for different enclosure components for each climate zone. The column 

highlighted in red shows the minimum “whole-wall” R-values that are used in this report as the 

current minimum standard for high R-value wall assemblies.
3

Table 1. Current Recommended "Whole-Wall" Minimum R-Valuea Including Thermal Bridging 

(Straube 2010)

Climate 

Zone Wall Vented Attic

Compact 

Roof

Basement 

Wall

Exposed 

Floor

Slab 

Edgeb

Windows

(U/SHGC)

Subslabc

1 10 40 35 5 10 none yes none

2 15 50 40 10 20 5 0.35/<.25 none

3 20 50 45 10 20 7.5 0.30/<.3 5

4 25 60 45 15 30 7.5 0.30/<.35 7.5

5 30 65 50 15 30 10 0.24/<.50 7.5

6 35 75 60 20 40 10 0.18/-- 10

7 40 90 65 25 45 15 0.15/-- 15

8 50 100 75 35 50 20 0.15/-- 20

a These are recommended values based on experience - see economics section

b Slab edge insulation includes all of stem wall or monolithic slab edge

c Full area coverage of slabs 

A broad classification of the approaches to high R-value enclosures for cold climate residential 

buildings was suggested in a recent Building America study (Lukachko et al. 2012). Figure 1

below describes two common approaches: adding insulation to the exterior of the building 

structure (i.e., the “exterior” approach), which may include insulation materials inside the 

structural cavity or none at all; and adding more insulation inside the structural cavity (i.e., the 

“inside” approach), which uses different types of insulation material and an increased width of 

the cavity to reach higher R-value levels. Walls are illustrated in Figure 1, but the classification 

could also apply to other enclosure components with a few modifications.

For each of the approaches in Figure 1, there are additional modifications depending on choice of 

insulation material and the thickness of the wall or of various layers in the assembly. Different 

enclosure assemblies will have different requirements to meet or exceed current durability 

expectations, but recommendations can be made at this level of classification. In general, drained 

and ventilated claddings, exterior insulating sheathing, and high airtightness combine to provide 

an enclosure that is more durable even when insulated to high levels. 
4

Exterior Approach Inside Approach

All Exterior Hybrid 

Insulation

Insulating 

Sheathing 

With Cavity 

Fill

2 × 8+ Double Stud or 

Truss

SIPs

Figure 1. Common approaches to high R-value enclosures

Notes for Figure 1:

1. The left-hand side is the “outside climate” and the right-hand side is the “inside climate.”

2. Dark gray is rigid insulation, medium gray is spray foam insulation, light gray is cavity fill insulation, and white 

is open structure.

1.1.2 Sources of Moisture for High R-Value Walls

Enclosure assemblies are subject to moisture loads from a number of sources including bulk 

water (introduced by leakage), built-in moisture, water vapor (introduced by vapor diffusion or 

air leakage), and capillary transport through materials in contact with water or in contact with the 

ground. Different approaches to high R-value construction are affected differently by each 

source. The moisture sources are described below.

1.1.2.1 Bulk Water

The largest potential moisture source in wall assemblies is bulk water leakage. Bulk water is 

introduced at the exterior of wall assemblies in the form of rainwater and meltwater (from ice 

and snow). The means and methods to prevent the bulk water penetration and moisture damage 

are well developed and understood. Roof overhangs and wall surface features prevent rainwater

from pooling or standing on the exterior surface. Flashings prevent bulk water penetration at 

interfaces, at openings (e.g., windows and hatches), and at service penetrations (plumbing and

electrical stacks, air intake and exhaust vents, etc.) (Lstiburek 2006). Exposure to bulk water can 

also be indirect: splash-back from hard surfaces at the base of the wall and surface runoff from 

grade or roof areas sloping toward the wall are common problems.

1.1.2.2 Built-in Moisture

Moisture is said to be “built-in” when damp or wet materials are enclosed in an assembly during 

construction. Built-in moisture can be introduced through the use of wet materials or through 

unprotected materials that are wet by rain or meltwater during construction. Builders in areas 
5

with high hours of annual rainfall are likely to have a high level of awareness of this issue and in 

some areas (the Pacific Norwest coast, for example), building regulations require spot 

measurements to verify that the MC of the wood framing is below critical levels before 

construction is allowed to be closed in.

1.1.2.3 Water Vapor

Another moisture source, water vapor, is often considered but not as well understood. Through 

the winter months in cold and mixed climates the indoor air can be a significant source of water 

vapor. Water vapor moves through and into the assembly by two mechanisms: vapor diffusion 

and airflow. Methods to control vapor diffusion and air movement are well documented (Latta 

1973; Hutcheon and Handegord 1985; Quirouette 1985; Straube and Burnett 2005) but are 

unfortunately rarely well executed. Airflow is capable of transporting hundreds of times more

moisture than vapor diffusion (Wilson 1961); hence, it is important to control airflow to prevent 

moisture problems and ensure the durability of the building enclosure.

1.1.2.4 Capillary Transport

Movement of moisture by capillary action occurs through the interconnected network of pores in 

a hygroscopic material or between two adjoining hydrophilic materials due to the attractive force 

of surface tension. Capillary transport through joints is significant only in gaps of less than about 

1⁄8 in. (3 mm) but can occur in a broad range of building materials such as concrete, clay brick 

masonry, and wood. Wall assemblies in direct contact with a concrete foundation can be at risk 

of wetting by this mechanism unless protected by a capillary break created by a nonporous or 

hydrophobic material.

Solutions exist to control these moisture sources and maximize assembly durability. Solutions 

include: use insulation exterior to any sheathing, use lower permeance insulating exterior 

sheathing, build a ventilation space outside of the sheathing or behind the cladding, build a more 

airtight enclosure, provide better rainwater management (e.g., drained subsill flashing), etc.

These solutions are considered and explained in Sections 3 and 4 of this report.

1.2 Past Building America Research

The increased risk for moisture damage in insulated wall assemblies is well understood by 

researchers (Rose 2005; Straube and Burnett 2005; Hutcheon and Handegord 1985), but it is not 

well understood by the code and building communities.

When the thermal resistance of a wall assembly is increased, wood-based sheathings and some 

sidings (particularly wood and fiber cement) are placed at a higher risk of moisture damage 

(Lstiburek 2010). Field experience with certain types of high-R enclosures (e.g., structural 

insulated panels (SIPs) and double stud walls and dense-pack roof assemblies) have shown that 

wetting due to small errors (for example, rain leaks or convective loops) can occur and, since 

drying is very slow (due to increased airtightness, decreased heat flux, and the introduction of 

vapor impermeable layers), high RH and moisture content (MC) persist for longer periods and 

there is a heightened risk of damage (Straube and Burnett, 2005).

In 2009 and 2010, BSC conducted a series of studies, each focusing on a different part of the 

building enclosure. These included reports for high R-value walls (Smegal and Straube 2009), 

high R-value foundations (Straube and Smegal 2010), and high R-value roofs (Straube and Grin 
6

2010). Each report looked at thermal control, but also moisture control, durability, buildability, 

cost, and material use, for common high R-value assembly designs. Analysis conducted for these 

reports sought to identify high R-value assemblies that were likely to be implemented at a 

production scale and that were also designed to minimize durability risks.

A study conducted by IBACOS in 2010 (Broniek et al. 2010) also evaluated different approaches 

to the construction of high R-value wall assemblies. This study included a comparison of 

simulation results using a whole-house energy model and collected experience with construction 

issues through consultation with builders and manufacturers, and through the construction of 

full-scale mockups. In addition to some of the same performance criteria examined in the BSC 

study above, IBACOS looked at architectural flexibility (i.e., the ability of the wall system to 

accommodate a wide range of floor plans and finishes) and scalability to mass production in 

multiple climate zones.

Field research projects have been conducted by Building America teams to assess different 

approaches to high R-value enclosures in number of climate zones. Some recent BSC examples 

include:

• The Westford Habitat for Humanity project in Westford, Massachusetts

• Research with Transformations, Inc. at three developments in Massachusetts 

• The NIST Net Zero Energy Lab House in Gaithersburg, Maryland

• The Neighborhood Stabilization Program 2 community in Wyandotte, Michigan.

The Westford Habitat project used a 4-in. layer of insulating sheathing outside of advance 

framed 2 × 6 walls and 4 in. of insulating sheathing over engineered wood rafters (i.e., the 

“exterior” insulating sheathing approach, see Figure 1). With the cavity fill insulation included, 

this prototype house has nominal R-44 walls, R-66 roof insulation, R-26 basement wall 

insulation, R-10 under the basement floor slab, and a whole-house airtightness of 1.5 ACH50. 

Important lessons were learned during the construction of wall and roof assemblies with 

thick layers of exterior insulation, including special details for window and door installation 

(Lstiburek 2009; BSC 2010a).

The Transformations, Inc. project involved three communities of houses that employ a 12-in.

thick double stud wall assembly to achieve a high R-value enclosure (i.e., the “inside” double 

stud approach, see Figure 1). The double stud approach is favored by some builders because it 

allows for the use of low-cost cavity fill insulation materials (instead of more expensive board 

foam and spray foam insulation materials). Double stud walls, however, are at a higher risk of 

moisture-related problems than walls constructed using the insulating sheathing (exterior)

approach. The moisture risks associated with this approach have been documented as part of the

high R-value Wall study (Straube and Smegal, 2009a) and the Transformations project continues 

to assess this issue with long-term field measurement of a side-by-side comparison between fullcavity ocSPF and full-cavity netted and dry blown-in cellulose (BSC 2010b; Ueno et al. 2012).

The NIST Net Zero Energy Lab House was designed primarily to test mechanical and renewable 

energy systems inside an ultra-low load enclosure. The enclosure was designed using current 

best practices for thermal control and airtightness. The walls and roof assemblies were fully clad 
7

in oriented strand board (OSB) sheathing to support a continuous self-adhered membrane. This 

membrane was detailed as the air barrier system, which was continuous over the roof/wall 

interface and integrated with windows and other enclosure penetrations. Insulating sheathing was 

added over this membrane: 4 in. of polyisocyanurate (PIC) (R-26) for the walls and 6 in. of PIC 

(R-39) for the roof (i.e., the “exterior” insulating sheathing approach). The final airtightness 

test result for this assembly was 0.61 ACH50. An extensive set of detail drawings was prepared 

for this project and construction and quality control processes were documented (Lukachko

et al. 2011).

In Wyandotte, 18 houses were constructed using a hybrid insulation approach consisting of 2 in.

of extruded polystyrene (XPS) insulating sheathing (R-10), with 2 in. of closed cell spray 

polyurethane foam (ccSPF) sprayed to the interior of the insulating sheathing (R-12), fiberglass 

batt insulation was used to fill the balance of the 2 × 6 wood stud cavity (R-12). There were two 

primary outcomes from this research. First, the airtightness measurements demonstrated that 

builders having little previous experience with energy-efficient construction techniques were

able to achieve consistent results that are < 1.5 ACH50. Second, the process changes 

implemented to help secure these results were straightforward and ended up encouraging better 

communication between designer, builder, and the officials supervising the project (Lukachko

et al. 2012).

1.3 Research Questions

Researchers and builders have gained experience with the detailing and construction of high Rvalue wall assemblies. Past work described in the section above has identified the moisture risks 

for high R-value walls; however, more work is needed to quantify the risk. Furthermore, a 

number of variables affect the risk (climate, cladding type, insulation type and location, etc.) and 

more information is needed to assess the impact of each of these. Finally, with the continued 

introduction of new materials, changing indoor environmental conditions, enhanced expectations 

from the occupants, and higher performance standards, there are a myriad of factors that need to 

be considered before moisture guidelines can be developed for high R-value wall assemblies. To 

further develop our understanding of these factors, the following research questions are

addressed in this report.

1. What is the role of insulation levels on the risk in different climates?

2. How resistant are the walls to air leakage and vapor diffusion in different climates?

3. What are the drying rate capacities of the proposed wall assemblies?

4. What are the high-level steps necessary to build moisture-resistant high-R wall 

assemblies?

To answer these questions, hygrothermal simulations were prepared to assess the performance of 

representative high R-value walls in a range of climate zones. The study was structured to assess 

the sensitivity and response to different factors. The simulations were calibrated against field 

experience with and laboratory research on similar high-R wall assemblies. The approach to this 

work is described in Section 2 below.
8

2 Modeling Methods

2.1 Technical Approach

In this project, hygrothermal modeling tools, field experience, and building science theory were

used to address the research questions. These research questions can be divided in to two main 

sections: (1) what are the limits for water vapor diffusion and air leakage in select climates, and 

(2) what are the moisture performance characteristics of the proposed high-R wall assemblies in 

a range of climates? Each section is considered separately from a modeling perspective.

To assess the limits of water vapor diffusion and air leakage in wall assemblies, a parametric 

study was devised. The study compares the response of two wall assemblies, one that is 

particularly sensitive to moisture loads and another that is more tolerant based on experience, to

bound the limits of the problem. These walls were simulated in a range of climates (very cold, 

cold-humid, hot-humid, and hot-dry) and subjected to differing levels of interior RH and air 

leakage rates. Certain climate zones required specialized treatment for vapor or thermal control

to maintain code compliance. In all cases, the walls were created to comply with the 2012 

International Residential Code (IRC).

To assess the moisture performance characteristics of the proposed high-R walls, a comparative 

modeling approach was used. Select cities, representative of a range of climate zones, were 

chosen to expose the proposed walls to a range of environmental conditions. A baseline 

simulation was then conducted to better compare the walls with added moisture loads. The 

proposed walls were then subjected to a series of moisture loadings from three major sources of 

moisture: construction moisture, air leakage condensation, and bulk water leakage. The degree of 

moisture loading was based primarily on experience, but also refers to published literature (i.e.,

ASHRAE 160P-09) (ASHRAE 2009). The results were recorded and analyzed.

The reader is cautioned that the research contained within this report is based largely on 

simulations and has not been verified empirically. It is based on assumptions derived from 

significant field experience and published literature and the results were compared with known 

performances of high-R buildings. However, to validate the models, empirical research on smalland full-scale assembly mockups and buildings, with monitored boundary conditions and 

instrumented with temperature and moisture sensors, needs to be undertaken to examine and 

fully quantify these risks—this will be identified in the conclusions as a future research need. 

2.1.1 Selection of High-R Walls

High-R walls, for the purpose of this report, are defined as walls that exhibit an effective R-value 

greater than double the code required thermal resistance of the wall assembly. In general, they 

provide an R-value that reaches, at a minimum, the recommended R-value in Table 1. An 

effective R-value is different than the clear wall R-value, which accounts for only a onedimensional section through the wall and does not consider the effects of thermal bridging. 

Consequently, the effective R-value better approximates the actual wall thermal performance by 

accounting for the effects of thermal bridging through wood studs, plates, joists, and other 

framing members.

A total of nine walls were considered for this report, of which five derive from the high-R walls 

report, discussed in greater detail below. It should be noted that some of these walls do not 
9

strictly comply with the high-R definition for cold climates. However, in warmer climates (i.e.,

climate zones 1 through 3), these would qualify as the effective R-value is greater than double 

the code requirement.

The five most common, high performance walls were selected based on five criteria: thermal 

control, durability, buildability, cost, and material use. Table 2, adapted from Straube and 

Smegal (2009a), shows the performance of the various walls to these selected criteria. The 

weight for each criterion was set to 1.0 for the purposes of this report. However, depending on 

the needs of the user, the weights can be modified to more accurately reflect their needs. To do 

this, a user would apply a weighting of > 1.0 if the criteria was deemed to be more important 

(e.g., optimized for thermal control) or < 1.0 if the criterion was deemed to be less important 

(e.g., a custom house might lower the importance of first cost in favor of performance). The 

other four wall types were not rated in the high-R report and are thus not scored.
10

Table 2. Wall Comparison Chart (Straube and Smegal, 2009a)

Thermal 

Control Durability Buildability Cost Material Use Total

Criteria Weighting 1 1 1 1 1

Wall 1: Advance Framed Wall With 4 in. of Exterior 

Insulating Sheathing 4 4 4 4 4 20

Wall 2: Hybrid Advance Framed Wall With 2 in. of 

Exterior Insulation, and 2-in. of ccSPF and 3.5 in. of 

Fiberglass Batt

– – – – – –

Wall 3: Advance Framed Wall With 7.25 in. of ccSPF – – – – – –

Wall 4: Double Stud Wall With 9.5 in. of Cellulose 4 3 3 3 2 15

Wall 5: A Hybrid-Insulated Double Stud Wall With 2 in.

of ccSPF and Cellulose 5 4 3 3 3 18

Wall 6: Double Stud Wall With 9.5 in. of ocSPFa – – – – – –

Wall 7: Truss Wall With 9.5 in. of Cellulose 4 3 2 3 3 15

Wall 8: SIPs With 11.5 in. of EPSb Insulation 4 4 3 3 3 17

Wall 9: CMUc Wall With 2 in. of EIFSd – – – – – – a Open cell spray polyurethane foam

b Expanded polyurethane

c Concrete masonry unit

d Exterior insulation finishing system
11

Four main wall types were analyzed in this report: an advance framed wall insulated on the 

exterior, a double stud wall with different cavity insulations, an SIP wall with 11.5 in. of EPS 

insulation, and a CMU wall with EIFS insulation. These wall types come in several variations, 

such as different insulation thicknesses, types of insulation, and the location of vapor and air 

control layers through the wall assembly. Each section will discuss any wall type variations that 

were made to ensure superior moisture performance. Sample sectional cutaways of the four wall 

types may be found below, in Figure 2 to Figure 10.

Figure 2. Wall 1: Exterior insulation on advance framed wall

(Straube and Smegal 2009a)
12

Figure 3. Wall 2: Exterior insulation on hybrid advance framed wall

(Straube and Smegal 2009a)
13

Figure 4. Wall 3: Advance framed wall with ccSPF cavity insulation

(Straube and Smegal 2009a)
14

Figure 5. Wall 4: Double stud wall with fully installed cellulose fiber insulation (CFI)

(Straube and Smegal 2009a)
15

Figure 6. Wall 5: Hybrid-insulated double stud wall with 2-in. of ccSPF and CFI fill

(Straube and Smegal 2009a)
16

Figure 7. Wall 6: Double stud wall with 9.5 in. of ocSPF

(Straube and Smegal 2009a)
17

Figure 8. Wall 7: Truss wall with 9.5 in. of CFI fill

(Straube and Smegal 2009a)
18

Figure 9. Wall 8: SIP wall

(Straube and Smegal 2009a)
19

Figure 10. Wall 9: CMU wall with EIFS

As several of the criteria reference buildability, cost, and material use, extremely high R-value 

products, such vacuum insulated panels, or aerogels, were not considered. These materials are 

considered to be cost prohibitive for most projects, not accessible to typical developers and 

builders, and beyond the scope of this work. It should be noted that the walls examined in this 

report will have some similarities with walls constructed with these more expensive materials.

2.1.2 Hygrothermal Simulations

Computer simulations were performed using WUFI Pro 5.1 to evaluate the thermal control 

and moisture durability (i.e. the hygrothermal performance) of the nine wall assemblies. WUFI 

Pro 5.1 was developed by the Fraunhofer Institute for Building Physics and Oak Ridge National 

Laboratory. 

Current moisture flow theory has difficulty in properly accommodating for the inhomogeneity, 

transient temperature and moisture characteristics, and anisotropic properties of building 

materials. Some programs attempt to capture fundamental physics at the microscopic level. This 

approach can lead to highly accurate models for very specific situations; however, the models do 

not cover an adequate portion of the range of problems encountered in applied building science.

Furthermore, such models require a significant number of detailed material properties and 
20

characteristics, and considerable effort and time are required to make the necessary 

measurements. Frequently, this information is often not available for the materials in question.

WUFI is somewhat unusual in that its underlying equations and algorithms are based upon 

macroscopic empirical behavior of organic and inorganic materials (Künzel 2002). This 

precludes the detailed testing required to generate topological material properties (e.g., pore size 

distribution, frequency of checks and cracks, etc.). The accuracy of the WUFI simulations has

been verified by the Fraunhofer Institut für Bauphysik in Holzkirchen, Germany, against 

numerous full-scale field studies of enclosures over a number of years. 

WUFI possesses the capacity to properly account for water vapor adsorption and the absorption

and redistribution of liquid water. The simulation is run for a given period, with the most 

common time step being 1 hour, considering the effects of sun, rain, temperature, and humidity. 

The quality of the results is extremely dependent on the quality and accuracy of the input 

material and condition data.

Sample cross-sectional images of the nine proposed high R-value wall types may be found in 

Figure 11 through Figure 19. Details of the boundary conditions (i.e., surface transfer films, 

including vapor diffusion resistance of paints), may be found in Section 2.1.4.
21

Figure 11. Wall section 1: Advance framed wall with 4 in. of exterior insulation

Water Leak

Air Leak
22

Figure 12. Wall section 2: Advance framed wall with 2 in. of exterior insulation, 

2 in. of ccSPF, and 3.5 in. of batt insulation

Water Leak

Air Leak
23

Figure 13. Wall section 3: Advance framed wall with 7.25 in. ccSPF cavity insulation

Water Leak

Air Leak
24

Figure 14. Wall section 4: Double stud wall with 9.5 in. of CFI

Water Leak

Air Leak
25

Figure 15. Wall section 5: Double stud wall with 2 in. of ccSPF and 7.5 in. of CFI

Water Leak

Air Leak
26

Figure 16. Wall section 6: Double stud wall with 9.5 in. of ocSPF

Water Leak

Air Leak
27

Figure 17. Wall section 7: Truss wall with 9.5 in. of CFI

Water Leak

Air Leak
28

Figure 18. Wall section 8a: SIP wall with 11.25 in. of EPS

Water Leak
29

Figure 19. Wall section 8b: SIP joint

Some parts of the wall construction are highly three-dimensional and, as a result, some aspects of 

hygrothermal performance can be difficult to model using a one-dimensional hygrothermal 

simulation tool. The modeling of air leakage is one such example. In a SIP wall, the air leakage 

will only occur around the joints. Hence, the wall section used for air leakage condensation in the 

SIP is based around the joint between these two panels. Other issues include correctly identifying 

the amount of moisture deposited by air leakage or through bulk water leakage. The values used 

in the simulations are based on estimates. 

Air Leak
30

2.1.3 Metrics for Analysis

There are no clear models for assessing the moisture durability of building materials. The 

moisture content to decay mechanism response is not well categorized and defined. Further, a 

range of decay mechanisms may occur. Table 3 summarizes the types of organisms, the damage 

type, the requirements for humidity or moisture, and the range of temperature over which these 

organisms act.

Table 3. Biodegradative Organisms and Mechanisms

(Viitanen and Salonvaara, 2001; Viitanan et al, 2003)

Type of 

Organism Damage/Problem Type Humidity or Moisture Range(RH or MC%)

Temperature 

Range

(°F [°C]) 

Bacteria

Biocorrosion of many 

different materials, smell, 

and health problems

Wet materials

RH > 97%

~23–140

(–5 to +60)

Mold Fungi

Surface growth on 

different materials, smell 

and health problems

Ambient RH > 75%,

Depends on duration, 

temperature, and mold species

~ 0-122

(0 to +50)

Blue-Stain 

Fungi

Blue-stain of wood and

permeability change of 

wood

Wood MC > 25%–0%

RH > 95%

~ 23–113

(–5 to +45)

Decay Fungi

Different types of decay in 

wood (soft rot, brown rot,

or white rot), also many 

other materials can be 

deteriorated. Strength loss 

of material.

Ambient RH > 95%,

MC > 25%–120%

depends on duration, 

temperature, fungus species, 

and materials

~ 32–113

(0 to +45)

Algae and 

Lichen

Surface growth of 

different materials on 

outside or weathered 

material

Wet materials, also nitrogen 

and low pH are required

~ 32-113

(0 to +45)

Insects

Different type of damage in 

organic materials, surface 

failures or strength loss

Ambient RH > 65%, depends on 

duration, temperature, species, 

and environment

~ 41–122

(5 to +50)

Due to high-level media attention from sick building syndrome and black mold, molds are 

frequently the primary concern of moisture durability for occupants. The difficulty with 

assessing moisture risk from mold sources is a range of factors that fundamentally affect the 

predisposition for its life cycle. These factors include the temperature and water activity (RH or 

MC) of the substrate, the presence and types of mold spores, the substrate quality (nutritional 

content, pH value, presence of biocides), and the duration and history of all the above factors. 

Molds are either allergenic (i.e., causing allergic reactions in the occupants from mold spores), or 

toxigenic (i.e., toxic substances created by the metabolic activity of the mold) (Sedlbauer 2004).

In general, toxigenic molds, such as Stachybotrys chartarum (i.e., black mold), require elevated 

MCs (i.e., in excess of 95% RH) just to start germination, much less growth (Ayerst 1968). 
31

Allergenic molds, on the other hand, are capable of starting germination at much lower RHs (i.e.,

70%–80%), although time to onset of germination and growth is frequently in excess of 21–36

weeks under ideal conditions (Wang and Morris 2012).

Many models exist to try to quantify moisture durability risk, such as those by Sedlbauer 

(biohygrothermal method, or WUFI BIO), Viitanen (the VTT biodeterioration model), the 

Institute for Research in Construction- National Research Council of Canada (the RHT index), 

and the ASHRAE 160 criterion; however, each is limited by its fundamental assumptions. 

Currently, none of the models are able to categorically account for transient or seasonal 

fluctuations to the fundamental properties required for mold growth. Further, extreme conditions 

may negatively affect mold health, either causing cessation of growth, recession, or even death 

of the fungi. 

It has been shown that short periods of time at high humidity conditions will not lead to fungal 

growth if the periods at low humidity preventing mold growths are sufficiently long (Viitanen 

and Bjurman 1995). If the period of high RH is longer than 24 hours, the cumulative effect of 

mold growth is more pronounced. However, a very low or negligible growth response is 

expected if the dry periods between wetting events are prolonged (Viitanen and Bjurman 1995).

Due to the limitations of the models, a simpler and more transparent approach is used. To 

compare the moisture durability of the wall assemblies, the MC of a thin slice (1⁄8 in. [3 mm]) of 

the structural sheathing on the interior and exterior faces is obtained from the simulations on an 

hourly basis. OSB sheathing moisture is used as the performance criteria because this is 

generally where moisture will accumulate and the wood sheathing is a moisture-susceptible 

material. The peak daily OSB sheathing MC was determined and the risk was assessed based on 

the following criteria:

• Peak OSB sheathing MC < 20%, no mold growth; very little risk

• Peak OSB sheathing MC 20%–28%; potential for mold growth eventually, depending on 

frequency and length of wetting, and temperatures during wetting. This design can be 

successful, but conservative durability assessments usually require corrective action

• Peak OSB sheathing MC > 28%; moisture-related problems are expected and this design 

is not recommended.

A thin slice moderates the surface MC such that the averaging function of the software does not 

artificially reduce the actual MC with the drier core. The MC and temperature are recorded and 

analyzed.

Predicted wood MCs of OSB are generally assessed with respect to relative risk as opposed to 

being judged on a pass/fail criterion. The predicted MC should be kept in context and good 

engineering judgment is required to determine the moisture risk to the sheathing. For example, 

elevated wood MCs in the cold winter months when the wood substrate is on the cold side of the 

assembly are much safer, from a mold growth perspective, than similar MCs in the summer, 

when mold will grow more quickly.
32

2.1.4 Climates, Locations, and Boundary Conditions

Different climate zones present a range of exterior conditions that can greatly affect the wall 

assembly. To encompass the range of environmental conditions that buildings may experience 

throughout the United States, select cities, representing a range of climate zones, as seen in Table 

4, were selected. The cities were selected based on a combination of city and population size, 

climate, and availability of climate data. The embedded weather files in WUFI were used for this 

modeling and, as a result, the climate data do not include extreme values usually associated with 

major disasters such as hurricanes or 1/100-year temperatures.

Table 4. Simulated Cities and Heating Degree Day and Cooling Degree Day Data

(ASHRAE 2009)

City Climate 

Zone

Description

Heating 

Degree Day

65°F (18.3°C)

Cooling 

Degree Day

65°F (18.3°C)

Coldest

3-Month 

Average 

Temperature

°F (°C)

International 

Falls 7 Very cold, dry 10,487 (5826) 248 (138) 1.0 (–17.2)

Chicago 5A Cold, moist 6,311 (3506) 842 (468) 20.3 (–6.5)

St. Louis 4 Temperate 4,504 (2502) 1,631 (906) 23.9 (–4.5)

Seattle 4C Maritime 

Temperate 4,729 (2627) 1,76 (98) 39.7 (4.3)

Atlanta 3 Temperate 2,990 (1661) 1,667 (926) 38.7 (3.7)

Houston 2 Hot, moist, 

coastal 130 (72) 4,459 (2477) 65.7 (18.7)

The interior temperature and RH utilized in the modeling are also included in Table 5. The 

temperature and RH vary sinusoidally, according to the peak values for the summer and 

winter seasons, as seen in Table 5. These values were selected based on BSC experience and 

recorded data.

Table 5. Summer and Winter Indoor Conditions

City Summer Conditions Winter Conditions

Temperature

°F (°C)

RH

(%)

Temperature

°F (°C)

RH

(%)

International Falls 70 (22) 60 68 (20) 30

Chicago 70 (22) 60 68 (20) 40

St. Louis 70 (22) 60 68 (20) 50

Seattle 70 (22) 60 68 (20) 60

Atlanta 70 (22) 60 68(20) 55

Houston 70 (22) 60 68 (20) 60

In this project higher indoor RHs are utilized to establish an anticipated “average upper limit” to 

what a high performance, airtight building would likely experience. Should the walls be able to 

accommodate such high moisture loading, the building enclosure will be able to safely handle 
33

any indoor RH below the designated RH. The analysis therefore should result in moisturetolerant high-R walls.

The boundary conditions were identical for each of the wall assemblies studied (see Table 6). 

Table 6. Boundary Conditions

Model Properties Value

Heat Resistance 0.33 (ft2

·°F·h/Btu)

Sd-Value None

Short-Wave Radiation Absorptivity 0.8

Long-Wave Radiation Emissivity 0.9

Effective Adhering Rain-Fraction 0.45

Interior Heat Resistance 0.71 (ft2

·°F·h/Btu)

Interior Sd-Value 1 (ft)

Vapor Permeance of the WRB 52 (U.S. perms)

The boundary condition values were selected based on standard published values (Straube and 

Burnett 2005). The interior Sd-value, or a linear vapor diffusion resistance factor, is equivalent to 

two layers of latex paint over a primer. Additional material properties used in the hygrothermal 

models may be found in Appendix B.

2.1.5 Building Enclosure Selection

A brief parametric study was undertaken to assess the effects of the cladding and ventilation 

rates of the drainage space. Water storing claddings place wall assemblies at greater risk, due to 

increase flows of moisture due to elevated vapor pressures. Similarly, very low or high 

ventilation rates of the drainage space immediately behind the cladding will mediate the amount 

of moisture to which the walls are exposed. All wall assemblies were assumed to use a spunbonded polyolefin water resistive barrier (WRB), due to the pervasiveness of its use within the

residential building industry. This WRB typically possess very high vapor permeance values, 

which enable outward drying, but may also result in large inward vapor flows. Interior vapor 

controls were not directly investigated, as these fall under the purview of the vapor diffusion and 

air leakage limits analysis.

For the purposes of the report, all wall assemblies used a fiber cement cladding, with ACH20 air 

space ventilation in a 1⁄2-in. air gap. The results of these simulations should not be extended to 

moisture storing claddings (i.e., brick or direct applied stucco), as these cladding types created a 

significant moisture load on the wall assembly.

2.1.6 Simulation of Moisture Loads

Four sets of hygrothermal simulations were prepared to assess the moisture risk associated with 

each wall assembly. The four simulation sets are discussed in Sections 2.1.6.1 to 2.1.6.4.

2.1.6.1 Base-Case Simulations

To determine optimal approaches to moisture management, first a base case model must be 

established. The base case comprises the wall to be tested with no additional modifications or 

moisture loads, aside from the established boundary conditions.
34 

2.1.6.2 Simulated Construction Moisture

Construction moisture is another source of moisture that can pose moisture problems to the 

building enclosure. Enclosures that are vapor tight may sometimes retain and hold moisture for 

prolonged periods of time. Usually, the moisture diffuses out of the wall over time, but certain 

wall assemblies dry out very slowly. The wood components of the wall assembly start the 

simulation at the fiber saturation point (28% MC) (FPL 1999). The drying starts on October 1. 

This creates a worst-case scenario, as the MC in wall assemblies starts to increase in the autumn 

and is unable to dry out until spring. If the walls are unable to reach an MC < 20% after several 

months, there is a good probability that mold growth would occur (FPL 1999; Sedlbauer 2004). 

2.1.6.3 Interior Air Leak

Interior air leaks can pose a serious problem to highly insulated walls. The use of thick layers of 

insulation, frequently with low vapor permeance, cannot only result in colder interstitial 

temperatures, but may also inhibit drying. 

A simulated air leak was modeled in all the proposed wall assemblies. The rate of the air leak 

was first obtained by collecting a range of air changes per hour from blower door tests of high 

performance homes. Three air leakage rates were selected to depict three levels of common 

performance: 0.5 ACH50, 1.5 ACH50, and 2.5 ACH50. These represent a range of air leakage 

rates for air tight, high performance homes. Based on the 2010 U.S. Census, the average home 

size was 2,392 ft2

. Dickerhoff et al. (1982) and Harrje (1982) have shown that approximately 

35% of all residential building air leakage occurs through the wall. By factoring the amount of 

wall component of the building air leakage, and assuming a surface area for the average home 

size (approximately 4,000 ft2), an air leakage rate per unit area at 50 Pascals of pressure of wall 

was obtained. To reduce air leakage at the standardized test pressure (50 Pa) down to natural 

levels (typically assumed to be 4 Pa), the Sherman-Grimsrud method was used (Sherman and

Grimsrud 1980), utilizing standard coefficients. The results are summarized in Table 7. 

Table 7. Air Leakage Rate Conversions: Whole Building Surface Area Values

Air Leakage Rate Results Air Leakage Rate

(cfm·ft-2 @ 50 Pa)

Air Leakage Rate

(cfm·ft-2 @ 4 Pa)

0.5 ACH50 0.041 0.008

1.5 ACH50 0.123 0.024

2.5 ACH50 0.205 0.040

These air leakages rates were used to bound the problem of air leakage limits for high-R, high 

performance walls. With regards to the wall comparisons, the upper air leakage rate (i.e., 2.5 

ACH50) was used to demonstrate the intrinsic resistance each wall possesses to incident air 

leakage. 

To simulate an air leak in WUFI, first, a small air layer must be created within the model. The 

location of this layer can significantly affect the results of the simulation, and thus experience in 

identification of air leakage paths through the wall assembly is required. Typically, the air 

leakage layer was located at the first upstream condensing plane with regards to the direction of 

the airflow, whether the air is exfiltrating or infiltrating. In cold climates, exfiltration is the 

primary concern with regards to air leakage condensation. Warm and humid interior air can leak 

through the wall assembly and upon contacting cold interstitial surfaces, condensation of the 
35

water vapor can occur. Contrarily, infiltration poses the greatest risk in hot climates, as moist 

exterior air can contact the cold surface of the gypsum wall board (GWB) in air-conditioned 

buildings—although the temperature differences over the assembly are typically much less than 

in cold climates. The air leakage layer was also created sufficiently small that it would not 

noticeably affect the effective R-value of the assembly. The unit area air leakage rate at natural 

pressures was converted into an equivalent air leakage rate that could be computed by the 

program. The MCs of the OSB sheathing in each simulation were collected and analyzed.

2.1.6.4 Simulated Bulk Water Penetration

A serious source of moisture is leaks caused by wall penetrations, such as windows, doors, and 

other utilities. To simulate a window leak, 2% of the wind-driven rain was assumed to bypass the 

wall assembly and deposit itself on to the inside surface of the OSB sheathing. This simulates 

rain striking the window and draining down into a small hole into the frame. While ASHRAE 

160 recommends using 1% of the wind-driven rain to represent a leak, the impermeability of a 

window increases the effective catchment area. Unfortunately, little research is available to 

quantify the amount or rates of water penetration in various types of leaks (roof, window, 

mechanical penetration, etc.).

A brief sensitivity analysis was conducted to compare the results of 1%, 2%, and 5% leaks. The 

results suggest that a 1% leak is insufficient to create the severe and localized damage that is 

observed in forensic investigations. However, at leakage rates of 2% or more, it was found that 

the MCs are appreciably high to recreate the observed levels of biodeterioration. Further, a value 

of 2% has been used by others (Kramer and Karagiozis 2004). However, after letting the 

simulations reach hygric equilibrium at a 2% leak (i.e., from reaching hygric equilibrium for the 

entire wall assembly), it was found that it closely approximated the results of using a 5% leak for 

a much shorter simulation interval. To reduce computation requirements, a 5% leak was used.

Should elevated moisture levels be present for a long period of time, it is possible that molds 

may establish themselves into the material. Thereafter, every wetting will provide additional 

moisture for mold growth. Consequently, the drying rate of the walls is important to ensuring 

safe functionality of the building enclosure.
36

3 Results and Discussion

In analyzing the results of the simulations, it was found that walls were subject to a much lower 

moisture risk in warmer climates (i.e., zones 1–3) than in colder climates (i.e., zones 4–8). For 

warmer climates, the predicted drying rates for construction moisture were measured in weeks, 

as opposed to months, and air leakage was not predicted to result in moisture problems in any of 

the walls, either in exfiltration or infiltration, provided that no Class I or II interior vapor barriers 

are present. In general, warmer climates allow for greater drying potentials in wall assemblies 

and hence, the assemblies are more likely to be able to manage adverse moisture loadings. The 

results are divided in to two sections, the first treating the limits of vapor diffusion and air 

leakage in two wall types in select climates, and the second dealing with the moisture 

performance of all the proposed high-R walls to different moisture loads in a range of climates.

3.1 Limits of Vapor Diffusion and Air Leakage

To assess the limits of vapor diffusion and air leakage in wall assemblies, two wall assemblies, 

from the list of high-R walls considered in this report, were selected to present moisture-sensitive 

and moisture-tolerant wall assemblies. This comparison is useful to bound the limits of possible 

MCs that may be experienced under the parameters of the simulations. The two walls selected 

are exemplary of the two main insulation strategies—exterior insulation versus cavity insulation. 

The moisture-tolerant wall selected was Wall 1: Advance Framed Wall with 4-in. of Exterior 

Insulation and 5.5 in. of Batt. The moisture-sensitive wall was Wall 4: Double Stud Wall with 

9.5 in. of CFI.

Wall 1 was selected due to the elevated substrate temperature provided by the exterior insulation. 

Further, due to a lack of Class I or II interior vapor retarders, this wall possesses the capacity to 

dry to the interior. Wall 2 was selected because the OSB sheathing was subjected to near-outdoor 

temperatures (more extreme temperature variations), and the wall possesses significant moisture 

storage. Once the wall reaches hygric equilibrium (i.e., is charged and reaches equilibrium with 

the ambient humidity), a small change in environmental conditions can rapidly create situations 

that place the moisture-sensitive materials at risk.

The walls were simulated in two cold climates: climate zone 5 (Chicago) and climate zone 7 

(International Falls); and two hot climates: climate zone 2C (Houston) and climate zone 2 

(Phoenix). The walls were simulated over a range of interior RHs (20%–40%), and from 0– 2.5

ACH50. With regards to air leakage, the critical air leakage path was identified and the air leak 

was inserted adjacent to the condensation plane. In cold climates, exfiltration indoor air can 

result in condensation on cold components of the wall assembly, and so the condensation plane is 

typically the interior side of the OSB sheathing. However, the reverse occurs in hot and humid 

climates. Warm outdoor air, infiltrating into the wall assembly, can collect behind interior vapor 

barriers and moisture may condense. The critical condensation plane in warm climates is on the 

exterior side of the GWB. Table 8 and Table 9 show the peak daily sheathing MCs in cold, very 

cold, hot-humid, and hot-dry climate zones, with Class I and III interior vapor retarders, for both 

wall assemblies. The cells highlighted in red show MCs in excess of 28% MC. Cells with a “D”

indicate that the worst-case scenario was modeled and the MC of the sheathing remained within 

safe limits. The “W” indicates that the best-case scenario was modeled and the MC was also 

dangerously high.
37

Table 8. Peak Daily MCs for Exterior Insulated Wall Assembly,

With Class I and III Vapor Barriers

Interior 

RH ACH Chicago International 

Falls Houston Phoenix

1 3 1 3 1 3 1 3

20%

0 10.8 8.9 10.5 13.7 47.8 D D D

0.5 8.5 8.9 10.9 13.6 40.1 D D D

1.5 8.4 8.6 12.0 13.3 50.5 D D D

2.5 12.6 8.5 12.2 13.0 56.8 D D D

30%

0 11.1 10.6 10.8 21.3 W D D D

0.5 9.2 10.8 13.3 23.0 W D D D

1.5 9.7 10.5 15.9 21.2 W D D D

2.5 9.7 10.2 16.0 18.0 W D D D

40%

0 11.3 14.4 11.1 36.8 W D D D

0.5 11.1 14.5 16.2 W W D D D

1.5 12.5 14.1 27.1 W W D D D

2.5 12.6 13.5 W W W 14 8.4 8.4

Table 9. Peak Daily MCs for Cavity Insulated Wall Assembly,

With Class I and III Vapor Barriers

Interior 

RH ACH Chicago International 

Falls Houston Phoenix

1 3 1 3 1 3 1 3

20%

0 15.3 16.0 15.4 N/A 28.5 D D D

0.5 15.4 19.0 23.5 N/A 30.1 D D D

1.5 17.6 20.8 W N/A W D D D

2.5 19.6 22.1 W N/A W D D D

30%

0 15.3 18.6 15.4 N/A W D D D

0.5 18.0 24.5 W N/A W D D D

1.5 23.0 23.8 W N/A W D D D

2.5 29.3 28.6 W N/A W D D D

40%

0 15.4 22.1 15.4 N/A W D D D

0.5 19.7 29.7 W N/A W D D D

1.5 30.1 33.5 W N/A W D D D

2.5 32.8 W W N/A W 14.8 8.4 8.4

Notes: 

W indicates that the simulation experienced a convergence failure due to extremely high moisture fluxes, or that the 

best-case scenario resulted in dangerously high MCs. 

D indicates that the worst-case simulations resulted in MCs that do not pose a risk to the wall assembly, and so all 

the other simulations would result in similar, if not improved, moisture performances.

N/A indicates that this assembly is not code compliant.

3.2 Indoor Relative Humidity and Air Leakage

In cold climates, wall components may fall below the critical indoor air temperature for 

condensation, either from vapor diffusion or air leakage. The colder the climate, the colder the 

wall components and thus, the more sensitive the assembly becomes to outward flowing 
38

moisture. Exterior insulated wall assemblies possess increased tolerance to elevated indoor RHs

since the exterior insulation raises the temperatures of the moisture-sensitive wall components. It 

is only at extreme interior RHs (i.e., 40% in International Falls), during very cold exterior 

temperatures that the limits of an exterior insulated wall are reached. On the other hand, cavity 

insulated walls are much more sensitive to interior RHs, since some of the moisture-sensitive 

components approach outdoor temperatures, which likely fall below the interior dew point.

Under idealized conditions (i.e., with no air leakage), these wall assemblies with Class I vapor 

retarders perform adequately. However, real wall assemblies all experience some level of air 

leakage. 

A stark contrast in performance can be seen by how the exterior insulated walls outperform the 

cavity insulated walls when air leakage is considered. Under idealized circumstances, both walls 

perform adequately in all indoor RHs. However, as soon as a nominal level of air leakage is 

introduced, the cavity-only insulated walls reached dangerously high MCs, even at 20% indoor

RH in climate zone 7. This highlights the importance of considering air leakage in all 

hygrothermal simulations. Failure to do so may result in false predictions from practitioners 

unaware of the significance of air leakage effects on the wall’s moisture durability.

3.3 Interior Vapor Barriers

This series of simulations show the effects of interior vapor control layers on the moisture 

performance of the wall assembly. In the exterior insulated wall, the use of a Class I interior 

vapor retarder in cold climates does not naturally create dangerous conditions for biodegradation

unless a bulk water leak occurs. Similarly, a Class I interior vapor retarder in cavity insulated 

assemblies creates an artificial result, suggesting that the assembly performs adequately, when in 

reality, when air leakage is considered, dangerously high MCs are predicted.

In hot and humid climates, the use of a Class I vapor retarder, in both exterior insulated and 

cavity insulated walls, creates extremely high MCs and RHs. These elevated MCs occurred 

regardless of interior RH and regardless of the insulation strategy. On the other hand, hot-dry 

climates do not possess sufficient ambient moisture to create significant air leakage 

condensation. Nonetheless, low vapor permeance interior membranes should be avoided in hothumid climates.

3.4 Wall Performance

To assess the moisture durability performance of high-R wall assemblies, a climate zone 

comparison study was undertaken. This enables comparisons to be made between wall types and 

climate zones to allow designers and builders to identify which wall types are best suited for 

their climates.

Each wall comprised identical interior and exterior construction (i.e., cladding and ventilation, 

interior finishes) and climatic boundary conditions for each respective climate zone; the primary

differences are the types and locations of the insulating materials, and the presence and locations

of any code-required vapor barriers. Each wall was also subjected to three types of moisture

loads: construction moisture, air leakage, and bulk water leakage. The results are presented

below.
39

3.4.1 Advance Framed Walls With Exterior Insulation

Advance framed walls employ efficient framing strategies to minimize the amount of thermal 

bridging through the wall and maximize the effective thermal resistance. The addition of exterior 

insulation acts to significantly reduce the effects of thermal bridging through the remaining 

framing members. For the purposes of this project it was assumed that all of the advance framed, 

exterior insulated walls employ sufficient exterior insulation to meet the requirements of the 

International Energy Conservation Code, as given in Table 10.

Table 10. Exterior Insulation Requirements and Alternates for the Use of Class III Vapor Retarders

(IECC 2012)

Climate Zones

Minimum Exterior 

R-Value for Class III 

Vapor Retarder

Alternatives to Exterior Insulation

4 2 × 4 2.5 Vented cladding over wood structural panels 2 × 6 3.75

5 2 × 4 5 Vented cladding over wood structural panels 2 × 6 7.5

6 2 × 4 7.5 Vented cladding over fiberboard of gypsum 2 × 6 11.25

7 and 8

2 × 4 10 N/A 2 × 6 15

The values obtained from the International Energy Conservation Code are based on the thermal 

resistance of typical homes. High performance homes are usually more airtight and may have 

higher indoor moisture levels, especially if HVAC equipment cannot provide dehumidification

or if humidifiers are used to maintain elevated humidity levels. Higher indoor RH levels 

necessitate the installation of greater amounts of exterior insulation (i.e., higher R-values) to 

keep the sheathing temperatures above the indoor dew point temperature. If the sheathing 

temperature does not fall below the interior dew point temperature, then air leakage condensation 

against the sheathing cannot occur. The ratio of the exterior insulation divided by the total wall 

R-value can be used to infer whether or not the sheathing will be at risk of air leakage 

condensation. Table 11 identifies the required insulation ratios such that air leakage moisture 

condensation does not occur for a range of indoor RHs and outdoor temperatures. For example, 

if an R-23 wall in an advance framed wall (not the effective R-value), building is located in a 

cold climate (–4°F) with an interior RH of 30%, then a minimum R-12.6 of exterior insulation is 

required to minimize interstitial condensation. The highlighted areas in Table 11 represent the 

required insulation ratio for the cities in climate zones 4, 5, and 7, based on the coldest 3-month 

average temperature (see Table 4) at the modeled, elevated interior RHs. The modeled advance 

framed wall has an insulation ratio of 0.46. Consequently, it is anticipated that International Falls 

and St. Louis, under their assumed higher indoor RHs, may have elevated sheathing MCs due to 

having a lower insulation ratio than required.
40

Table 11. Insulation Ratio for a Given Interior RH and Outdoor Temperature

(Straube 2010)

3.4.1.1 Advance Framed, Exterior Insulated Walls—Baseline

The baseline case follows from the boundary conditions listed in Section 2.1.4. If the peak MC

never exceeds 20%, then it is very likely that decay will not happen to the sheathing. Figure 20

shows the peak daily sheathing MC for the selected cities. 

Figure 20. Peak daily MCs for advance framed wall insulated walls in modeled locations

Only in International Falls does the peak MC exceed 20%. However, if the insulation ratio in 

Table 11 were followed and a higher ratio of exterior insulation were installed, the peak MC

would decrease to a safe 14%. In an R-30 wall, a minimum of R-16.5 of exterior of insulation is 

required to maintain a peak 14% MC for International Falls. The insulation ratio is an important 

factor when considering the surface temperatures of the interstitial surfaces of a wall assembly. 

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

A.F. w/ 4in. IS, 5.5in Batt

International Falls Chicago St. Louis Seattle
41

Maintaining elevated surface temperatures is of utmost importance for the durability of moisturesensitive components in wall assemblies.

3.4.1.2 Advance Framed, Exterior Insulated Walls—Construction Moisture

Advance framed walls sometimes restrict outward drying, depending on the thickness and type 

of exterior insulation, the type and vapor permeance of the WRB, and whether or not a hygric, or 

moisture redistribution space is provided. Figure 21 shows the construction moisture drying rates 

for the advance framed walls. 

Figure 21. Advance framed wall insulated walls construction moisture drying

The MCs in the sheathing of the walls all increase in the winter due to outward vapor diffusion

and decreased sheathing temperatures (which results in increased RH and thus, higher MC). The

very cold temperatures in International Falls yield sheathing temperatures that are below 

freezing—any residual moisture in the sheathing would turn to ice. This ice will melt in the 

spring and should not pose any problems to the sheathing; however, field experience suggests 

that mold and decay can occur on the top surface of the bottom plate as the result of sheathing 

meltwater. The Seattle wall experiences elevated MCs throughout the winter. In this situation, it 

is probable that mold will form on the sheathing. The times required for the sheathing to dry to 

20% MC are shown in Table 12. In general, mold growth requires around 20–30 weeks in ideal 

conditions. While temperature fluctuations may interfere with any mold growth, it is likely that 

some degree of mold will be found in advance framed walls in International Falls and Seattle if 

the walls are sealed at the beginning of October and the wood materials were sealed at the 

simulated 28% MC. However, these results should not be taken without consideration of the 

inherent assumptions in these simulations. If these walls were finalized in the spring, the walls 

could dry out throughout the summer and the required drying times would be greatly reduced. 

These simulations represent a worst-case scenario. 

0

5

10

15

20

25

30

35

40

13/10/01 13/12/31 14/04/01 14/07/01 14/10/01

Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
42

Table 12. Advance Framed Wall With Exterior Insulation Drying Times To Reach 20% MC

Climate Zone Time To Reach 20% MC

(in Weeks)

International Falls 25.8

Chicago 2.2

St. Louis 0.4

Seattle 31.6

Atlanta 0.3

Houston 0.3

The general conclusion that may be drawn from this simulation is that environments that are very 

cold (e.g., International Falls) or that are considered very wet (e.g., Seattle) negatively affect the 

drying times for the OSB sheathing. Hot climates, such as Houston, allow for greater drying 

capacities. 

The reader should note that all of these simulations are based on idealized conditions. Empirical 

testing and forensic investigations are required to validate the durability of sheathing durability 

when exposed to construction moisture. 

3.4.1.3 Advance Framed, Exterior Insulated Walls—Air Leakage

When 0.04 cfm/ft2 at 4 Pa of indoor air is introduced immediately behind the sheathing, 

condensation can occur and the MC at the inside face of the sheathing can become elevated. 

Figure 22 demonstrates that advance framed walls are tolerant to air leakage.

Figure 22. Advance framed wall insulated walls peak MC with air leakage condensation

Air leakage only marginally increases the sheathing MC. A balance of thermal energy and 

moisture deposits occur when a certain rate of air is injected into a cavity. Too much flow and 

the heat of the air will warm the surface; too little and there is insufficient moisture to 

appreciably increase the MC of the sheathing. In advance framed walls, the sheathing is typically 

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

A.F. w/ 4in. IS, 5.5in Batt Air Leak
43

warmer than the dew point of the interior air and thus no significant amount of air leakage 

condensation occurs. The differences in sheathing MCs among the different climate zones are a 

function of the differing thermal resistances of the sheathing in each climate zone.

3.4.1.4 Advance Framed, Exterior Insulated Walls—Bulk Water Leakage

Some problems were encountered when simulating a bulk water leak. The results of the 

simulation, with a leak of 5% of wind-driven rain, correlate well with forensic evidence of 

advanced moisture deterioration at leak sites. However, this added water sometimes causes 

convergence errors due to the enormity of the leak. In general, bulk water leaks totaling 5% of 

wind-driven rain must be avoided by using proper flashing and weather proofing techniques. 

Figure 23. Bulk water leak in advance framed walls with exterior insulation

MCs exceeding 50% will not only provide ample moisture for mold and rot growth, but can also 

damage the adhesives used to bind the OSB. A 5% water leak will cause significant moisture 

damage to the walls. Consequently, proper water shedding details, and especially adequate 

overhangs, are of utmost importance.

3.4.2 Hybrid Advance Framed Walls With 2 in. of Exterior Insulation, 2 in. of 

ccSPF, and 3.5 in. of Batt Insulation

The hybrid advance framed wall is characterized by an advance framed wall with 2 in. of

exterior insulation (XPS), to help reduce thermal bridging, 2 in. of ccSPF cavity insulation to 

help control air leakage and condensation, and finished with 3.5 in. of batt insulation for added 

insulation. While this wall does not provide the same level of thermal insulation as Wall 1: 

Advance Framed Wall with 4-in. of Exterior Insulation, it does meet the high-R criteria for 

climate zones 1 through 5.

The greatest concern with this wall assembly is the possibility to capture moisture in the OSB, as 

both the interior and exterior drying pathways are limited by closed cell foams (i.e., XPS exterior 

insulation or ccSPF on the interior). However, these concerns can be alleviated by using a more 

0

10

20

30

40

50

60

70

13/10/01 13/12/31 14/04/01 14/07/01 14/10/01

Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
44

vapor-permeable exterior insulation, such as mineral board or EPS board insulation, and ensuring 

that the OSB sheathing does not get wet subsequent to the installation of the ccSPF insulation.

3.4.2.1 Hybrid Advance Framed Walls—Baseline

The baseline MCs for the hybrid advance framed wall all fall below 20%, and are therefore 

considered low risk. The peak daily MCs for the modeled cities may be found in Figure 24.

Figure 24. Peak daily MC for hybrid advance framed wall insulated walls in modeled cities

Despite a lower exterior board insulation ratio compared to the advance framed wall with 4-in. of 

exterior insulation, the MC of the OSB in International Falls is lower. The wall assembly

benefits from the ccSPF’s ability to behave as an exterior insulation, by acting as the 

condensation plane instead of the moisture-sensitive OSB. 

3.4.2.2 Hybrid Advance Framed Walls—Construction Moisture

Concerns related to inhibiting the drying capacity of the walls are justified when the drying rates 

are plotted for the hybrid wall in all climate zones. It should be noted that if the exterior OSB is 

protected from precipitation and moisture sources subsequent to the installation of the ccSPF, the 

conditions assumed (i.e., initial MC of 28%) are invalid. The intent of the model is to present 

potential drying rates for these wall assemblies and present worst-case scenarios. The results may 

be found in Figure 25.

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

A.F. w/ 2in IS, 2in ccSPF, 3.5in Batt
45

Figure 25. Hybrid advance framed wall insulated walls construction moisture drying

Due to the inhibited drying routes, the moisture in the OSB sheathing remains trapped for 

prolonged periods of time. The required times to dry down to 20% MC are summarized in 

Table 13.

Table 13. Hybrid Advance Framed Wall Drying Times To Reach 20% MC

Climate Zone Time To Reach 20% MC 

(in Weeks)

International Falls 40.8

Chicago 37.4

St. Louis 34.1

Seattle 44.7

Atlanta 32.3

Houston 28.6

As a result of these prolonged times at elevated MC, it is very probable that some degree of mold 

growth, or other moisture deterioration, would be found in such assemblies. OSB that is exposed 

to elevated MCs may experience adhesive failure and delamination. Consequently, hybrid wall 

assemblies should be allowed to dry prior to installation of either the interior ccSPF, or exterior 

insulation, such that at least unidirectional drying is provided. 

3.4.2.3 Hybrid Advance Framed Walls—Air Leakage

When exfiltrating air is introduced into the system (adjacent to the interior surface of the ccSPF), 

the additional heat helps to slightly raise the temperature of the OSB. This is expressed by lower 

MCs in the OSB sheathing, as seen in Figure 26.

0

5

10

15

20

25

30

35

13/10/01 13/12/31 14/04/01 14/07/01 14/10/01

Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
46

Figure 26. Peak daily MC for hybrid advance framed wall insulated walls 

with air leakage in modeled cities

The vapor resistance of the ccSPF effectively isolates the OSB from the interior conditions, as 

seen by the relatively dry MCs. Also, the additional heat source from the exfiltrating air helps 

raise the temperature of ccSPF just enough that the temperatures in the OSB are slightly 

elevated. Regardless, the MC of the sheathing does not exceed 20% at any point in any of the 

modeled climate zones.

3.4.2.4 Hybrid Advance Framed Walls—Bulk Water Leakage

Similar to the bulk water leakage values in Wall 1: Advance Framed Wall with Exterior 

Insulation, the added moisture results in extremely high MCs throughout the entire year. The 

MCs are sufficiently high to not only harbor mold growth, but to likely cause adhesive failure of 

the OSB. The drying rates for such a leak may be inferred from the drying rates from the 

construction moisture simulations. The results suggest that if the OSB gets wet in such a hybrid 

system, it will remain wet for prolonged periods of time. This emphasized the importance of 

proper flashing and moisture details in walls built with vapor-resistant materials, such as ccSPF 

and board foams.

3.4.3 Advance Framed Walls With 7.25 in. of ccSPF Cavity Insulation

The use of an advance framed wall with 7.25 in. of ccSPF insulation is not very common.

Combining the, albeit thermally superior, ccSPF in an already expensive structure (2 in. × 8 in.) 

may be cost prohibitive to certain builders, designers, and owners. However, the air sealing 

capacity of this assembly can be superior, provided that other air leakage pathways not covered 

by the spray foam (i.e., between the plates and the rim joists, plates and trusses, window and 

mechanical penetrations) are also sealed. Furthermore, the structural capacities of this wall 

system should be significant, as the structural contributions of spray foam can be significant 

(tensile strength of approximately 35 psi). Further, no additional vapor controls are required in 

this assembly, as the vapor permeance of 7.25 in. of ccSPF insulation as a Class I vapor retarder.

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

A.F. w/ 2in IS, 2in ccSPF, 3.5in Batt Air Leak
47

Due to the adhesive bond between the OSB and the ccSPF, it is extremely unlikely that any mold 

growth can occur on the interior side of the OSB—any mold that would grow would be 

exclusively on the exterior of the sheathing. This mold would hence be on the exterior of the air 

barrier and should pose no problems to the occupants.

3.4.3.1 Advance Framed, ccSPF Cavity Insulated Walls—Baseline

The baseline MC values are all quite similar for all climate zones. Despite the OSB sheathing 

experiencing near-outdoor temperatures year round, it is isolated from interior moisture from the 

vapor-resistant ccSPF insulation. The results of the peak daily MCs of the sheathing may be 

found in Figure 27.

Figure 27. Peak daily MCs for advance framed wall insulated walls 

with ccSPF cavity insulation in modeled cities

As can be seen above, the MC of this assembly never exceeds 20%. These baseline moisture 

predictions suggest that this wall is low risk from a biodegradative standpoint.

3.4.3.2 Advance Framed, ccSPF Cavity Insulated Walls—Construction Moisture

The benefit of using ccSPF cavity insulation can also negatively affect the functionality of the 

building if construction moisture is present in the assembly. The ccSPF allows only one-way 

drying to the exterior. 

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

A.F. 2x8 w/ 7.25in ccSPF
48

Figure 28. Advance framed wall with ccSPF cavity insulation construction moisture drying

Even with unidirectional drying, most of the walls were able to dry to 20% MC in less than 20 

weeks. The required drying times to achieve this MC are shown in Table 14.

Table 14. Advance Framed Wall With ccSPF Cavity Insulation Drying Times To Reach 20% MC

Climate Zone Time to Reach 20% MC 

(in Weeks)

International Falls N/A

Chicago 15.6

St. Louis 3.9

Seattle 19.6

Atlanta 8.5

Houston 4.6

The wall located in International Falls experienced convergence errors due to the extremely low 

temperatures and elevated MCs of the sheathing. The results for International Falls should not be 

considered in evaluating the estimated drying times. It is imperative that once the ccSPF 

insulation is installed that the OSB sheathing is protected from any moisture sources, particularly 

precipitation. A saturated OSB sheathing with ccSPF insulation can result in moisture damage to 

the OSB.

3.4.3.3 Advance Framed, ccSPF Cavity Insulated Walls—Air Leakage

Since the ccSPF insulation effectively eliminates any convection between the insulation and the 

sheathing, the only plane for air leakage condensation is on the surface of the ccSPF, 

immediately behind the GWB. The results are shown in Figure 29.

0

5

10

15

20

25

30

13/10/01 13/12/31 14/04/01 14/07/01 14/10/01

Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
49

Figure 29. Peak daily MCs for advanced frame wall insulated walls with ccSPF cavity insulation 

and air leakage in modeled cities

Since the plane of condensation is close to indoor temperatures, there is limited possibility for 

condensation. As can be seen above, the OSB is virtually isolated from the indoor conditions. 

3.4.3.4 Advance Framed, ccSPF Cavity Insulated Walls—Bulk Water Leakage

The advance framed wall with 7.25 in. of ccSPF insulation is as equally susceptible to bulk water 

leaks as all the other walls. A leaking window may deposit water on the window subsill 

structure, but it is unlikely that this water will leak down the interior side of the OSB. Instead, 

this water is likely to leak down the exterior face of the OSB. This will result in decreased drying 

times, as the required travel distance for drying is reduced. Essentially, the ccSPF helps contains 

and limits the potential leak. However, the leak will still cause significant localized deterioration 

of moisture-sensitive materials.

3.4.4 Double Stud Walls With CFI and Polyethylene Vapor Barrier

Double stud walls are used as high-R walls due to the low cost of insulation. Further, the 

structure is thermally broken, by having interior and exterior stud walls. A Class I or II vapor 

retarder is required in these walls in cold climates. Frequently, a polyethylene sheet will be 

suspended on the exterior side of the indoor framing structure. However, in climate zones 1 

through 4 (not including climate zone 4: marine), no additional vapor control layers were 

included beyond the Class III interior latex paint.

The thermal benefits provided by double stud walls are also a source of concern regarding the 

moisture durability of the sheathing. A wall with significant cavity insulation will maintain 

sheathing temperatures close to those of the outdoor environment. This can pose a problem when 

sheathing temperatures drop below the dew point temperature of the indoor air. Air leakage can 

result in condensation formation on the sheathing, and the significant thermal resistance 

minimizes any thermal drying. The amount of cellulose used in double stud walls helps alleviate 

minor moisture concerns due to the significant moisture storing capacity of the cellulose. 

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

A.F. 2x8 w/ 7.25in ccSPF Air Leak
50

However, if the cellulose’s storing capacity is exceeded, significant decay to the structure can 

occur. Cellulose treated with fungicides should experience biological growth only if sufficient 

liquid water is present to leach away these treatments. Cellulose without treatment would start to 

experience biodegradation with RHs in excess of 80%.

3.4.4.1 Double Stud Walls With CFI and Polyethylene Vapor Barrier—Baseline

The baseline MC readings for the modeled cities are shown in Figure 30.

Figure 30. Peak MCs for double stud walls in modeled cities

Double stud walls with polyethylene sheets perform very well under ideal circumstances (e.g., no 

air leakage, no construction moisture, etc.). However, once real-world effects are introduced, 

such as an air leak, the material and design of double stud walls result in nonideal performances.

3.4.4.2 Double Stud Walls With CFI and Polyethylene Vapor Barrier—

Construction Moisture

Some concerns exist that the use of a vapor barrier may inhibit the drying of walls should they 

get wet during construction. The following figure demonstrates the drying rate of the period of a 

year for these walls when exposed to construction moisture (28% MC for the OSB sheathing).

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

Dbl. Std. w/ 9.5in CFI
51

Figure 31. Double stud walls construction moisture drying

The immediate drying effects have a noticeable impact. This is due to the cellulose absorbing the 

construction moisture out of the OSB. However, once the cellulose reaches a saturated MC, it no 

longer absorbs moisture out of the OSB. Consequently, the OSB MC starts to rise during the 

colder winter months as it no longer has a proximate moisture sink to maintain a lower MC. The 

rapidity at which the OSB dries is demonstrated in Table 15.

Table 15. Double Stud Walls Drying Times To Reach 20% MC

Climate Zone Time to Reach 20% MC 

(in Weeks)

International Falls 27.9

Chicago 1.9

St. Louis 0.6

Seattle 3.1

Atlanta 0.4

Houston 0.3

International Falls experiences prolonged durations (approximately 5 months) of nearly 28% 

MC in the OSB. This is a concern, as this elevated MC is sufficiently high to promote mold 

growth. Further, the elevated MC occurs over months where the ambient outdoor temperature is 

above 32°F.

While the drying rates are quite prolonged, and the peak MCs approach 28%, the borate 

treatment of the cellulose should be sufficient to mitigate any mold growth. Additional testing is 

required to verify the resistance of cellulose for a given duration at MCs in excess of 28%. 

Further research is required to verify if the anti-fungal treatment of the cellulose also provides 

benefits to the OSB.

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Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
52

In these simulations all of the construction moisture is assumed to be present in the OSB and 

none in the cellulose fiber insulation. The simulations might reflect a scenario in which the OSB 

is wet by rain water prior to application of the WRB on the outside and installation of the (dry 

installed) cellulose on the inside. Another less likely—but still plausible—scenario might be one 

in which the cellulose is installed prior to wetting. Under this second scenario a much larger 

volume of construction moisture could be built into the assembly (distributed between the OSB 

and the cellulose fibers located nearer to the outside of the assembly. Further research should be 

conducted to estimate the potential amount, distribution, and impact of moisture in this scenario.

3.4.4.3 Double Stud Walls With CFI and Polyethylene Vapor Barrier—Air Leakage

A sheet of polyethylene may be utilized to reduce wetting from outward vapor diffusion during 

colder weather but, unless it is properly sealed and detailed as an air barrier, it may not provide 

any benefit to air leakage condensation. Figure 32 demonstrates the difference in sheathing 

moisture content when air leakage is considered. Climate zones that do not include an MC for 

the sheathing when subjected to air leakage indicate that a convergence failure of the 

hygrothermal model occurred due to excessive moisture fluxes. It is to be assumed that the MC

of the sheathing would exceed 28%.

Figure 32. Peak MCs for double stud walls in modeled cities, with and without air leakage

When exfiltration air leakage is introduced into a double-stud wall, dangerously high moisture 

contents occur. The sheathing is outside of all of the insulation and tracks the outdoor 

temperatures closely. In the winter the sheathing is cold and air leakage condensation results in

frost accumulation on the inside face of the OSB. The frost accumulation is expected to be much 

more significant than the frost accumulation predicted for the advance framed, exterior insulated 

wall assembly in International Falls (see Section 3.2). While the frost may not pose a direct 

problem, once sheathing temperatures rise above freezing, the meltwater leak can create elevated 

MCs within the sheathing and on the top surface of the bottom plate (as a result of run down).

Significant amounts of moisture can be quickly absorbed into the cellulose insulation and may be 

present for long periods when thermal gradients are small (e.g., during swing seasons). All 

0

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30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

Dbl. Std. w/ 9.5in CFI Air Leak
53

efforts should be made to eliminate any air leakage into the wall cavity in double stud walls. An 

alternative is to consider using ccSPF as an air barrier to eliminate airflow to the back of the 

sheathing (considered later in this report).

3.4.4.4 Double Stud Walls With CFI and Polyethylene Vapor Barrier—

Bulk Water Leakage

Bulk water leakage is a serious concern for all wall assemblies. However, the predicted severity 

of the simulated leak in the double stud wall with cellulose is not as bad as in other simulated 

walls. The results from the simulation are found in Figure 33. 

Figure 33. Bulk water leak in double stud walls with exterior insulation

The bulk water leak in the double stud walls does not appear as serious as in the advance framed

walls with exterior insulation. This is due to the significant moisture-storing capacity of the 

cellulose. However, throughout the 3-year duration of the simulations, the MC of the cellulose 

increased every year. This suggests that the moisture from the OSB is absorbed by the cellulose.

If the double stud wall is subjected to frequent and repeated wetting due to a bulk water leak, the 

limiting factor for the durability is no longer exclusively the OSB sheathing, but also the 

moisture durability of the cellulose. The MC rise in the cellulose over a 3-year simulation period 

may be seen in Figure 34.

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70

80

12/10/01 12/12/31 13/04/01 13/07/01 13/10/01

Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis Seattle
54

Figure 34. Cellulose MC for bulk water leak in Chicago

The polyethylene sheet divides the interior frame cellulose and the exterior frame cellulose. The 

increasing MC of the exterior cellulose demonstrates that the polyethylene sheet is inhibiting 

inward drying, resulting in ever-increasing MCs.

If the polyethylene vapor barrier is eliminated, the wall assembly will be able to dry to the indoor 

side and the effects of the bulk water leak will be somewhat reduced; however, field experience 

suggests that bulk water leaks often overwhelm most drying mechanisms. Adequate water 

control details are paramount to ensuring the proper functioning of double stud walls.

3.4.5 Hybrid-Insulated Double Stud Walls With ccSPF and CFI

The standard double stud wall concept can be improved through use ccSPF installed directly 

against the sheathing. The remainder of the framing cavity is filled with dry cellulose fiber 

insulation. This approach is said to employ a hybrid insulation strategy. By using at least 2 in. of 

ccSPF, air leakage condensation on the sheathing is eliminated and vapor diffusion is also 

controlled, as 2 in. of ccSPF is considered a Class II vapor retarder. Table R601.3.1 in the IRC 

states that for a 2 × 6 frame in climate zones 5, 7, and 8, you require at least R-7.5 to R-15 of 

ccSPF, respectively. As double stud walls possess more insulation, additional thicknesses of 

ccSPF are required to ensure the surface of the spray foam is maintained above the dew point 

temperature of the indoor air. In practice this is not typically done with double stud wall 

assemblies. If code ratios are followed to determine ccSPF thickness double stud wall assemblies 

perform well. However, if code table listed R-values are used instead double stud walls do not 

perform well. For the purposes of this report, the minimum R-value as specified by the code 

tables for 2 × 6 assemblies will be used to demonstrate a worst-case scenario. International Falls 

(climate zone 7) was modeled with 3 in. of ccSPF, while the other cities had 2 in. of ccSPF. 

3.4.5.1 Hybrid-Insulated Double Stud Walls With ccSPF and CFI—Baseline

The results from the baseline simulation of a double stud wall with ccSPF insulation may be 

found in Figure 35.
55

Figure 35. Peak MCs for hybrid double stud walls with ccSPF in modeled cities after 3 years

Under ideal conditions, the double stud wall with ccSPF performs only marginally better than the 

double stud wall with polyethylene sheet, a difference of only about 2% MC for each respective 

city. However, the benefits of using ccSPF become apparent when air leakage was considered 

(see Section 3.3.3).

3.4.5.2 Hybrid-Insulated Double Stud Walls With ccSPF and CFI—

Construction Moisture

A double stud wall with 2 in. of ccSPF insulation on the interior of the sheathing will inhibit any 

inward drying. A spray foam installer following manufacturer’s recommendations should not 

install ccSPF on wood substrates in excess of 18% MC. However, this simulation creates a 

worst-case scenario wherein the OSB gets wet prior to the ccSPF installation. The results may be 

found in Figure 36.

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CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

D.Std. w/ 2in ccSPF, 7.5in CFI
56

Figure 36. Hybrid-insulated double stud walls construction moisture drying

The ccSPF clearly limits inward drying of the sheathing. MCs in excess of 20% last for 

approximately 6 months in climate zones 4C, 5, and 7. The required times to dry to 20% MC are 

shown in Table 16.

Table 16. Hybrid-Insulated Double Stud Wall Drying Times To Reach 20% MC

Climate Zone Time To Reach 20% MC 

(in Weeks)

International Falls 31.9

Chicago 21.5

St. Louis 3.8

Seattle 23.1

Atlanta 8.5

Houston 4.5

The prolonged time of wetness poses a significant moisture risk. It is possible that the sheathing 

will experience moisture damage in the form of delamination and failure of the adhesives used to 

hold the wood fibers; mold and rot may also be problems, but the ccSPF will likely limit the 

mold growth on the exterior of the OSB. Additional research is required to verify the severity of 

the damage based on the quantity and duration of the moisture loads.

3.4.5.3 Hybrid-Insulated Double Stud Walls With ccSPF and CFI—Air Leakage

The use of ccSPF in double stud walls minimizes the effects of air leakage by providing a 

continuous air barrier that is also resistant to vapor diffusion. Figure 37 shows the peak sheathing 

MCs for the selected cities.

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Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
57

Figure 37: Peak MC for double stud walls with ccSPF in modeled cities, 

with and without air leakage

The use of ccSPF effectively minimizes air leakage condensation. Further, if the surface of the 

ccSPF is the condensing plane, the ccSPF is tolerant to moisture. It is possible that if sufficient 

condensation occurs on the ccSPF that it may run down and collect on the sill plate of the stud 

wall. However, dimensional lumber requires prolonged and very elevated MCs to be susceptible 

to mold. Furthermore, the borate treatment of the CFI functions as an anti-fungal. 

In comparison with the standard double stud wall, the increased R-value of the spray foam 

results in warmer surface temperatures at the condensing plane. In the hybrid wall, this is the 

innermost surface of the ccSPF. Consequently, the use of ccSPF to prevent air leakage 

condensation is a sound method to improve sheathing durability by minimizing the exposure to 

warm interior air laden with moisture.

3.4.5.4 Hybrid-Insulated Double Stud Walls With ccSPF and CFI—

Bulk Water Leakage

When a small water leak is introduced into a double stud wall with ccSPF, the MC of the 

sheathing increases well beyond the fiber saturation point. Figure 38 demonstrates the severity of 

the bulk water leakage. 

0

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15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

D.Std. w/ 2in ccSPF, 7.5in CFI Air Leak
58

Figure 38. Bulk water leak in hybrid double stud walls

In a hybrid-insulated double stud wall with ccSPF, the sheathing is maintained at near outdoor 

conditions throughout the year. Wet sheathing may not dry quickly in cities that have climates 

with high outdoor moisture loads (i.e., high year-round RH with lots of precipitation). This is 

contrasted with cities that have climates more amenable to drying. This is exemplified by 

comparing St. Louis (climate zone 4) with Seattle (climate zone 4 marine). The sheathing in 

Seattle experiences MCs that would result in significant deterioration and failure. The bulk water 

leak in St. Louis resulted in a computation failure at the first major rain event. The performance 

of St. Louis, under the simulated conditions, for the rest of the year remains an unknown. 

However, it is anticipated that MCs of around 30% would occur, as in all the other cities.

The results of these simulations suggest that the sheathing in the other cities might be able to 

accommodate brief leaks, but due to the problems of the software being unable to account for 

liquid moisture on the surface of the materials, real-world conditions may not be adequately 

captured by the limitations of the software. Proper flashing and water barrier details, and 

especially properly sized overhangs, are required to minimize damage from bulk water leaks.

Real-world testing is recommended.

3.4.6 Double Stud Wall With ocSPF Cavity Insulation

A double stud wall filled with ocSPF is an inexpensive way to decrease the air leakage 

susceptibility of double stud walls commonly filled with cellulose insulation. The ocSPF acts to 

seal the OSB from any sources of air leaks between the exterior and interior stud walls. Further, 

while ocSPF is not considered vapor resistant, 9.5 in. of ocSPF behaves like a Class III vapor 

retarder (i.e., approximately 6 U.S. perms). While this is insufficient in colder climates, the 

ocSPF provides sufficient vapor resistance in climate zones 1 through 5, provided that there is a 

ventilated cladding, according to table R601.3.1. of the IRC 2012 building code. However, for 

the modeled walls, a Class I or Class II vapor retarder is required in climate zones 6 through 8. A 

polyethylene vapor barrier was therefore included in the International Falls simulation.
59

3.4.6.1 Double Stud Wall With ocSPF Insulation– Baseline

The results from the simulations suggest that ocSPF cavity insulation in a double stud wall can 

create conditions of elevated moisture content in the OSB sheathing. The high vapor 

permeability of the ocSPF enables water vapor to diffuse through the foams whereby it 

accumulates where it encounters a more vapor restricting surface; in this situation, the OSB. 

As seen in Figure 39, the peak daily MCs are all dangerously high in all climate zones, with the 

exception of International Falls where a Class I vapor control layer was used. This is due to the 

significant moisture diffusion fluxes from the elevated indoor RHs. If the RHs were significantly 

decreased (i.e., from a less airtight building, or for a building with a well-designed HVAC 

system, a monitored and properly operated interior), the MC of the OSB sheathing would 

decrease significantly. The difference from 40% RH to 20% RH in Chicago may be seen in 

Figure 40.

Figure 39. Peak MC for double stud walls with ocSPF in modeled cities

Figure 40. Annual peak daily MCs for 40% and 20% indoor RH in Chicago

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CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

Dbl.Std. w/ 9.5in ocSPF

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Chicago-20% Chicago-40%
60

The conclusion to be drawn from this analysis is that additional levels of vapor control are 

required for ocSPF walls in cold climates, unless the interior RH can be maintained at very low 

levels throughout the year. BSC recommends the use of a smart vapor retarder, which allows for 

drying by enabling increased vapor flows at high RHs or the application of a Class II vapor 

control coating to the interior face of the ocSPF to limit outward vapor drives in cold climates as 

is required for roof systems using ocSPF in the 2012 IRC.

3.4.6.2 Double Stud Wall With ocSPF Insulation—Construction Moisture

The vapor permeance of the ocSPF allows for interior drying of the wet OSB. However, in the 

colder climates, the outward flowing moisture from the humid indoor air easily overcomes the 

effects of inward drying, resulting in increasing MCs. The severity of this is such that it takes in 

excess of 28 weeks (7 months) before the International Falls, Chicago, and Seattle walls finally 

reach 20% MC (Figure 41).

Figure 41. Construction moisture in double stud walls with ocSPF

The required times for the walls to reach 20% MC are included in Table 17.

Table 17. Double Stud Wall With ocSPF Drying Times To Reach 20% MC

Climate Zone Time To Reach 20% MC 

(in Weeks)

International Falls 28.9

Chicago 32.5

St. Louis 1.4

Seattle 34.8

Atlanta 2.0

Houston 0.9

While rapid drying rates are experience in warmer or drier climates, the lack of outward flowing 

vapor control creates extremely long periods of time at elevated MCs in the colder climates. 

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Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
61

While mold growth is unlikely on the interior surface of the OSB, other forms of deterioration 

are likely to occur.

3.4.6.3 Double Stud Wall With ocSPF Insulation—Air Leakage

The air seal afforded by the ocSPF ensures that the only plane for air leakage condensation is the 

innermost surface of the foam. This foam is maintained near interior temperatures, and therefore 

air leakage condensation levels are essentially eliminated. However, the air leakage carries with 

it additional moisture. This moisture is not restricted by the Class III vapor retarding paint, and 

therefore more vapor diffusion occurs through the foam, elevating the OSB MC.

Figure 42. Peak MCs for double stud walls with ocSPF in modeled cities, 

with and without air leakage

With the exception of International Falls, which benefits from a vapor barrier, all of the wall 

assemblies experience dangerously high MCs. The advantage of spray foam, however, is that 

mold will have difficulty establishing itself between the spray foam and the OSB. It is much 

more likely that structural issues in the sheathing may occur due to adhesive and bond failure 

between the wood chips.

3.4.6.4 Double Stud Wall With ocSPF Insulation—Bulk Water Leakage

The high vapor permeance of the ocSPF provides ample drying capacity, both to the interior and 

exterior. This is demonstrated in the very fast drying rates for construction moisture. However, 

the inability of the ocSPF to control outward flowing water vapor negates the benefits of inward 

drying.

The benefits afforded by the Class I vapor barrier in International Falls eliminates the benefits of 

the inward drying of the ocSPF. However, in all the other walls, drying rates of 30% MC within 

the span of a month are possible during the summer months. However, elevated MCs recur 

during the winter. Relying on the drying capacity of ocSPF to accommodate for inadequate water 

detailing is not advised (Figure 43).

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25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

Dbl.Std. w/ 9.5in ocSPF Air Leak
62

Figure 43. Bulk water leak in double stud walls with ocSPF

3.4.7 Truss Wall With CFI Insulation and Polyethylene Vapor Barrier

The truss wall is a variation on the double stud wall approach, with the exception that thermal 

bridging is further reduced by intermittent gusset plates, which attach the exterior framing to the 

interior support. This helps reduce the thermal bridging effect of the floor structure to the 

exterior by cantilevering the insulation beyond the limits of the floor structure. However, in all 

other respects, the behavior of a truss wall closely approximates that of a double stud wall, 

including the sensitivity to air leakage condensation. A Class I vapor barrier was included in the 

models for climate zones 4C and higher. However, in climate zones 4 and lower, no additional 

vapor control layers were included beyond the Class III interior latex paint.

3.4.7.1 Truss Wall With CFI Insulation—Baseline

The primary difference between the truss wall and a double stud wall with cellulose insulation is 

the location of the polyethylene vapor barrier. In double stud walls, the polyethylene sheet is 

sometimes placed on the exterior side of the interior stud wall. On truss walls, however, the only 

location to place the vapor barrier is immediately behind the gypsum. This can aggravate the 

moisture durability of the wall when hot, humid outdoor conditions persist, as the moisture 

storage capacity of the cellulose fiber is not mediated by the vapor control layer (Figure 44).

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Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
63

Figure 44. Peak MCs for truss walls in modeled cities

The baseline MCs for the truss wall are similar to those of the double stud wall, with the 

exception that the peak daily average MCs are slightly higher. This is likely due to the moisturestoring potential of the truss wall being greater than that of the double stud wall, due entirely to 

the placement of the vapor barrier within the assembly.

3.4.7.2 Truss Wall With CFI Insulation—Construction Moisture

The truss wall construction drying follows nearly the same patterns as the double stud wall, with 

the exception that the MCs are all slightly more elevated. The cause of this difference is the 

location of the vapor barrier. In the truss wall, a greater amount of the cellulose is exposed to the 

initial drying of the OSB, thereby absorbing a greater amount of water. The consequence of this 

is that over the winter, an increased amount of moisture is forced to the exterior due to thermal 

gradients through the insulation (Figure 45).

Figure 45. Truss walls construction moisture drying

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CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

Truss Wall with 9.5in CFI

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Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
64

The drying rates and patterns of the truss wall are, again, very similar to those of the double stud 

wall, with the exception that the MC levels are slightly more elevated. The drying times to 

achieve 20% MC are provided in Table 18, below.

Table 18. Truss Wall Drying Times To Reach 20% MC

Climate Zone Time To Reach 20% MC 

(in Weeks)

International Falls 30.7

Chicago 3.5

St. Louis 0.6

Seattle 21.7

Atlanta 0.4

Houston 0.3

3.4.7.3 Truss Wall With CFI Insulation—Air Leakage

As the sheathing of the truss wall is not thermally insulated from the outdoor conditions, the 

sheathing is frequently one of the coldest elements in the wall assembly. When air leakage is 

considered, significant condensation may form on the sheathing, as demonstrated in Figure 46.

Figure 46. Peak MCs for truss walls with in modeled cities, with and without air leakage

The degree of air leakage condensation can be so severe that in climate zones where a value is 

not provided indicate that significant convergence errors occurred in the simulation due to 

excessive moisture fluxes. For all intents and purposes, it can be assumed that the MC of the 

sheathing would exceed 28%. Even in hot climates (i.e., climate zones 2 and 3), the MCs in the 

assembly approach what is considered risky limits for the sheathing.

3.4.7.4 Truss Wall With CFI Insulation—Bulk Water Leakage

Similar to the double stud wall, the significant moisture storage of the cellulose in the truss wall 

partially masks the severity of a moisture leak if only analyzed over short periods. However, 

0

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20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

Truss Wall with 9.5in CFI Air Leak
65

once the cellulose insulation is charged, even relatively small changes in outdoor conditions (i.e. 

a cold snap) can cause the safely stored moisture to condense on cold surfaces. This results in 

elevated MCs of the sheathing and may, depending on the temperature, promote mold growth.

3.4.8 Structural Insulated Panels

SIPs are created by adhering structural sheathing (usually OSB) as a skin on a core of insulation

(usually EPS). The benefits of SIPs pertain to possessing a continuous layer of insulation. The 

primary concern with SIPs is not the center of the panel performance, but at the joints between 

the panels. Forensic experience has identified numerous cases in which air leakage condensation 

caused significant damage along the edges of the panels. Air sealing methods have been 

developed to minimize this moisture risk; however, complex geometries and installer 

inexperience often result in imperfect joint air seals. Due to the limitations of using a onedimensional model to simulate a two-dimensional problem, the SIPs were simulated in two 

ways. A clear panel simulation (i.e., center of the panel) was used to model the baseline 

simulation, construction moisture, and bulk water leakage. A joint between two SIPs panels was 

modeled to simulate air leakage. 

3.4.8.1 Structural Insulated Panels—Baseline

The baseline simulations were conducted for a clear section of the panels. The effects of the 

edges and joints in the panels were not considered. The baseline results from these simulations 

are shown in Figure 47.

Figure 47. Peak MCs for SIPs walls in modeled cities

Due in large part to the significant thickness of the EPS core in the panels, which restricts vapor 

flows, very low MCs occur in the center of the panels. In ideal circumstances, SIPs panels 

perform well in a range of climates.

0

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25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

SIP w/ 11.25in EPS
66

3.4.8.2 Structural Insulated Panels—Construction Moisture

The model used to assess construction moisture drying assumed a section through the center of 

the panel. Due to the thickness of the EPS core, drying to the interior is restricted. Consequently, 

only exterior drying occurs. As exterior drying is highly dependent on the exterior environment, 

a strong correlation between drying rate and outdoor temperature and RH occur. This is 

demonstrated in Figure 48, with a slow drying rate for International Falls and the fast drying rate 

for St. Louis.

Figure 48. SIP walls construction moisture drying

Due to the cold temperature, the SIPs in International Falls take nearly 6 months to dry. When 

construction moisture is considered, the SIP’s exterior OSB skin appears to be subject to the 

same moisture risk as the OSB sheathing in the hybrid-insulated double stud wall. Some damage 

may occur to the OSB sheathing. The drying times for the OSB to reach an MC of 20% are 

shown in Table 19.

Table 19. Hybrid Advanced Frame Wall Drying Times To Reach 20% MC

Climate Zone Time To Reach 20% MC 

(in Weeks)

International Falls 29.1

Chicago 15.5

St. Louis 4.9

Seattle 18.6

Atlanta 9.5

Houston 5.3

Additional field research is required to verify the severity of this damage. The panels in Chicago 

and Seattle are predicted to also be at risk from prolonged times at elevated MCs, but the effects 

of the severity and duration of the moisture load are not well known or researched. Additional 

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Moisture Content (%)

Date (year/month/day)

I. Falls Chicago St. Louis

Seattle Atlanta Houston
67

research is required to verify the moisture durability of SIPs when exposed to elevated MCs in 

the central areas of the panel membranes.

3.4.8.3 Structural Insulated Panels—Air Leakage

The primary issue with SIPs is not air leakage through the middle of the panels, but at the joints. 

Significant moisture damage has occurred in buildings constructed with SIPs that were not 

properly air sealed along the joints. Consequently, the simulations modeled air leakage through a 

section at the joint. It was assumed that a small air leak bypassed the foam air barrier and 

deposited itself immediately behind the exterior OSB spline. The results may be found in 

Figure 49.

Figure 49. Peak MCs for SIP walls in modeled cities, with and without air leakage

Climate zones that do not include an MC value indicate that a convergence failure of the 

hygrothermal model occurred due to excessive moisture fluxes. It is to be assumed that the MC 

of the sheathing would exceed 28%.

No air leakage value was included in International Falls due to convergence errors. The air 

leakage condensation was so significant that could not be absorbed by the OSB and the 

simulation crashed. In all the other climates, with the exception of Atlanta (where the outdoor 

temperature infrequently dips below the dew point of the interior air), dangerously high MCs

occur. The results of the simulation suggest that SIPs that are not adequately air sealed are prone 

to risk of air leakage damage.

No moisture risk was predicted at the SIP joint when exfiltration air leakage is simulated using 

the Miami climate; however, in Miami, infiltration air leakage can introduce moisture to and 

cause condensation at the outdoor side of the interior OSB skin and spline at the indoor side of 

the panel joint. Further research should be conducted to assess this potential risk.

0

5

10

15

20

25

30

CZ 2C

(Houston)

CZ 3

(Atlanta)

CZ 4C

(Seattle)

CZ 4

(St.Louis)

CZ 5

(Chicago)

CZ 7

(Int'l Falls)

Moisture Content (%)

SIP w/ 11.25in EPS Air Leak
68

3.4.8.4 Structural Insulated Panels—Bulk Water Leakage

Significant calculation errors occurred when bulk water leakage was simulated in the SIPs. Due 

to restrictions toward inward drying, and minimal moisture storage capacity of the EPS 

insulation, water quantities that exceed the storage capacity of the OSB occurred. The errors are 

a result of the modeling software being unable to account for liquid water on the surface of the 

material.

The only pertinent information obtained from the simulations is that bulk water leakage into a 

SIP wall can result in significant moisture accumulation. This water can cause a range of damage 

from delamination of the adhesives in the OSB or plywood skins of the panels, to the potential of 

mold and rot growth.

3.4.9 Concrete Block Wall With 3.5 in. of Exterior Insulated Finishing System

The benefits of a high-R value assembly are typically of more value in cold climates. Also, the 

majority of residential buildings in U.S. cold climates tend to be of wood stick framed 

construction. Consequently, this report is primarily focused on high-R value, wood structure 

residential buildings. However, moisture-tolerant, high-R walls in hot climates may also provide 

benefits in reduced energy costs for air conditioning, although the required level of insulation is 

much less than in cold climates. Many hot and humid coastal areas of the United States use 

concrete blocks as their primary building structures. Therefore, a high-R value assembly, with R14 of exterior insulation, featuring a CMU supporting wall, was analyzed. 

Since very little wood is used in masonry construction, the area of foremost concern for moisture 

damage now becomes the GWB. In general, 80% RH is the lower limit for mold growth on 

paper-faced gypsum, but these conditions do not necessarily indicate that mold growth will 

occur. In general, significant mold growth will only start to occur once RHs approach the 90% 

level. For the purposes of this report, if sustained levels of 90%+ RH occur on the back side of 

the GWB, then this is considered extremely risky. However, if the RH surpasses 80%, but then 

falls below the lower threshold, this will be considered as only a moderate level of risk. If the RH

never exceeds 80%, then it is considered safe.

3.4.9.1 CMU With 3.5 in. of EIFS—Baseline, Air Leakage, and Vinyl Wallpaper

The RHs and temperatures were recorded immediately on the outside and inside surfaces of 

the GWB Houston, in climate zone 2C. The results are plotted throughout the year, as seen in 

Figure 50.
69

Figure 50. Yearly peak daily RHs at the GWB surface for CMU walls—

baseline, air leakage, and air leakage and vinyl wallpaper

Even with air leaking into the cavity space of the assembly, the RH does not exceed 60%. This is 

a result of not including any types of interior vapor resisting layers, such as vinyl wallpaper, or at 

areas of low vapor permeance finishes, such as cupboards, mirrors, and glazed tiles. As soon as a 

low permeance layer such as vinyl wallpaper is included in the simulation, the RH on the paper 

faces of the GWB reach up to 90%. It is very probable that mold will germinate and grow under 

such conditions. It is therefore extremely important that no vapor-retarding interior finishes are 

installed in this assembly in this type of climate.

3.4.9.2 CMU With 3.5 in. of EIFS—Construction Moisture

By constructing a wall system that does not inhibit inward drying, very fast drying times may be 

achieved. The MC rapidly drops from 80% to 60%, which matches the modeled indoor 

conditions. Based on the results of the drying simulation, the risk of mold growth on the GWB is 

very low (see Figure 51). 

0

10

20

30

40

50

60

70

80

90

100

13/10/01 13/12/31 14/04/01 14/07/01 14/10/01

Relative Humidity (%)

Date (year/month/day)

Baseline Air Leakage Air Leakage and Vinyl Wallpaper
70

Figure 51. CMU wall with 3.5 in. of EIFS construction moisture drying

3.5 Wall Summary

The results of these simulations provide the anticipated moisture performance of the proposed 

wall assemblies when exposed to a range of simulated moisture loads. While extensive effort 

was undertaken to ensure that the parameters and values used are representative of actual, realworld conditions, no large-scale, empirical testing of the moisture durability of wall assemblies 

has been undertaken. 

Consequently, to summarize the results of simulations, a table was produced with a general risk 

assessment for each proposed wall type. These values are all based on a 2.5 ACH50 wholebuilding air leakage, as the idealized simulations that do not include air leakage do not provide 

realistic results. 

A “low risk” assembly in a given climate is not likely to experience a single peak average day 

MC in excess of 20%. A moderate risk assembly is not likely to experience a daily peak MC in 

excess of 28%. While there is a high probability that mold will grow on these assemblies, it is 

likely to be limited and/or negligible. A high risk assembly is likely to experience multiple days 

of peak daily MC in excess of 28%. Not only is this assembly likely to experience mold growth, 

the mold may be toxigenic. In general, walls with exterior insulation, or cavity insulation that 

behaves like exterior insulation (i.e., ccSPF), have a much lower risk than walls with exclusive 

cavity insulation (see Table 20).

0

10

20

30

40

50

60

70

80

90

13/10/01 13/12/31 14/04/01 14/07/01 14/10/01

Relative Humidity (%)

Date (year/month/day)

CMU with 3.5" of EIFS
71

Table 20. Moisture Durability Risk for Proposed Wall Assemblies in 

Select Climate Zones, Including Air Leakage

Modeled Climate Zone

Modeled City

Wall 1: Advance Frame

With

4 in. IS, 5.5 in. Batt

Wall 2: Advance Frame 

With

2 in. IS, 2 in. ccSPF, 

3.5 in. Batt

Wall 3: Advance Frame 

2 × 8 With 7.25 in. ccSPF

Wall 4: Double Stud 

With 9.5 in. CFI

Wall 5: Double Stud With

2 in. ccSPF, 7.5 in. CFI

Wall 6: Double Stud 

With 9.5 in. ocSPF

Wall 7: SIP With

11.25 in. EPS

Wall 8: Truss Wall 

With 9.5 in. CFI

2C Houston Low Moderate Low Moderate Low High Low Moderate

3 Atlanta Low Low Low High Low High Low High

4C Seattle Low Low Low High Low High Moderate High

4 St. Louis Moderate Low Low High Low High High High

5 Chicago Low Low Low High Low High High High

7 Int’l

Falls Low Low Low High Low Moderate High High

3.6 Cost Considerations

High-R walls are specialized assemblies designed to provide significantly better thermal 

performance, comfort, and moisture durability relative to standard or conventional wall 

assemblies. High-R walls have not been adopted by any large production builders; however, a 

number of smaller (i.e., fewer than 50 units/yr) builders have successfully developed near netzero energy houses that employ high-R wall assemblies (Ueno et al. 2012).

When analyzed from a strict capital cost or energy payback period (neglecting ancillary costs, 

such as space heating costs during construction), high-R walls are not cost competitive with the 

code minimum wall. The reason is because the standard code wall does not provide the same 

level of R-value than high-R walls. Table 21 and Table 22 summarize the costs for a 

conventional code-built wall assembly and the four walls analyzed in this report. Conventional 

walls appear to be less expensive because they use fewer building materials and have lower labor 

costs. To make the comparison more relevant, analyzing the cost per R-value normalizes for the 

differing thermal resistances of the walls.
72

Table 21. Total and Area Cost for Advance Framed High-R Walls1

Conventional 

“Code-Built” 

Wall

Exterior 

Insulated,

Advance

Framed Wall

Exterior 

Insulated, 

Hybrid

Advance

Framed Wall

Advance Framed 

Wall- 7.25 in.

ccSPF Cavity

Product Cost

($) Product Cost Product Cost ($) Product Cost($)

WRB Tyvek 0.35 Tyvek 0.35 Tyvek 0.35 Tyvek 0.35

Exterior 

Insulation NA

4-in. FF 

PIC (R24)3.54

2-in. FF 

PIC (R12)1.77 NA

Exterior

Sheathing

7/16 in.

OSB 0.98 7/16 in.OSB 0.98 7/16 in.OSB 0.98 7/16 in.OSB 0.98

Framing

2 × 6 @ 

16 in.

o.c.

4.32

2 × 6

Adv @ 

24-in.

o.c.

3.57

2 × 6

Adv @ 

24-in.

o.c.

3.57

2 × 8

Adv @ 

24-in.

o.c.

3.89

Frame 

Insulation

1

FG batt 

(R-22) 1.17 FG batt (R-22) 1.17

2-in.

ccSPF 

(R-10)

1.66

7.25 in.

ccSPF 

(R-36)

6.64

Frame 

Insulation

2

NA NA FG batt 

(R-13) 0.85 NA

Interior

Sheathing NA NA

Interior

Finish

1⁄2-in.

GWB 1.94 1⁄2-in.GWB 1.94 1⁄2-in.GWB 1.94

Total 

$/ft2 8.76 11.55 11.12 13.48

Nominal

R-Value 22 46 35 36

Total 

$/ft2/R 0.40 0.25 0.32 0.37

Notes:

1 Data from RS Means 2011 for Boston

2 CFI cost is for loose fill, not dense-pack 3 SIP cost includes materials + 30% for fabrication. RSMeans data not available for 11.25 in. EPS.
73

Table 22. Total and Area Cost for Double Stud High-R Walls1

Conventional 

“Code-Built” 

Wall

Cellulose 

Insulated

Double Stud 

Wall

Hybrid 

Insulated

Double Stud 

Wall

Double Stud 

Wall With 

ocSPF

11.25 in. EPS 

SIP Wall

Product Cost Product Cost Product Cost Product Cost Product Cost

WRB Tyvek 0.35 Tyvek 0.35 Tyvek 0.35 Tyvek 0.35 Tyvek 0.35

Exterior 

Insulation NA NA NA NA NA

Exterior

Sheathing

7/16 in. 

OSB 0.98 7/16 in. OSB 0.98 7/16 in. OSB 0.98 7/16 in. OSB 0.98

11.25

in. SIP3

7/16 in. 

OSB

11.25

in. EPS 

(R-45)

7/16 in. 

OSB

11.02

Framing

2 × 6 @ 

16 in.

o.c.

4.32

2 × 4 @ 

16 in.

OC 

outer

2 × 4 @ 

24-in. 

o.c.

inner

6.36

2 × 4 @ 

16 in.

OC 

outer

2 × 4 @ 

24-in. 

o.c.

inner

6.36 6.36

Frame 

Insulation

1

FG batt 

(R-22) 1.17

9.25 in.

CFI2

(R-35)

3.28

2-in. 

ccSPF 

(R-10)

1.66 9.5 in. 

ocSPF 3.94

Frame 

Insulation

2

NA NA

7.25 in. 

CFI 

(R-27.5)

2.69 NA

Interior

Sheathing NA NA NA NA

Interior

Finish

1⁄2-in. 

GWB 1.94 1⁄2-in. GWB 1.94 1⁄2-in. GWB 1.94 1⁄2-in. GWB 1.94 1⁄2-in. GWB 1.94

Total 

$/ft2 8.76 12.91 15.64 13.57 13.31

Nominal

R-Value 22 35 37.5 35 45

Total 

$/ft2/R 0.40 0.37 0.42 0.39 0.30

Notes:

1 Data from RS Means 2011 for Boston

2 CFI cost is for loose fill, not dense-pack 3 SIP cost includes materials + 30% for fabrication. RSMeans data not available for 11.25 in. EPS.

Based on the comparisons, all four of the high-R walls cost less than the conventional “codebuilt” walls when normalized for the nominal R-value. The exterior insulated advance framed

wall is more cost competitive than the other high-R walls. It should be noted that the associated 

unit area costs are susceptible to regional cost variations and availability of the material. The data 

above represent only a general cost comparison and may be significantly different depending on 

the region. Region specific cost calculations should be conducted prior to adoption of a high-R 

wall insulation strategy.
74

4 Recommendations and Conclusions 

Modern high R-value walls—walls that are constructed to have a significantly higher than code 

prescribed thermal performance—are more at risk of moisture-related problems than walls that 

have a higher thermal conductance and elevated levels of air leakage. There are several factors 

that contribute to this risk (Lstiburek 2010). First, greater control of heat and air movement 

reduces the potential for drying to occur. Thus, walls that get wet tend to stay wet for longer and 

accumulating moisture may exceed the capacity of the wall assembly materials to store or 

redistribute moisture safely. Second, modern buildings are built using lighter-weight nonabsorptive assemblies, which have less potential for safe storage of moisture, and engineered 

materials, which are often less tolerant to moisture. Additionally, interior and exterior finish 

layers in modern construction tend to have lower vapor permeance and therefore further restrict 

potential drying. Buildings constructed this way are more energy efficient, more resource 

efficient, and typically can be constructed more quickly and at lower cost—but require greater 

attention to detail when it comes to the control of moisture.

Any construction assembly that is part of the building enclosure must be designed and built to 

manage the risk of moisture-related damage. Moisture in buildings or in building enclosure 

elements may degrade building materials, support mold growth, and, of course, may damage the 

contents enclosed within. Moisture may come from several sources: bulk water intrusion from 

rain, melting snow, surface runoff, or groundwater; bulk water from interior flooding; moisture 

in the exterior environment; moisture generated internally by occupants; moisture moving 

through materials in contact with the ground; and moisture that has been “built-in” to the 

building during construction. Moisture from all of these sources must be managed to ensure 

building durability and occupant health.

In this study, hygrothermal simulations were prepared to assess the moisture performance of 

representative high R-value walls in a range of climate zones and a number of variables affect 

the risk were examined (climate, cladding type, insulation type and location, etc.). Four research 

questions were addressed by this project. Each is discussed below.

1. What is the role of insulation levels on the risk in different climates?

Increasing amounts of insulation increase the risk of moisture problems in nearly all climate 

zones. Decreased vapor permeance of the insulation and decreased heat fluxes combine to 

exacerbate any moisture susceptibilities inherent in the system. However, the location of the 

insulation is more important than the quantity of insulation. A given amount of insulation 

installed outboard of the wood sheathing intrinsically grants more moisture resistance to the wall 

assembly than the equivalent amount in a wall cavity. The elevated sheathing temperature

provided by the exterior insulation protects the sheathing from moisture damage by enabling 

higher rates of evaporation and lower rates of condensation. Cavity insulation hampers drying of 

the sheathing by keeping it at a lower temperature, developing conditions prone to air leakage 

condensation and minimized evaporation.

Colder climates require a greater amount of insulation than warmer climates, which makes the 

location of the insulation even more imperative for the proper functioning and moisture 
75

durability of the wall assembly. Exterior insulation is the best approach for insulating buildings 

in cold climates.

2. How resistant are the walls to air leakage and vapor diffusion in different climates?

Two wall types, exemplifying the exterior insulation versus cavity insulation strategy, were 

simulated in four climates to assess the limits of vapor diffusion and air leakage. The two wall

types were selected to represent a moisture-tolerant (advance framed wall with 4 in. of exterior 

insulation) and a moisture-sensitive assembly (double stud wall with 9.5 in. of CFI). The tested

walls all perform well in ideal circumstances (i.e., no imposed sources of moisture) in all 

climates, with the exception of extremely high MCs in very cold climates (i.e., 40% indoor RH 

in International Falls). In following the building code requirements, vapor diffusion is not an 

issue for any of the walls. However, air leakage—which is an assembly property and varies by 

construction—leads to air leakage condensation and deposits an order of magnitude more water 

than vapor diffusion while bypassing the vapor retarding layers. Further, vapor retarding layers 

also inhibit drying in one direction, and in hot and humid climate, generate significant 

condensation from moisture-laden outdoor air infiltrating into the wall assembly.

Air leakage condensation occurs only if the sheathing temperature falls below the indoor air dew

point temperature and if air contacts the sheathing. Only the exterior insulated wall assembly 

effectively manages air leakage condensation. The advance framed wall mitigates air leakage 

condensation by keeping the sheathing temperature closer to indoor temperatures. 

The most resistant walls to air leakage and vapor diffusion are exterior insulated walls, or walls 

with cavity insulation that behave as exterior insulation (i.e., ccSPF insulation eliminating 

connectivity of the exfiltrating indoor air with the cold sheathing), while maintaining interior 

RHs to adequate levels given the local climate (typically, < 30% RH in cold climates, and 40% 

in temperature climates).

3. What are the drying rate capacities of the proposed wall assemblies?

To simulate the drying rate capacities of the walls, the simulations started with the sheathing set 

to the fiber saturation point (28% MC). The simulations started on October 1 and were left to run 

for an entire year.

The most important aspect learned from the simulations is that the drying capacities of the walls 

are highly dependent on the outdoor environment. A wall with a wet and cool climate will not be 

able to dry as fast as the equivalent wall in a warm and dry climate. The second key finding is 

that walls that incorporate vapor-impermeable membranes generally dry out slower than those 

without. This is a result of throttling or elimination of the wall’s capacity to dry out to either the 

interior or the exterior.

Overall, the advance framed wall dries at a faster rate than the other remaining walls, provided 

that no greater than a Class III vapor retarder is used. The double stud wall with cellulose 

decreases the MC of the OSB at the expense of elevated MC of the cellulose. Effectively, the 

moisture is redistributed within the wall only and does not actually dry. The cellulose acts as a 

sink for both construction moisture and bulk water leakage. The additional moisture is trapped 

within the system when polyethylene is used. This may be a potential source of moisture damage 
76

to the double stud walls. The sheathing in both the SIP wall and the double stud wall with ccSPF 

can dry only to the exterior. Consequently, the drying capacities are dependent on the permeance 

of the WRB and the climate.

4. What are the high-level steps necessary to build moisture-resistant high-R wall 

assemblies?

There are few challenges to building high-R walls that are moisture durable. Much of the 

information is publicly available on a range of websites and publications. Further, the IRC 

enables designers, builders, and architects to design and build wall assemblies that possess high 

thermal resistance and are moisture resistant.

The biggest problem facing high-R wall assemblies is the initial capital investment required. In 

many areas, the least expensive option that complies with the building code will be used. Further, 

high-R walls are only especially beneficial in colder climates where heating costs are high and 

the risk of occupant discomfort is considerable. High-R walls are frequently seen only in custom 

homes where the owner desires high levels of thermal resistance. There are no significant 

barriers on making these walls moisture tolerant aside from proper design, detailing, and 

construction in adherence with the code and any manufacturer’s instructions.

4.1 Future Considerations

Looking forward, more of this type of analysis needs to be done on other approaches to high Rvalue walls. However the models need to be verified empirically. To validate the models, 

empirical research on small- and full-scale assembly mockups and buildings, with monitored 

boundary conditions and instrumented with temperature and moisture sensors, needs to be 

undertaken to examine and fully quantify these risks.

The analysis conducted for this report did not include extremely high R-value products, such as

vacuum insulated panels, or aerogels. These materials are considered to be cost prohibitive for 

most projects and not accessible to typical developers and builders, but this may not be the case 

in the future. As a general comment, moisture control for new systems and new materials should 

be carefully considered—particularly when little practical field experience exists.
77

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80

Appendix A: Representative Detail Drawings

The following water management detail drawings are included in this appendix.

Exterior Insulation Approach (e.g., Insulating Sheathing over Advanced Framing)

• IS-1 Above Grade Wall to Roof

• IS-2 Foundation Wall to Above Grade Wall

• IS-3 Window Head Detail

• IS-4 Window Sill Detail

• IS-5 Window Jamb Detail

• IS-6 Window Sequence

• IS-7 Window Sequence

• IS-8 Mechanical Penetration

• IS-9 Mechanical Penetration Sequence

Interior Insulation Approach (e.g., Double Stud Wall)

• DSW-1 Above Grade Wall to Roof

• DSW-2 Foundation Wall to Above Grade Wall

• DSW-3 Window Head Detail

• DSW-4 Window Sill Detail

• DSW-5 Window Jamb Detail

• DSW-6 Mechanical Penetration

Concrete Masonry Unit Approach

• CMU-1 Above Grade Wall to Roof

• CMU-2 Foundation Wall to Above Grade Wall

Structural Insulated Panel Approach

• SIPS-1a Above Grade Wall to Roof

• SIPS-1b Above Grade Wall to Roof

• SIPS-2 Foundation wall to Above Grade Wall




















101

Appendix B: Inputs for Hygrothermal Simulations
102

Building America- 2.2- Moisture Management in High-R Walls
103
104
105
106
107
108
109
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buildingamerica.gov

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