Moisture Research—

Optimizing Wall Assemblies

Lois Arena and Pallavi Mantha

Consortium for Advanced Residential Buildings

May 2013
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iii

Moisture Research—Optimization of Wall Assemblies

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

Lois Arena and Pallavi Mantha

Steven Winter Associates, Inc.

of the 

Consortium for Advanced Residential Buildings (CARB) 

61 Washington Street 

Norwalk, CT 06854 

NREL Technical Monitor: Cheryn Metzger 

Prepared under Subcontract No. KNDJ-0-40342-00 

May 2013 
iv

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v 

Contents

Contents....................................................................................................................................................... v

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................ viii

Definitions ................................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

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

1.1 Background ..........................................................................................................................2

1.2 Report Organization .............................................................................................................3

2 Method................................................................................................................................................... 4

2.1 Modeling ..............................................................................................................................4

2.1.1 Tools ........................................................................................................................4

2.1.2 Boundary Conditions ...............................................................................................4

2.1.3 Basic Assumptions ...................................................................................................5

2.2 Failure Criteria .....................................................................................................................6

2.2.1 Moisture Content in Oriented Strand Board (Rot/Decay) .......................................6

2.2.2 Assembly Moisture Content ....................................................................................6

2.2.3 Condensation Potential ............................................................................................6

2.2.4 ASHRAE Criteria ....................................................................................................7

2.2.5 Isopleths ...................................................................................................................7

3 Code Minimum Walls ........................................................................................................................... 9

3.1 Introduction ..........................................................................................................................9

3.2 Wall Construction Details ....................................................................................................9

3.3 Results ................................................................................................................................12

3.3.1 Moisture Content in Oriented Strand Board (Rot/Decay) .....................................12

3.3.2 Assembly Moisture Content ..................................................................................14

3.3.3 Condensation Potential ..........................................................................................15

3.3.4 ASHRAE Criteria ..................................................................................................17

3.3.5 Isopleths .................................................................................................................19

3.3.6 Results Summary for Code Built Walls .................................................................19

3.4 Discussion ..........................................................................................................................23

3.4.1 Comparison to Fiberglass-Only Insulation ............................................................23

3.5 Conclusions ........................................................................................................................24

4 High-R Walls........................................................................................................................................ 26

4.1 Wall Construction Details ..................................................................................................26

4.1.1 Vented Cladding Assumption ................................................................................28

4.2 Results ................................................................................................................................29

4.2.1 Moisture Content in Oriented Strand Board (Rot/Decay) .....................................29

4.2.2 Assembly Moisture Content ..................................................................................29

4.2.3 Condensation Potential ..........................................................................................30

4.2.4 ASHRAE Criteria ..................................................................................................31

4.2.5 Isopleths .................................................................................................................32

4.3 Discussion ..........................................................................................................................32

5 Brick Walls With Interior Insulation.................................................................................................. 33

5.1 Introduction ........................................................................................................................33

5.2 Wall Construction Details ..................................................................................................33

5.3 Material Properties .............................................................................................................35
vi 

5.4 Additional Failure Criteria (Masonry Walls) .....................................................................36

5.4.1 Freeze-Thaw Damage ............................................................................................36

5.4.2 Drying of Masonry Walls ......................................................................................37

5.5 Results ................................................................................................................................37

5.5.1 ASHRAE Criteria ..................................................................................................37

5.5.2 Condensation Potential ..........................................................................................38

5.5.3 Freeze-Thaw Damage ............................................................................................39

5.5.4 Maximum Moisture Content ..................................................................................42

5.5.5 Drying of Masonry Walls ......................................................................................43

5.5.6 Isopleths .................................................................................................................46

5.6 Discussion ..........................................................................................................................46

6 General Discussion............................................................................................................................ 47

6.1 Condensation Potential ......................................................................................................47

6.2 ASHRAE 160-2009 Failure Criteria ..................................................................................47

7 General Conclusions.......................................................................................................................... 48

References................................................................................................................................................. 49
vii 

List of Figures

Figure 1. Isopleth graph for wall 4A1 predicting no potential for mold growth on the interior face 

of the drywall over a 3-year period ..................................................................................................... 8

Figure 2 Typical wall construction for the walls built to code; cavity depth and foam levels vary 

depending on the climate .................................................................................................................... 9

Figure 3. Predicted moisture content in OSB: ASHRAE 160-2009 interior conditions...................... 13

Figure 4. Predicted moisture content in OSB: sine curve interior conditions ................................... 13

Figure 5. Comparison of OSB MC using ASHRAE 160-2009 and sine curve interior conditions..... 14

Figure 6. Difference in predicted assembly MC from beginning to end of modeling period: 

ASHRAE 160-2009 versus sine curve interior conditions.............................................................. 15

Figure 7. Condensation potential for the different levels of MDSPF for two different sets of indoor 

conditions: sine curve and the ASHRAE 160-2009 method........................................................... 16

Figure 8. Percentage of the total number of averages that each wall failed the ASHRAE 160-2009 

criteria: ASHRAE 160-2009 interior conditions ............................................................................... 18

Figure 9. Percentage of the total number of averages that each wall failed the ASHRAE 160-2009 

criteria: sine curve interior conditions............................................................................................. 19

Figure 10. Typical high-R wall with exterior XPS insulation................................................................. 26

Figure 11. Typical high-R wall with SPF insulation ............................................................................... 26

Figure 12. Typical double stud high-R wall with cellulose insulation ................................................. 27

Figure 13. High-R walls: predicted % MC in OSB when using the ASHRAE 160-2009 method to 

generate the indoor conditions (cooling was assumed in all climates) ....................................... 29

Figure 14. High-R walls: percentage of averages that each wall failed the ASHRAE 160-2009 

criteria.................................................................................................................................................. 32

Figure 15. Cross sections of the 12-in.-thick brick walls evaluated: (A) uninsulated, interior plaster 

finish; (B) 1-in. air gap, 2 × 4 steel studs with R-13 fiberglass batt insulation and gypsum board 

interior finish; and (C) 1-in. R-5 foam, 2 × 4 steel studs with R-13 fiberglass batt insulation and 

gypsum board interior finish............................................................................................................. 34

Figure 16. Percent of averages that each wall failed the ASHRAE 160-2009 criteria ........................ 37

Figure 17. Condensation potential for the masonry walls.................................................................... 38

Figure 18. Temperature gradient across the wall sections (highlighted dotted line indicates 

potential condensation plane)........................................................................................................... 39

Figure 19. Freeze-thaw potential of an uninsulated wall with high-density, low-permeability face 

brick ..................................................................................................................................................... 40

Figure 20. Freeze-thaw potential of uninsulated wall with medium-density, high-permeability face 

brick ..................................................................................................................................................... 40

Figure 21. Freeze-thaw potential of an uninsulated wall with high-density, low-permeability face 

brick ..................................................................................................................................................... 41

Figure 22. Freeze-thaw potential of a wall with fiberglass cavity insulation and high-density, lowpermeability face brick....................................................................................................................... 42

Figure 23. Comparison of the maximum MC in each of the three wythes of all the walls ................ 43

Figure 24. Effect of interior insulation on the drying time of wythe 2 in all the walls with highdensity, low-permeability face brick................................................................................................. 44

Figure 25. Effect of interior insulation on the drying time of wythe 3 in all the walls with highdensity, low-permeability face brick................................................................................................. 44

Figure 26. Effect of interior insulation on the drying time of wythe 2 in all the walls with mediumdensity, high-permeability face brick ............................................................................................... 45

Figure 27. Effect of interior insulation on the drying time of wythe 3 in all the walls with mediumdensity, high-permeability face brick ............................................................................................... 45

Unless otherwise noted, all figures were created by CARB.
viii

List of Tables

Table 1. ASHRAE Standard 160-2009 Interior Boundary Conditions .................................................... 5

Table 2. Exterior and Interior Surface Conditions Used in WUFI Analysis........................................... 5

Table 3. Climate Zones and Representative Cities.................................................................................. 5

Table 4. Average Outdoor Winter Temperature for Climate Files Evaluateda ...................................... 7

Table 5. Code Built Wall Assemblies Evaluated .................................................................................... 10

Table 6. THERM Analysis of Temperatures at the Interior Face of the MDSPF at the Center of the 

Cavity and at the Stud........................................................................................................................ 17

Table 7. Results Summary for Code Built Walls.................................................................................... 20

Table 8. Comparison of Failure Criteria Results for Climate Zone 5A: R-20, Fiberglass Batt Code 

Wall Versus R-25 Hybrid Wall with MDSPF and Blown Fiberglass ............................................... 23

Table 9. High-R Wall Assemblies Evaluated .......................................................................................... 28

Table 10. High-R Walls: Difference in Predicted Assembly Moisture Content From Beginning to 

End of Modeling Period ..................................................................................................................... 30

Table 11. Condensation Potential for High R-Value Walls.................................................................... 31

Table 12. THERM Analysis of Temperatures at the Interior Face of the Foam................................... 31

Table 13. Masonry Wall Assemblies Evaluated ..................................................................................... 34

Table 14. Material Properties for Brick from WUFI Pro 5.1................................................................... 36

Table 15. Free Saturation and Critical Saturation of Brick Analyzed .................................................. 36

Unless otherwise noted, all tables were created by CARB.
ix

Definitions

ACH Air changes per hour

ASTM American Society for Testing and Materials

Btu British thermal unit

CARB Consortium for Advanced Residential Buildings

ft2 Square foot

ft3 Cubic foot

h Hour

IBP Institute for Building Physics

in. Inch

IRC International Residential Code

lb Pound

LIM Lowest isopleth for mold

MC Moisture content

MDSPF Medium-density spray polyurethane foam

NRCC National Research Council Canada

OSB Oriented strand board

RH Relative humidity

SPF Spray polyurethane foam

THERM Two-dimensional heat transfer tool 

vol % Volume percent

WUFI Wärme- und Feuchtetransport Instationär 

XPS Extruded polystyrene
x 

Executive Summary

The Consortium for Advanced Residential Buildings (CARB) evaluated several different 

configurations of wall assemblies to determine the accuracy of moisture modeling and make 

recommendations to ensure durable, efficient assemblies. WUFI1 and THERM2 were used to 

model the hygrothermal and heat transfer characteristics of these walls. Assemblies evaluated 

include the following: 

• Code minimum walls using spray foam insulation and fiberglass batts 

• High R-value walls at least 12 in. thick: R-40 and R-60 assemblies

• Brick walls with interior insulation. 

Code minimum walls were evaluated in climate zones 4 through 7 and in the dry, moist, and 

marine moisture regimes. Three different classes of vapor retarder were analyzed along with 

varying thicknesses of exterior and cavity insulation. Three different methods of constructing 

high R-value (high-R) walls were analyzed—two employing hybrid insulation strategies and one 

double-stud cellulose assembly. Varying combinations of foam and fibrous insulation were 

evaluated with and without vented cladding in climate zones 4, 5, 6, and 7, all in moisture regime 

A. Finally, two different brick assemblies with different brick densities and insulation strategies 

were modeled in climate zone 4A. 

All walls were analyzed against currently accepted failure criteria including condensation 

potential, moisture content thresholds, drying capacity of the assembly, potential for mold 

growth, and freeze-thaw damage with respect to brick. 

While the majority of the code minimum walls analyzed fail the ASHRAE 160-2009 30-day 

criteria, the walls with Class III vapor retarders and the lowest levels of foam required by code 

fail the ASHRAE criteria by a much higher margin than the rest. In addition, condensation 

potential is near or above 20% for most of the walls with that configuration. 

The analysis of high R-value walls indicates that assemblies constructed with the oriented strand 

board (OSB) on the exterior of all the insulation should employ a vented cladding to assist in 

drying of that layer regardless of the type of insulation in the cavity. Drying to the interior is 

severely limited in these walls, therefore, drying to the exterior must be enhanced. 

High R-value walls constructed with the OSB sandwiched between a moisture permeable 

insulation on the interior and an impermeable insulation on the exterior should maximize the 

ratio of impermeable to permeable insulation. This keeps the OSB as warm as possible, reduces

the potential for condensation, and promotes drying to the interior. Based on modeling results, a 

minimum of 50% of the total cavity wall R-value should be provided by the impermeable 

insulation in climate zones 4 through 6, and 60% is recommended in climate zone 7. 

 

1

 Wärme-und Feuchtetransport instationär (Transient Heat and Moisture Transport), a moisture prediction tool 

created by the Fraunhofer Institute for Building Physics and Oak Ridge National Laboratory 2 Two-dimensional heat transfer tool created by Lawrence Berkeley National Laboratory
xi 

Before insulation retrofits are undertaken in buildings with structural brick walls, the 

hygrothermal properties of the brick should be determined. Properties vary so widely across the 

country that no generic recommendations for insulation strategies can be given here. Analysis 

shows that the insulation strategies appropriate for one type of brick are not necessarily 

appropriate for another. Criteria that should be analyzed include freeze-thaw cycles in 

conjunction with the critical saturation threshold and drying potential of the masonry. Analysis 

periods longer than 3 years are recommended when modeling. 

Further research into the appropriateness of the ASHRAE 160-2009 interior conditions in moist 

climates is needed. Interior relative humidity levels generated with this method are higher than 

recorded in actual studies (Arena et al. 2010) and result in overly pessimistic predictions for 

mold growth on the interior of the assembly. 

Considering that almost every wall in this study failed the ASHRAE 30-day criteria, it is 

recommended that this threshold be reevaluated by industry professionals. 
1 

1 Introduction

Moisture problems within the building shell can be caused by a number of factors including 

excess interior moisture that is transported into the wall through air leakage and vapor drive, bulk 

water intrusion from leaks and wind-driven rain, capillary action from concrete to wood 

connections, and through wetted building materials such as siding wetted from rain splash back. 

Depending on the temperature of the surfaces and the permeability of the materials, that moisture 

can get trapped inside the walls, potentially leading to mold growth, decay of the building 

materials, or both. 

For mold to grow on the surface, the mean monthly relative humidity (RH) of the air next to the 

surface must be approximately 80% or greater. Decay of wooden building components, also 

caused by fungi, requires temperatures between 23°F and 113°F (there is little growth below 

50°F), exposure to air, and a wood moisture content (MC) greater than 30% (ASTM 2001). 

Because most untreated wood is dryer than this, it usually takes wetting by some other means 

such as window or roof leaks, foundation moisture, splash back, or excessive condensation for 

the wood to reach this level of MC. 

In an effort to provide a better understanding of the various mechanisms and interactions of 

moisture and heat transfer in building envelopes, transient mathematical models have been 

developed. There has been a rapid improvement in the capabilities of computer-based moisture 

analysis tools that can predict the movement and accumulation of moisture in building 

components and materials. One of those tools, WUFI, was created by the Fraunhofer Institute for 

Building Physics and Oak Ridge National Laboratory. WUFI predicts moisture transfer by 

diffusion and capillary flow. It allows users to assess the effectiveness of the wall materials and 

construction assemblies against moisture flow and indicates area where condensation can occur.

According to ASTM (2001), although WUFI is a highly validated model for hygrothermal 

applications, it has several limitations, which include the following: 

• Because WUFI deals only with one-dimensional processes, it cannot adequately model 

multidimensional thermal and moisture bridges.

• Airflows in the component, uptake of groundwater, and gravity effects have been 

neglected. 

• Some materials like wood and concrete can change their material properties as a function 

of their present and past MC and, as a result, do not lend themselves to simplified 

transport equations.

This research study is intended to analyze the hygrothermal performance of different wall 

assemblies that are gaining popularity in the market, but have yet to be comprehensively 

monitored with respect to moisture and heat transfer. In addition to the modeling results 

presented here, CARB will be collecting moisture data on a high-R, double-stud cellulose wall in 

climate zone 5 and a retrofit brick assembly using a hybrid insulation strategy in climate zone 4. 

Results of the data collection will be published at the end of 2012. 
2 

1.1 Background

Over the past 30 years, significant research has been conducted on moisture levels in homes and 

inside building components. In Chapter 13 of ASTM Manual 18, Moisture Control in Buildings: 

The Key Factor in Mold Prevention (ASTM 2009), George Tsongas reviews some of the most 

relevant case studies of moisture problems and research activities in residential buildings over 

the last several decades. But, in his summary, Tsongas states, “In mixed use climates, especially 

those with high humidity in the summer, best practices are often debated. Unfortunately, there 

are very few studies involving buildings in mixed climates.” 

Viitanen and Salonvaara echo this need for field research in Chapter 4 of ASTM Manual 40, 

Moisture Analysis and Condensation Control in Building Envelopes (ASTM 2001), in their 

discussion of failure criteria. They argue that there are several standards for testing the resistance 

or durability of materials against mold and decay fungi, but that the results of the tests are often 

relative. In reality, conditions are often different in the field and many factors affect durability 

and performance. Like Tsongas, they feel that field tests in real conditions and in different 

climates should be performed. 

With the increased use of foam insulation, various vapor barrier applications, the drastic increase 

in retrofit activities, and the increasing thickness of the walls (all factors that can reduce the 

drying potential of the walls) moisture issues could potentially become much bigger problems. 

High-R wall assemblies and assemblies employing a combination of insulation products are 

gaining popularity in the market because of programs like Passive House, net zero energy home 

challenges in several states, and highly incentivized retrofit programs. New insulation products, 

code changes, and the desire to reduce costs to achieve these new efficiency levels are also 

factors behind the various assemblies available today. 

Although several people have performed extensive analysis on some of these assemblies using 

moisture modeling software, little field research has been conducted on high-R walls and 

masonry retrofits to validate the results. CARB intends to monitor different wall assemblies that 

have not been previously monitored to determine the accuracy of moisture modeling and make 

recommendations to ensure durable, efficient assemblies. Climate zones of the greatest interest 

are 4A, 5A, 6A, and 7. These zones experience both cooling and heating seasons as well as 

considerable humidity during the summer which will likely reduce the drying potential of the 

wall assemblies. Assemblies of the greatest interest include the following: 

• Brick walls plus interior insulation. Builders are faced with the proper way to insulate 

brick masonry walls in both new construction and retrofit applications. Typical 

applications usually include an air space, framing, and batt insulation between the studs. 

Increasing insulation requirements and stricter air leakage targets are forcing builders to 

evaluate the best strategies while balancing costs and maximizing living space. Common 

solutions are to install foam (spray or rigid boards) between the studs and the brick, 

thereby increasing the R-value while maintaining the interior footprint. Several modeling 

studies have examined freeze-thaw cycles and condensation and moisture potential in 

these assemblies (Wilkinson et al. 2009; Khudaverdian 2007; Straube 2007; Tariku and 

Jumaran 2006; Sedlbauer and Kunzel 2000), but other than a study conducted by Said 

and colleagues (2003) from 1995 to 1997 on a brick wall retrofitted with interior 
3 

insulation, little monitored data are available. Said’s retrofit study did not involve any 

foam between the interior frame wall and the brick, as is being proposed here. 

• Superinsulated walls. Walls with R-values in the R-40 to R-60 range are becoming more 

prevalent. Passive house and net zero energy home programs are a couple of driving 

forces behind the increased number of superinsulated walls. These walls can be 

constructed with a single insulation product or a hybrid of two or more products such as 

spray foam and blown-in cellulose or fiberglass. In a previous Building America special 

research project conducted by Straube and Smegal (2009), several different walls with Rvalues near 40 were evaluated in Climate Zone 6 using WUFI. The National Research 

Council Canada (NRCC) and the Cold Climate Housing Research Center are conducting 

research and performing monitoring in extreme cold climates on a variety of wall 

assemblies and R-values, but the research is limited to those climates, which affect a very 

small percentage of homeowners in the United States3. 

• 2009 International Energy Conservation Code compliant walls (spray foam 

insulation plus fiberglass batts). In recent editions of the International Residential Code 

(IRC), minimum R-values for foam insulation have been specified when employing 

hybrid insulation strategies in ceilings and walls (IRC 2006). This is intended to prevent 

condensation from forming on the inside face of the foam. Table 601.3.1 in the 2009 IRC 

lists prescriptive R-values for exterior rigid insulation that must be met to eliminate an 

interior vapor barrier. A note under the table states that these same levels should be met if 

using spray foam inside the cavity. This has raised some questions in the industry. Some 

feel that these levels are too high; others believe that they are too low and could result in 

moisture problems within the wall cavities. The Spray Polyurethane Foam Alliance has 

teamed with CARB to analyze the condensation potential of these assemblies in climate 

zones 4–7 using WUFI. Field verification of these results is intended. 

The goals of this study are to (1) evaluate predictions from WUFI and develop recommendations 

for the best combination of components for the listed wall assemblies and (2) to supplement 

existing guidelines for producing energy efficient, durable wall systems that meet the changing 

requirements of codes, new programs, and the construction industry. 

1.2 Report Organization

This research report is organized into one main section and three subsequent subsections for each 

of the wall categories described in the previous section. The main section discusses the research 

methodology, basic software assumptions, limitations, and failure criteria. The main section aims 

to set the context for the reader before presenting a detailed explanation of wall constructions, 

analysis, and results.

Each subsequent section is based on a wall type: walls constructed to the minimum code 

requirements, superinsulated walls, and brick masonry walls. Wall construction details, relevant 

assumptions, results, and discussion specific to each assembly can be found in each of the 

respective sections. 

 

3 http://www.gwscientific.com/cchrc/rtf_data/REMOTE/remote.html
4 

2 Method

2.1 Modeling

2.1.1 Tools

Hygrothermal modeling was performed with WUFI, a computer-based moisture analysis tool 

that can predict the movement and accumulation of moisture in building components and 

materials. WUFI predicts moisture transfer by diffusion and capillary flow. It allows users to 

assess the effectiveness of the wall materials and constructions with respect to moisture flow and 

areas where condensation can occur.

To evaluate the thermal bridging effects of the framing members and the condensation potential 

at the intersection of the insulation and framing, CARB used THERM, which can model twodimensional heat-transfer effects in buildings. Local temperature patterns in an assembly can be 

mapped in THERM, allowing the user to analyze the potential for condensation, moisture 

damage, and structural integrity. Using THERM, CARB evaluated the condensation potential of 

the framing/insulation intersections. 

2.1.2 Boundary Conditions

WUFI offers several different methods for generating interior temperature and RH levels. For 

this study, the interior conditions for all three wall types were generated using the ASHRAE 160-

2009 method. It should be noted that, in all climates, the interior RH levels predicted by this 

method reach 90% even though cooling was assumed. Using these interior conditions, WUFI 

predicts that there is the potential for mold growth on the interior surface of the drywall in all 

climates. Realistically, we know that this is not true. Except for bathrooms with inadequate 

ventilation, little mold is found on interior walls in any of these climates if no unusual 

circumstances are present. Because of this, several other methods of generating the interior 

conditions were analyzed, but all have drawbacks. 

The most popular method used in other studies is to generate a sine curve where the interior 

conditions typically range from 68°F–72°F and 40% to 60% RH. A comparison of the results 

between ASHRAE 160-2009 interior conditions and the standard sine curve is provided for the 

code minimum walls as a point of reference. In dry climates this is probably too high an estimate 

of annual RH, and in moist climates it might be too low. It can be argued that in cold climates, 

hygrothermal analysis of these building assemblies should assume interior wintertime RH levels 

below 40% (Straube and Smegal 2009; Lstiburek 2004). Where this can be the case in older, 

leakier homes, homes built to today’s code requirements or better could be seeing higher RH 

levels because of the tightness of the building shell, even when constructed with mechanical 

ventilation levels as prescribed in ASHRAE Standard 62.2 (2010). Data collected in an effort to 

validate the assumptions for the interior conditions in ASHRAE Standard 160-2009 (Arena et al. 

2010) show a range of wintertime indoor RH levels in 20 different homes in climate zone 5 

(mid-30% to mid-50% RH levels). Also, several professionals consulted recently who are 

actively monitoring homes built with lower than average natural air change rates have noted RH

levels above 40%. Therefore, a conservative analysis has been provided in this study and sine 

curves with RH levels below 40% were not evaluated.

Table 1 displays the values input for the ASHRAE 160-2009 option in WUFI. Air conditioning 

was assumed in all climates. 
5 

Table 1. ASHRAE Standard 160-2009 Interior Boundary Conditions 

Parameter Assumption

Heating Set Point 70°F

Cooling Set Point 75°F

Floating Indoor Temperature Shift 5°F

Moisture Generation Rate (lb/h) 1.3

Air Exchange Rate (1/h) 0.2 (equivalent of standard construction )

The boundaries for the since curve were set to 68°F to 72°F and 40% to 60% RH. In addition to 

these boundary conditions, exterior and interior surface conditions were also defined. Table 2

displays the values used in the analysis.

Table 2. Exterior and Interior Surface Conditions Used in WUFI Analysis

Surface Condition External Surface Conditions Internal Surface Conditions

Heat Resistance

(h∙ft2∙°F/Btu) 0.334 (default) 0.71 (default)

Moisture Resistance (perm) Zero (no coating) 10.7 (latex paint)

Radiation Absorption/ 

Emission

0.5/0.95

(oil paint, green, light) n/a 

Rainwater Absorption 0.7 (default) n/a

2.1.3 Basic Assumptions

All simulations assume a 2,400-ft2 house with three bedrooms and a volume of 19,200 ft3. In 

order to maintain uniformity among the different climates and generate a worst-case scenario, all 

frame wall assemblies were assumed to be oriented north, eliminating the drying effects of the 

sun. The brick walls were oriented according to the worst-case scenario for driving rain. WUFI 

allows the user to select a representative warm year weather file or a representative cold year. 

These files are based on the years in the warmest and coldest tenth percentile of the weather data 

available. This analysis was based on the cold year weather files.

Table 3. Climate Zones and Representative Cities

Climate Zone Representative City

4A Nashville, Tennessee

4B Albuquerque, New Mexico

4C Seattle, Washington

5A Detroit, Michigan

5B Elko, Nevada

6A Madison, Wisconsin

6B Billings, Montana

7A International Falls, Minnesota

The temperature and moisture content of the construction layers were assumed to be constant 

across the layers at the start of the simulation. Initial temperature and relative humidity within 
6 

each component were assumed to be 68°F and 80% respectively (representative of building 

materials during the construction phase).

All the simulations were run for a period of three consecutive years beginning on October 1, 

2011 (1,095 days/26,280 h). This run time allows for the wall to acclimatize and reduces the 

effects of the assumed initial moisture content and temperature within the wall assembly.

2.2 Failure Criteria 

CARB researched and analyzed currently accepted failure criteria for moisture levels in walls. 

While these are the cited in research and manuals on moisture performance in building 

assemblies, there are concerns with many of them. These concerns are explained in the General 

Discussion section at the end of this report. For completeness, all walls were evaluated with 

respect to the following thresholds and/or considerations. 

2.2.1 Moisture Content in Oriented Strand Board (Rot/Decay)

It is often quoted that that the minimum MC requirement for the growth of fungi is 

approximately 20% in wood corresponding to about 80%- 90% RH (Siau 1984). Decay generally 

occurs above 90%–95% at 68°F (ASTM 2001). The minimum, maximum and average MC of the 

OSB sheathing was calculated for each wall and evaluated against these thresholds. 

2.2.2 Assembly Moisture Content

The initial and final moisture content for the entire assembly was evaluated for each wall over a 

3-year period. The desired result was that the final moisture content was less than the initial at 

the end of the modeling period indicating that the walls have the potential to dry out over time. 

2.2.3 Condensation Potential

2.2.3.1 WUFI

Condensation potential within the wall was evaluated by comparing the interior air dew point 

temperature to the surface temperature of the potential condensing layer(s). If the surface 

temperature of the material is lower than the dew point temperature of the air (Straube and 

Smegal 2009), condensation is likely to occur: the longer the period during which the surface 

temperature falls below the air dew point temperature, the greater the risk for damage. The 

critical juncture analyzed was the first condensing surface in each assembly. 

Based on the indoor boundary conditions, the dew point was calculated on an hourly basis for the 

entire 3-year modeling period. The surface temperature of the condensing surface was then 

compared to that dew point. The percentage of time that the surface temperature fell below the 

dew point was calculated and tabulated for each wall. 

2.2.3.2 THERM

THERM was used to evaluate the condensation potential at the studs. The analysis was 

conducted at interior temperatures of 70°F and 68°F in order to compare the results to both the 

ASHRAE 160-2009 and the sine curve interior conditions. Exterior boundary conditions used for 

this analysis were calculated by averaging the mean temperatures for each climate for the months 

of December, January and February. Outdoor average temperatures for each of the three months 

and the corresponding overall averages are shown in Table 4. 
7 

Table 4. Average Outdoor Winter Temperature for Climate Files Evaluateda

Climate 

Zone Climate File December(°F)

January

(°F)

February

(°F)

Average 

(°F) 

4A Nashville, Tennessee 36.3 36.5 41.4 38.1 

4B Albuquerque, New Mexico 36.7 39.7 35.8 37.4 

4C Seattle, Washington 40.1 42.4 41.7 41.4 

5A Detroit, Michigan 24.3 25.3 30.0 26.5 

5B Elko, Nevada 23.2 32.5 25.7 27.1 

6A Madison, Wisconsin 14.7 19.6 19.8 18.0 

6B Billings, Montana 23.9 27.9 28.4 26.7 

7 International Falls, Minnesota 1.6 11.3 5.7 6.2 a See http://apps1.eere.energy.gov/buildings/energyplus/cfm/ 

weather_data2.cfm/region=4_north_and_central_america_wmo_region_4. 

2.2.4 ASHRAE Criteria

In 2009, ASHRAE published ASHRAE Standard 160-2009 Criteria for Moisture-Control 

Design Analysis in Buildings. This standard sets the performance criteria to minimize problems 

associated with moisture in building envelope assemblies. 

The standard specifies that the following conditions be met: A 30-day running average surface 

RH <80% when the 30-day running average surface temperature is between 41°F and 104°F. 

This threshold applies to all materials and surfaces in the building envelope except the exterior 

surface. The first condensing surface in each assembly analyzed was chosen as the surface to be 

evaluated with this criterion. 

Surface temperature and RH of the first condensing surface was averaged over a 30-day period 

and compared to the ASHRAE 160-2009 limits.

2.2.5 Isopleths

Graphs called isopleths were created for each of the walls to identify potential mold growth on 

the interior surface of the wall assembly and the interior face of the condensing surface. An 

isopleth system captures the germination time and growth rates of mold based on humidity and 

temperature. WUFI assigns a lowest isopleth for mold (LIM), which is the temperaturedependent, lowest RH under which no fungus activity is expected. 

Figure 1 shows a graph with limiting isopleths. Each point in this graph represents the 

hygrothermal conditions at the interior surface of the assembly at a certain time. The color of the 

dots changes with time. For the isopleth in Figure 1, at the start of the calculation the dot color is 

red. It turns to green and finally blue at the end of the 3-year calculation period. 

LIM B I and LIM B II refer to limiting isopleth for specific fungi and substrate classes. If the 

conditions lie above the limiting isopleths, mold growth could be possible, but additional criteria 

evaluation is required for a firm assessment.
8 

Figure 1. Isopleth graph for wall 4A1 predicting no potential for mold growth on the 

interior face of the drywall over a 3-year period
9 

3 Code Minimum Walls

3.1 Introduction

In recent editions of the IRC, minimum R-values for foam insulation have been specified when 

employing hybrid insulation strategies in ceilings and walls. These levels are intended to prevent 

condensation from forming on the inside face of the foam. Table 601.3.1 in the 2009 IRC lists 

prescriptive R-values for exterior rigid insulation that must be met if a Class III vapor retarder is 

intended. A footnote under the table states that these same levels should be met if using spray 

foam (with a minimum density of 2 lb/ft3) inside the cavity. 

These required levels have raised some questions in the industry. Some feel that these levels are 

too high; others believe that they are too low and could result in moisture problems within the 

wall cavities. The Spray Polyurethane Foam Alliance hired Steven Winter Associates, Inc. 

(leader of the BA team CARB) to analyze the potential for moisture problems in hybrid wall 

assemblies in climate zones 4–7. Various levels of medium-density (2 lb/ft3) spray polyurethane 

foam (MDSPF; installed against the inside face of the exterior sheathing) in combination with 

blown fiberglass cavity insulation were analyzed. All analysis methods and results of this study 

are explained in detail in this report.

3.2 Wall Construction Details

Figure 2 shows the typical detail for the walls analyzed in this section. The stud size and 

minimum level of MDSPF vary depending on the climate zone being analyzed. 

Figure 2 Typical wall construction for the walls built to code; 

cavity depth and foam levels vary depending on the climate

Table 5 lists each wall analyzed in WUFI and shows the specific climate and weather file used. 

The R-values of the MDSPF, fiberglass, and the total cavity R-value are also provided along 

with the percentage of the total R-value that the MDSPF contributes. The MDSPF R-value was 
10 

based on 2 lb foam with an R-value of 6.5/in. The minimum level of foam was determined from 

Table 601.3.1 in the IRC. Fiberglass insulation with an R-value of 4/in. was then assumed to fill 

the remainder of the cavity. Each of the three vapor retarder classes was analyzed. Values 

assumed for modeling were as follows: 

• Class III: 10.7 perm (latex paint)

• Class II: 1.0 perm (kraft-faced batt)

• Class I: 0.1 perm (sheet polyethylene). 

Table 5. Code Built Wall Assemblies Evaluated

Wall ID 

Vapor 

Retarder 

Perm Rating

MDSPF 

R-Value

Fiberglass 

R-Value

Total 

R-Value

MDSPF % 

of Total 

R-Value

(perm-in) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu)

4A: Nashville, Tennessee

4A-1 10.7 2.50 12.46 15.0 17

4A-2 10.7 4.13 11.46 15.6 26

4A-3 10.7 5.75 10.46 16.2 35

4A-4 10.7 7.38 9.46 16.8 44

4A-5 1.0 2.50 12.46 15.0 17

4A-6 1.0 4.13 11.46 15.6 26

4A-7 1.0 5.75 10.46 16.2 35

4A-8 1.0 7.38 9.46 16.8 44

4A-9 0.1 2.50 12.46 15.0 17

4A-10 0.1 4.13 11.46 15.6 26

4A-11 0.1 5.75 10.46 16.2 35

4A-12 0.1 7.38 9.46 16.8 44

4B: Albuquerque, New Mexico

4B-1 10.7 2.50 12.46 15.0 17

4B-2 10.7 4.13 11.46 15.6 26

4B-3 10.7 5.75 10.46 16.2 35

4B-4 10.7 7.38 9.46 16.8 44

4B-5 1.0 2.50 12.46 15.0 17

4B-6 1.0 4.13 11.46 15.6 26

4B-7 1.0 5.75 10.46 16.2 35

4B-8 1.0 7.38 9.46 16.8 44

4B-9 0.1 2.50 12.46 15.0 17

4B-10 0.1 4.13 11.46 15.6 26

4B-11 0.1 5.75 10.46 16.2 35

4B-12 0.1 7.38 9.46 16.8 44
11 

Wall ID 

Vapor 

Retarder 

Perm Rating

MDSPF 

R-Value

Fiberglass 

R-Value

Total 

R-Value

MDSPF % 

of Total 

R-value

(perm-in) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu)

4C: Seattle, Washington3

4C-1 10.7 3.75 19.69 23.4 16

4C-2 10.7 5.38 18.69 24.1 22

4C-3 10.7 7.00 17.69 24.7 28

4C-4 10.7 8.63 16.69 25.3 34

4C-5 1.0 3.75 19.69 23.4 16

4C-6 1.0 5.38 18.69 24.1 22

4C-7 1.0 7.00 17.69 24.7 28

4C-8 1.0 8.63 16.69 25.3 34

4C-9 0.1 3.75 19.69 23.4 16

4C-10 0.1 5.38 18.69 24.1 22

4C-11 0.1 7.00 17.69 24.7 28

4C-12 0.1 8.63 16.69 25.3 34

5A: Detroit, Michigan

5A-1 10.7 7.50 17.38 24.9 30

5A-2 10.7 9.13 16.38 25.5 36

5A-3 10.7 10.75 15.38 26.1 41

5A-4 10.7 12.38 14.38 26.8 46

5A-5 1.0 7.50 17.38 24.9 30

5A-6 1.0 9.13 16.38 25.5 36

5A-7 1.0 10.75 15.38 26.1 41

5A-8 1.0 12.38 14.38 26.8 46

5A-9 0.1 7.50 17.38 24.9 30

5A-10 0.1 9.13 16.38 25.5 36

5A-11 0.1 10.75 15.38 26.1 41

5A-12 0.1 12.38 14.38 26.8 46

5B: Elko, Nevada

5B-1 10.7 7.50 17.38 24.9 30

5B-2 10.7 9.13 16.38 25.5 36

5B-3 10.7 10.75 15.38 26.1 41

5B-4 10.7 12.38 14.38 26.8 46

5B-5 1.0 7.50 17.38 24.9 30

5B-6 1.0 9.13 16.38 25.5 36

5B-7 1.0 10.75 15.38 26.1 41

5B-8 1.0 12.38 14.38 26.8 46

5B-9 0.1 7.50 17.38 24.9 30

5B-10 0.1 9.13 16.38 25.5 36

5B-11 0.1 10.75 15.38 26.1 41

5B-12 0.1 12.38 14.38 26.8 46
12 

Wall ID 

Vapor 

Retarder 

Perm Rating

MDSPF 

R-Value

Fiberglass RValueTotal 

R-Value

MDSPF % 

of Total 

R-Value

(perm-in) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu)

6A: Madison, Wisconsin

6A-1 10.7 11.25 15.08 26.3 43

6A-2 10.7 12.88 14.08 27.0 48

6A-3 10.7 14.50 13.08 27.6 53

6A-4 1.0 11.25 15.08 26.3 43

6A-5 1.0 12.88 14.08 27.0 48

6A-6 1.0 14.50 13.08 27.6 53

6A-7 0.1 11.25 15.08 26.3 43

6A-8 0.1 12.88 14.08 27.0 48

6A-9 0.1 14.50 13.08 27.6 53

6B: Billings, Montana

6B-1 10.7 11.25 15.08 26.3 43

6B-2 10.7 12.88 14.08 27.0 48

6B-3 10.7 14.50 13.08 27.6 53

6B-4 1.0 11.25 15.08 26.3 43

6B-5 1.0 12.88 14.08 27.0 48

6B-6 1.0 14.50 13.08 27.6 53

6B-7 0.1 11.25 15.08 26.3 43

6B-8 0.1 12.88 14.08 27.0 48

6B-9 0.1 14.50 13.08 27.6 53

7: International Falls, Minnesota

7-1 10.7 15.00 12.77 27.8 54

7-2 10.7 16.63 11.77 28.4 59

7-3 10.7 18.25 10.77 29.0 63

7-4 1.0 15.00 12.77 27.8 54

7-5 1.0 16.63 11.77 28.4 59

7-6 1.0 18.25 10.77 29.0 63

7-7 0.1 15.00 12.77 27.8 54

7-8 0.1 16.63 11.77 28.4 59

7-9 0.1 18.25 10.77 29.0 63

3.3 Results

3.3.1 Moisture Content in Oriented Strand Board (Rot/Decay)

Based on the construction and conditions outlined in the previous section, the maximum MC of 

the OSB in most of the cases modeled did not exceed 20%. Drying was seen over the 3-year 

period as shown in Figure 3 and Figure 4. Failures only occurred in climate zone 4C in the wall 

with the lowest level of foam and Class III vapor retarders (4C-1) when using the ASHRAE 160-

2009 interior conditions. The OSB in this wall also increased in MC over the 3-year period. 

The highest MCs were recorded in the marine C and moist A zones with only 4C exceeding 20% 

during the 3-year period (Figure 3). The highest MC levels occurred during the first year of the 
13

simulation, in the assemblies with the lowest levels of foam, and in the walls with the Class III 

vapor retarder. 

Figure 3. Predicted moisture content in OSB: ASHRAE 160-2009 interior conditions 

Figure 4. Predicted moisture content in OSB: sine curve interior conditions 

0%

5%

10%

15%

20%

25%

4A-14A-54A-9

4B-14B-54B-9

4C-14C-54C-9

5A-15A-55A-9

5B-15B-55B-9

6A-16A-56A-9

6B-46B-8

 7-3 7-7

Moisture Content 

Wall ID

Predicted MC in OSB: ASHRAE 160 

Indoor Conditions

1st Year Max 3rd Year Max Average

0%

5%

10%

15%

20%

25%

00000000000000000000000000000000000000000000

Moisture Content

Wall ID

Predicted MC in OSB: Sine Curve Indoor 

Conditions

Year 1 Year 3 Average
14 

Figure 5 shows a direct comparison of the results for the ASHRAE 160-2009 and the sine curve 

interior conditions. Although there is fairly good agreement between the two methods for the dry 

B zones, there is a significant difference in predictions in the moist A and marine C zones. The 

largest disparity is in the marine climate. In general, using the ASHRAE 160-2009 interior 

conditions results in MC predictions that are significantly higher in the A zones and slightly 

lower in the dry climates than the MC predictions using the typical sine curve. This difference 

becomes less significant as the climates get colder.

Figure 5. Comparison of OSB MC using ASHRAE 160-2009 and sine curve interior conditions

3.3.2 Assembly Moisture Content

The initial and final MCs for the entire assembly were evaluated for each case. Using both 

methods for generating the interior conditions, the final MC was lower than the initial for all the 

walls evaluated, indicating that the assemblies dried out over the 3-year period. 

Figure 6 shows the difference in predicted assembly MC over the 3-year modeling period for 

both the ASHRAE 160-2009 and the sine curve interior conditions. Results are consistent with 

those of the OSB MC predictions. Using the ASHRAE 160-2009 method, less drying is 

predicted over the 3-year period for the moist A regimes than when using the sine curve. 

Inversely, more drying is predicted for the dry regimes when using ASHRAE 160-2009 than 

when using the sine curve. Again, the differences in predictions between the two methods are 

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

4A-14A-54A-9

4B-14B-54B-9

4C-14C-54C-9

5A-15A-55A-9

5B-15B-55B-9

6A-16A-56A-9

6B-46B-8

 7-3 7-7

Moisture Content 

Wall ID

Predicted 3 Year Average MC in OSB 

ASHRAE 160 Indoor Conditions vs. Sine 

Curve

Average Sine Curve
15 

more severe in the moist climates. The differences in the marine C zone are by far the most 

drastic.

Figure 6. Difference in predicted assembly MC from beginning to end of modeling period: 

ASHRAE 160-2009 versus sine curve interior conditions

3.3.3 Condensation Potential

Using the hourly results from WUFI and isotherms from THERM, CARB analyzed the 

condensation potential for each wall. The MDSPF/fiberglass interface was the critical juncture 

analyzed. Results from WUFI indicate that all walls in the study show some potential for 

condensation at the interior face of the MDSPF. Figure 7 illustrates the condensation potential 

for each case as predicted by WUFI when the sine curves are used to predict indoor conditions 

versus the ASHRAE 160-2009 indoor conditions. 

0.00

0.05

0.10

0.15

0.20

0.25

4A-14A-54A-9

4B-14B-54B-9

4C-14C-54C-9

5A-15A-55A-9

5B-15B-55B-9

6A-16A-56A-9

6B-46B-8

 7-3 7-7

lbs/ft2 

Wall ID

Difference in Assembly MC Over 3 Year 

Modeling Period 

ASHRAE 160 vs. Sine Curve

ASHRAE Sine Curve
16 

Figure 7. Condensation potential for the different levels of MDSPF for two different sets of indoor 

conditions: sine curve and the ASHRAE 160-2009 method 

Perm rating of the vapor retarder had little effect on the condensation potential because this 

analysis assumes that interior air is the vehicle for supplying moisture to the condensing surface, 

not diffusion. Interior moisture conditions and R-value of the spray foam were the driving 

factors.

Again, agreement in the dry climates is fairly good. Unlike all the other zones, however, the 

ASHRAE 160-2009 conditions result in a lower predicted condensation potential than the sine 

curve in climate zones 6B and 7. Interior RH levels in those two zones dip into the upper 20% 

range using ASHRAE 160-2009, but are held to a minimum of 40% with the sine curve. 

Disparity between the two methods of generating the interior conditions is quite high in the moist 

climates, especially the warmer climates and the marine climate.

Although little guidance is available on what level of condensation is acceptable, walls in climate 

zones 4A, 4C, 5A, and 6A appear to be at the highest risk. 

Table 6 summarizes the temperatures used in THERM and the resulting temperatures of the 

condensing surfaces: the interior face of the MDSPF in the center of the cavity and at the stud. 

Using the hourly results from the WUFI files with the ASHRAE 160-2009 interior conditions, 

the average dew point temperature for each climate was calculated for the 3 coldest months.

0%

10%

20%

30%

40%

50%

60%

4A-14A-54A-9

4B-14B-54B-9

4C-14C-54C-9

5A-15A-55A-9

5B-15B-55B-9

6A-16A-56A-9

6B-46B-8

 7-3 7-7

Percent of Modeling Period

Wall ID

Condensation Potential: ASHRAE 160 vs. 

Sine Curve Interior Conditions

Sine Curve ASHRAE 160
17 

Climate zones 4A, 4C, 5A, 5B, and 6A show risks of condensation potential with these boundary 

conditions because the interior face of the MDSPF and the stud are predicted to be below the 

dew point temperatures. The interior air dew point from the sine curve analysis is approximately 

45°F. Based on that temperature, walls in 4A and 4C see a reduced risk for condensation, but the 

walls in 6A, 6B, and 7 would be at greater risk. 

Table 6. THERM Analysis of Temperatures at the Interior Face of the MDSPF 

at the Center of the Cavity and at the Stud

Climate 

Zone Climate File

Average 

Outdoor 

Temperature

(°F) 

Average Dew Point 

Temperature 

(December,

January, February) 

(°F) 

Interior 

Surface 

of 

MDSPF

(°F) 

Stud 

Temperature 

at Interior 

Surface of 

MDSFP

(°F)

4A Nashville, 

Tennessee

38 51 46 46 

4B Albuquerque, 

New Mexico 37 45 46 46 

4C Seattle, 

Washington 41 52 48 48 

5A Detroit, 

Michigan 27 45 42 40 

5B Elko, Nevada 27 45 42 41

6A Madison, 

Wisconsin 18 42 41 39 

6B Billings, 

Montana 27 43 46 44 

7 

International 

Falls, 

Minnesota

6 39 41 38 

Considering that these results are based on average values for the 3 coldest months, it can be 

concluded that 50% of the time, the outdoor temperatures will fall below the averages listed, 

resulting in lower interior wall surface temperatures than those listed. These values support the 

WUFI results that all the walls have some potential for condensation based on the lowest levels 

of foam required by the code. 

According to both WUFI and THERM, walls of particular concern are 4A, 4C, 5A, 5B, and 6A. 

For these five sites, the interior surface of the MDSPF in the walls with the lowest levels of 

MDSPF is predicted to condense close to or above 30% of the year when the dew point is 

calculated based on the interior air temperature and RH.

3.3.4 ASHRAE Criteria

Based on the raw data, only 7 of the 87 walls pass the ASHRAE conditions using the customized 

sine curves and only 3 pass using ASHRAE 160-2009 conditions when cooling is assumed. 
18 

After looking at the results in more detail, it was determined that a significant percentage of the 

failures occurred in the first year at the very beginning of the modeling period. This is partly 

because the initial MC of the walls is assumed to be high at the time of construction. In addition 

to that, the modeling period begins in October, a cool month, reducing drying potential. Many of 

the walls dry out enough to pass the criteria in the first month or so. It should be noted that 

almost all the walls with the Class III vapor retarder fail the 30-day criteria using both methods 

to generate the interior conditions. Figure 8 shows which walls fail the ASHRAE 160-2009 

criteria and to what extent.

Figure 8. Percentage of the total number of averages that each wall failed the ASHRAE 160-2009

criteria: ASHRAE 160-2009 interior conditions 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

4A-14A-54A-9

4B-14B-54B-9

4C-14C-54C-9

5A-15A-55A-9

5B-15B-55B-9

6A-16A-56A-9

6B-46B-8

7A-37A-7

Wall ID

ASHRAE Criteria Failures

ASHRAE 160 Interior Conditions
19 

Figure 9. Percentage of the total number of averages that each wall failed the 

ASHRAE 160-2009 criteria: sine curve interior conditions 

3.3.5 Isopleths

For all walls in climate zones 4A, 4C, 5A, 6A, and 7, the ASHRAE 160-2009 indoor conditions 

result in isopleths that indicate the potential for mold on both the interior surface of the drywall 

and the interior face of the MDSPF. For climates 4B, 5B, and 6B no mold growth is predicted for 

the interior of the drywall. Mold growth is predicted on the interior face of the MDSPF for all 

walls with a Class III vapor retarder.

3.3.6 Results Summary for Code Built Walls

Table 7 summarizes all the results of the analysis performed on the walls described in this 

section. Entries located in white rows were modeled with a Class III vapor retarder, light blue 

indicates a Class II vapor retarder, and dark blue represents a Class I vapor retarder. A red “X” 

denotes a failure in that category. As noted earlier, when evaluating the hygrothermal 

performance of these walls in the dry B climate zones, there is fairly good agreement between 

the predictions from the two methods of generating the interior boundary conditions. The 

difference between the two methods in the moist A and marine C zones, however, is fairly 

drastic. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

4A-14A-54A-9

4B-14B-54B-9

4C-14C-54C-9

5A-15A-55A-9

5B-15B-55B-9

6A-16A-56A-9

6B-46B-8

7A-37A-7

Wall ID

ASHRAE Criteria Failures

Sine Curve Interior Conditions
20 

Table 7. Results Summary for Code Built Walls

 Sine Wave 

(69.8°F ± 1.8°F, 50% RH ± 10%) ASHRAE 160-2009, Cooling Assumed 

 

ASHRAE 

Criteria

OSB MC

Assembly 

MC

Isopleths, 

Interior

Isopleths, 

MDSPF

Condensation 

Potential (%)

ASHRAE 

Criteria

OSB MC

Assembly 

MC

Isopleths, 

Interior

Isopleths,

MDSPF

Condensation 

Potential (%)

4A-1 X √ √ √ X 13 X √ √ X X 36

4A-2 X √ √ √ X 10 X √ √ X X 30

4A-3 √ √ √ √ X 5 X √ √ X X 18

4A-4 √ √ √ √ X 2 X √ √ X X 10

4A-5 √ √ √ √ X 13 X √ √ X X 37

4A-6 √ √ √ √ X 10 √ √ √ X X 32

4A-7 √ √ √ √ √ 5 √ √ √ X X 21

4A-8 √ √ √ √ √ 3 √ √ √ X X 13

4A-9 √ √ √ √ X 13 √ √ √ X X 37

4A10 √ √ √ √ X 10 √ √ √ X X 32

4A11 √ √ √ √ X 5 √ √ √ X X 21

4A12 √ √ √ √ X 3 √ √ √ X X 13

4B-1 X √ √ √ X 19 X √ √ √ X 23

4B-2 X √ √ √ X 14 X √ √ √ X 18

4B-3 X √ √ √ X 7 X √ √ √ X 9

4B-4 √ √ √ √ X 3 X √ √ √ X 4

4B-5 √ √ √ √ √ 20 √ √ √ √ √ 23

4B-6 √ √ √ √ √ 15 √ √ √ √ √ 19

4B-7 √ √ √ √ √ 8 √ √ √ √ √ 10

4B-8 √ √ √ √ √ 4 √ √ √ √ √ 5

4B-9 √ √ √ √ X 20 √ √ √ √ X 23

4B10 √ √ √ √ √ 15 √ √ √ √ X 19

4B11 √ √ √ √ √ 8 √ √ √ √ √ 10

4B12 √ √ √ √ √ 4 √ √ √ √ √ 5

4C-1 X √ √ √ X 13 X X √ X X 62

4C-2 X √ √ √ X 8 X √ √ X X 58

4C-3 √ √ √ √ X 2 X √ √ X X 49

4C-4 √ √ √ √ X 1 X √ √ X X 41
21 

 Sine Wave 

(69.8°F ± 1.8°F, 50% RH ± 10%) ASHRAE 160-2009, Cooling Assumed 

 

ASHRAE 

Criteria

OSB MC

Assembly 

MC

Isopleths, 

Interior

Isopleths, 

MDSPF

Condensation 

Potential (%)

ASHRAE 

Criteria

OSB MC

Assembly 

MC

Isopleths, 

Interior

Isopleths,

MDSPF

Condensation 

Potential (%)

4C-5 X √ √ √ X 14 X √ √ X X 62

4C-6 √ √ √ √ √ 9 √ √ √ X X 58

4C-7 √ √ √ √ √ 3 √ √ √ X X 50

4C-8 √ √ √ √ √ 1 √ √ √ X X 42

4C-9 √ √ √ √ X 14 √ √ √ X X 63

4C10 √ √ √ √ X 9 √ √ √ X X 59

4C11 √ √ √ √ X 3 √ √ √ X X 50

4C12 √ √ √ √ √ 1 √ √ √ X X 43

5A-1 X √ √ √ X 24 X √ √ X X 49

5A-2 X √ √ √ X 18 X √ √ X X 40

5A-3 X √ √ √ X 10 X √ √ X X 22

5A-4 X √ √ √ X 6 X √ √ X X 10

5A-5 X √ √ √ X 24 X √ √ X X 50

5A-6 √ √ √ √ X 19 X √ √ X X 42

5A-7 √ √ √ √ X 10 X √ √ X X 24

5A-8 √ √ √ √ √ 7 X √ √ X X 12

5A-9 X √ √ √ X 24 X √ √ X X 50

5A10 √ √ √ √ X 19 √ √ √ X X 42

5A11 √ √ √ √ X 11 √ √ √ X X 25

5A12 √ √ √ √ X 7 √ √ √ X X 13

5B-1 X √ √ √ X 27 X √ √ √ X 29

5B-2 X √ √ √ X 22 X √ √ √ X 21

5B-3 X √ √ √ X 14 X √ √ √ X 9

5B-4 X √ √ √ X 10 X √ √ √ X 3

5B-5 √ √ √ √ X 28 √ √ √ √ X 30

5B-6 √ √ √ √ X 23 √ √ √ √ X 23

5B-7 √ √ √ √ X 15 √ √ √ √ √ 11

5B-8 √ √ √ √ √ 11 √ √ √ √ √ 5

5B-9 √ √ √ √ X 28 √ √ √ √ X 30
22 

 Sine Wave 

(69.8°F ± 1.8°F, 50% RH ± 10%) ASHRAE 160-2009, Cooling Assumed 

 

ASHRAE 

Criteria

OSB MC

Assembly 

MC

Isopleths, 

Interior

Isopleths, 

MDSPF

Condensation 

Potential (%)

ASHRAE 

Criteria

OSB MC

Assembly 

MC

Isopleths, 

Interior

Isopleths,

MDSPF

Condensation 

Potential (%)

5B10 √ √ √ √ X 18 √ √ √ √ √ 15

5B11 √ √ √ √ X 16 √ √ √ √ √ 12

5B12 √ √ √ √ X 11 √ √ √ √ √ 5

6A-1 X √ √ √ X 23 X √ √ X X 33

6A-2 X √ √ √ X 15 X √ √ X X 12

6A-3 X √ √ √ X 10 X √ √ X X 5

6A-4 √ √ √ √ X 23 X √ √ X X 35

6A-5 √ √ √ √ √ 16 √ √ √ X X 15

6A-6 √ √ √ √ √ 10 √ √ √ X X 7

6A-7 √ √ √ √ X 23 √ √ √ X X 35

6A-8 √ √ √ √ X 16 √ √ √ X X 16

6A-9 √ √ √ √ X 11 √ √ √ X X 8

6B-1 X √ √ √ X 17 X √ √ √ X 10

6B-2 X √ √ √ X 10 X √ √ √ X 3

6B-3 X √ √ √ X 7 √ √ √ √ X 0

6B-4 √ √ √ √ √ 18 √ √ √ √ √ 11

6B-5 √ √ √ √ √ 11 √ √ √ √ √ 3

6B-6 √ √ √ √ √ 7 √ √ √ √ √ 0

6B-7 √ √ √ √ X 18 √ √ √ √ √ 12

6B-8 √ √ √ √ X 12 √ √ √ √ √ 3

6B-9 √ √ √ √ X 8 √ √ √ √ √ 0

7-1 X √ √ √ X 21 X √ √ X X 8

7-2 X √ √ √ X 15 X √ √ X X 2

7-3 X √ √ √ X 9 X √ √ X X 1

7-4 √ √ √ √ √ 23 √ √ √ X X 11

7-5 √ √ √ √ √ 16 √ √ √ X X 4

7-6 √ √ √ √ √ 10 √ √ √ X X 2

7-7 √ √ √ √ X 23 √ √ √ X X 11

7-8 √ √ √ √ √ 16 √ √ √ X X 4

7-9 √ √ √ √ √ 10 √ √ √ X X 2
23 

3.4 Discussion

3.4.1 Comparison to Fiberglass-Only Insulation

Although the results show many failures, conclusions should not be drawn without comparison 

to a typical code wall. How do commonly constructed, 2 × 6 walls with only fiberglass batts in 

the cavities compare to the walls analyzed in this study? 

To answer this question, CARB ran the same type of analysis on a code wall in climate zone 5A, 

which is one of the more problematic zones with respect to moisture control. Condensation 

potential, ASHRAE 30-day averages, total assembly MC, and MC in the OSB were evaluated for 

a code built, 2 × 6 wall with R-20, kraft-faced fiberglass batts in the cavity and no MDSPF. The 

kraft facing on the fiberglass is considered a Class II vapor retarder and was modeled as having a 

rating of 1 perm. It is mandatory in climate zone 5A that walls without exterior rigid insulation 

or 2 lb foam in the cavity be constructed with either a Class I or Class II vapor retarder if they do 

not have an approved vented cladding. Other than this, the construction is similar to the other 

walls in this section. The results are displayed in Table 8. 

Table 8. Comparison of Failure Criteria Results for Climate Zone 5A: R-20, Fiberglass Batt Code 

Wall Versus R-25 Hybrid Wall with MDSPF and Blown Fiberglass

Fiberglass 

Only MDSPF + Fiberglass

Wall Details

Wall ID 5A-R20 5A-1 5A-5

MDSPF R-value 0 7.5 7.5

Fiberglass batt R-value 20 17.38 17.38

Perm Rating 1 10.7 1

ASHRAE Criterion 30-day moving average Fail Fail Fail

OSB MC % average over 3 years 14 15 13

ASHRAE Criteria

(%) Maximum over 3 years 17 18 16 

OSB MC %

Assembly MC (lb/ft2) 

Decreasing? Yes Yes Yes

Start 0.380 0.375 0.375

Finish 0.326 0.333 0.316

Assembly MC (lb/ft2) 

Condensation 

Potential 

Decreasing? Yes Yes Yes

Number of hours over 3 

years 18,084 12,995 13,113 

% over 3 years 69 49 50

Isopleths Interior and condensing 

surfaces

Yes Yes Yes

Note that the interior face of the OSB on the fiberglass-only case was assumed to be the 

condensing surface. On the hybrid walls, the interior face of the MDSPF was assumed to be the 

condensing surface. Consequently, condensation potential and ASHRAE 160-2009 criteria were 

evaluated at those respective surfaces.

Two walls containing MDSPF were compared to the fiberglass-only case. The first, wall 5A-1, is 

a code minimum wall, containing the least amount of MDSPF that can be installed when 
24 

combined with a Class III vapor retarder. Wall 5A-5 also has the minimum level of spray foam 

(R-7.5 MDSPF), but is coupled with a Class II (1 perm) vapor retarder. 

Note that all fail the 30-day average ASHRAE 160-2009 criteria, but the MDSPF wall with the 

Class III retarder fails all 3 of the 160 averages. In fact, 66% of the 30-day averages fail, while 

the FG-only wall fails 39% of the time. This is because the OSB on the code wall gets so much 

colder than the interior face of the MDSPF, producing unfavorable conditions for mold growth. 

The surface RH of the OSB on the fiberglass-only wall is actually predicted to be greater than 

80%—more than 75% of the 30-day averages—but because the OSB gets so cold, it fails the 30-

day criteria only 39% of the time. That is why, even though the condensation potential for the 

fiberglass-only wall is much higher than the MDSPF wall, it does better in the ASHRAE 160-

2009 criteria.

Other than these two criteria, the two code minimum walls—5A-R20 and 5A-1—perform fairly 

similarly. They both show drying over time in both the assembly MC and the OSB MC % 

categories, and neither exceeds the 20% threshold for OSB MC. Again, all the isopleths show the 

potential for mold growth both on the interior surface of the drywall and the condensing surface, 

but these are thought to be overly pessimistic when using the ASHRAE 160-2009 indoor 

conditions as was done here. 

Wall 5A-5 performs the best of all three, although it is not predicted to pass the ASHRAE 30-day 

criteria and the condensation potential is quite high. Installing a vapor retarder on the interior, 

however, while having a vapor semi-impermeable insulation on the exterior of the cavity, is not 

recommended unless a smart vapor retarder is employed. A smart vapor retarder changes its 

permeance with humidity, becoming more vapor open as humidity inside the building cavity 

increases. Without this capability, if moisture did get into the cavity, it would have a difficult 

time getting out with both an interior and an exterior vapor retarder.

3.5 Conclusions

After extensive analysis of the predicted hygrothermal performance for walls in climate zones 

4A through 7 using different combinations of vapor barrier strategies and levels of MDSPF, the 

analysis team came to the following conclusions: 

1. Although the majority of the walls analyzed fail the ASHRAE 160-2009 30-day criteria, 

the walls with Class III vapor retarders and the lowest levels of MDSPF required by code 

fail the ASHRAE criteria by a much higher margin than the rest. Note that a codecompliant wall with only fiberglass batts in the cavity also fails these criteria.

2. All walls modeled in the B regimes show a consistent tendency to dry out over time. 

3. The predicted hygrothermal performance in the dry B climate zones was slightly worse 

when generating the interior boundary conditions using a typical sine curve than the 

predicted performance using the ASHRAE 160-2009 interior conditions.

4. The predicted hygrothermal performance in the moist A and marine C climate zones was 

drastically worse when generating the interior boundary conditions using the ASHRAE 

160-2009 interior conditions as opposed to a typical sine curve. 
25 

Although many failures were predicted, when hybrid walls in climate zone 5A were compared to 

standard, commonly built walls that are insulated with fiberglass only, predicted hygrothermal 

performance was quite similar. The fiberglass-only wall performed slightly better with respect to 

the ASHRAE 160-2009 criteria, indicating a slightly reduced potential for mold growth, but 

according to these methods of predicting hygrothermal performance, the potential still exists. 

These predictions need to be tempered with the fact that millions of homes in the United States 

are built with only fiberglass insulation inside the wall cavities. These homes are not rotting or 

experiencing discernible mold growth. Although these methods provide valuable feedback when 

comparing different wall constructions to each other and the effects of different operating 

conditions, they should not be taken as absolute predictions of hygrothermal performance. 
26 

4 High-R Walls

4.1 Wall Construction Details

CARB evaluated three different methods of constructing high-R walls. In climate zones 4A and 

5A, R-40 walls were evaluated, and in climate zones 6A and 7, R-60 walls were modeled. Two 

of the high-R walls analyzed consisted of wood framing, wood clapboard siding (vented and 

unvented), a weather resistant layer of spun bonded polyolefin, OSB sheathing, closed cell spray 

polyurethane foam (SPF) or extruded polystyrene (XPS) at varying thicknesses, blown fiberglass 

insulation in the wall cavity, gypsum board, and a latex paint coating on the inside. These walls 

can be seen in Figure 10 and Figure 11. 

Figure 10. Typical high-R wall with exterior XPS insulation 

Figure 11. Typical high-R wall with SPF insulation 
27 

Given the results of the code wall analysis and previous modeling performed for other projects, 

CARB set the minimum level of foam to 50% of the cavity R-value. 

In addition to these options using hybrid insulation strategies, double stud walls dense-packed 

with cellulose, another commonly constructed high R-value wall, were evaluated for zones 5A 

and 7 (Figure 12). 

Figure 12. Typical double stud high-R wall with cellulose insulation 

Insulation resistances per inch used for this analysis were R-6.9 for MDSPF, R-5 for XPS, R-3.6 

for cellulose, and R-4 for fiberglass insulation. A Class III vapor retarder with a 10.7 perm rating 

(equivalent to latex paint) was analyzed for the assemblies with foam. A Class II vapor retarder 

was assumed for the cellulose walls. Table 9 lists each high-R wall analyzed in WUFI.
28 

Table 9. High-R Wall Assemblies Evaluated 

Wall 

Identification

Vapor 

Retarder 

Perm 

Rating

XPS/ MDSPF 

R-value 

Fiberglass/ 

Cell 

R-value 

Cavity 

 R-value 

XPS/MDSPF 

% of Total 

R-value 

 (permin) (h∙ft2

∙°F/Btu) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu) 

4A-XPS 10.7 19.6 19.8 39.4 50 

4A-SPF 10.7 20.82 19.8 40.6 51

5A-XPS 10.7 19.6 19.8 39.4 50 

5A-SPF 10.7 20.82 19.8 40.6 51

5A-Cell 1.0 n/a 40 40 n/a 

6A-XPS 10.7 29.4 29.7 59.1 50

6A-SPF 10.7 31.23 29.7 60.9 51 

7-XPS 10.7 29.4 29.7 59.1 50

7-SPF 10.7 31.23 29.7 60.9 64 

7-Cell 1.0 n/a 60 60 n/a

4.1.1 Vented Cladding Assumption

Previous hygrothermal modeling conducted by CARB and others of high R-value walls has 

indicated a serious potential for moisture damage to the wood sheathing if located on the exterior 

of the wall just beneath the cladding, especially in cold, moist climates (Straube and Smegal 

2009). Therefore, high-R walls with and without ventilated cladding were analyzed for this 

report.

Ventilated claddings are often recommended for wall assemblies to aid in the drying of the 

exterior layers. They also act as a capillary break and a drainage plane. Although 

vented/ventilated cavities within wall assemblies have been researched extensively, contradictory 

conclusions have been encountered. Several studies concluded that vented cladding has little to 

no effect on the drying rate of the exterior components of the wall (Hansen et al. 2002; Jung 

1985; Kunzel and Mayer 1983). More current studies, however, conclude that vented or 

ventilated claddings can significantly increase the drying rate of the exterior components of the 

building assembly (Karagiozis and Kunzel 2009; Straube and Finch 2009; Shi and Burnett 2007; 

ASHRAE 1091 2004a). 

To emulate a vented cladding in WUFI, a constant air change source of 10 air changes per hour 

(ACH) was introduced in a 0.75-in. air cavity behind the cladding. This value was chosen based 

on results from ASHRAE research report 1091(2004), which states that the equivalent 

permeance of the wall assembly is drastically increased once the minimum threshold of 10 ACH 

is reached. This value is also seen as a conservative estimate based on research conducted by 

Straube and Finch (2009b), which concluded that ventilation rates in a 0.75-in. vented cavity can 
29 

range from 12 ACH to 100 ACH depending on vent size and location, solar exposure, wind 

direction and speed, and temperature gradients. 

4.2 Results

4.2.1 Moisture Content in Oriented Strand Board (Rot/Decay)

Figure 13 shows the OSB % MC for each wall analyzed. Based on the construction and 

conditions outlined in the previous sections, maximum MC of the OSB is generally higher in 

walls with XPS insulation on the exterior side of the OSB. 

In climates 5A, 6A, and 7, the MC in the OSB in walls with SPF plus fiberglass insulation 

increased over the 3-year modeling period, as did the double cellulose walls in zones 5A and 7. 

Additionally, the walls with XPS in climate zone 7 also increased over time and surpassed the 

20% threshold in year 3. The walls with SPF and the double cellulose walls perform much better 

in all climates with the addition of a vented cladding. The vented cladding has little to no effect 

on the XPS walls, because, unlike the other two, the OSB is sandwiched in the center of the wall 

reducing the effect of the ventilation on that component. 

Figure 13. High-R walls: predicted % MC in OSB when using the ASHRAE 160-2009 method to 

generate the indoor conditions (cooling was assumed in all climates) 

4.2.2 Assembly Moisture Content 

The initial and final MC for the entire assembly was evaluated for each case (Table 10). For 

ASHRAE 160-2009 interior conditions, the 5-SPF and 5-DBL CELL wall and the 7-XPS, 7-SPF, 

and 7-DBL CELL walls modeling predicts an increase in assembly MC over the 3-year period. 

0

5

10

15

20

25

30

XPS

SPF

SPF+10ACH

XPS

SPF

SPF+10ACH

R40 DBL CELL

R40 DBL CELL+10ACH

XPS

SPF

SPF+10ACH

XPS

SPF

SPF50%+10ACH

R60 DBL CELL

R60 DBL CELL+10ACH

4A 5A 6A 7

Percent MC

Wall ID

Moisture Content in OSB Over 3 year Period for High-R Walls

 1st Year Max 2nd Year Max 3rd Year Max Average
30 

When modeled with a vented cladding, the SPF and double cellulose wall assemblies show 

drying over the 3-year period. The assemblies that increase in MC over the 3 year period are 

highlighted in red in Table 10. 

Table 10. High-R Walls: Difference in Predicted Assembly Moisture Content 

From Beginning to End of Modeling Period 

Assembly MC (lb/ft2)

Climate 

Zone Wall Identification Start Finish Delta % 

4A XPS 0.38 0.31 –0.08 –20

SPF 0.39 0.36 –0.03 –8

SPF+10ACH 0.39 0.33 –0.06 –15

5A XPS 0.38 0.36 –0.02 –5

SPF 0.39 0.41 0.01 4

SPF+10ACH 0.39 0.36 –0.03 –8

R40 DBL CELL 0.60 0.64 0.03 6

R40 DBL 

CELL+10ACH 0.60 0.52 –0.08 –13 

6A XPS 0.41 0.37 –0.04 –10

SPF 0.42 0.41 –0.01 –2

SPF+10ACH 0.42 0.37 –0.05 –12

7 XPS 0.41 0.45 0.04 10

SPF 0.42 0.47 0.05 12

SPF50%+10ACH 0.42 0.41 –0.01 –3

R60 DBL CELL 0.76 0.96 0.21 27

R60 DBL 

CELL+10ACH 0.76 0.72 –0.04 –5 

4.2.3 Condensation Potential

Using the hourly results from WUFI and isotherms from THERM, CARB analyzed the 

condensation potential for each wall. The critical junctures analyzed were the MDSPF/fiberglass

interface in the SPF walls, the OSB/fiberglass interface in the XPS walls, and the OSB/cellulose 

interface in the double stud walls. Table 11 displays the condensation potential for each case as 

predicted by WUFI. Assemblies with a high potential for condensation have been highlighted 

in red. 
31 

Table 11. Condensation Potential for High R-Value Walls

Condensation Potential

Climate Zone Wall Identification Hours Over 3 Years % Over 3 Years

4A XPS 2,278 9

SPF 2,151 8

5A XPS 4,149 16

SPF 3,875 15

R40 DBL CELL 20,082 76

6A XPS 5,055 19

SPF 3,957 15

7 XPS 7,711 29

SPF 6,931 26

R60 DBL CELL 25,022 95

SPF 60% 345 1

Results from WUFI indicate that condensation potential for the double cellulose walls is 

extremely high because the OSB in those wall assemblies is entirely outside of the insulation. All 

walls in climate zone 7 exhibit elevated condensation potential as well.

To evaluate the condensation potential using THERM, the average outdoor temperature for the 

coldest 3 months of the year (December, January, and February) was calculated for each 

location. These values are displayed in Table 12. The temperature of the first condensing surface 

in the foam walls is above the average dew point temperature of the interior air. The interior air 

winter temperature was assumed to be 70°F as was used in WUFI.

Table 12. THERM Analysis of Temperatures at the Interior Face of the Foam

Climate 

Zone Climate File

Average 

Outdoor 

Temperature

(°F) 

Average Dew Point 

Temperature 

(December, 

January, February) 

(°F)

Interior of 

First

Condensing 

Surface 

(°F)

4A Nashville, Tennessee 38 51 55

5A Detroit, Michigan 27 45 50

6A Madison, Wisconsin 18 42 45

7 International Falls, 

Minnesota 6 39 40 

4.2.4 ASHRAE Criteria

The ASHRAE 160-2009 criteria were applied to the first condensing surface in each wall 

assembly: OSB for both the double cellulose and XPS walls, and the interior surface of the 

MDSPF for the spray foam walls. All the high-R walls fail this criterion by a significant margin 

when the ASHRAE 160-2009 interior conditions with cooling are assumed. All the cases fail the 
32 

30-day running average. Figure 14 shows which walls fail the ASHRAE 160-2009 criteria and to 

what extent.

Figure 14. High-R walls: percentage of averages that each wall failed the ASHRAE 160-2009

criteria. 

4.2.5 Isopleths

For all high-R walls, the ASHRAE 160-2009 indoor conditions result in isopleths that indicate 

the potential for mold on both the interior surface of the drywall and the interior face of the first 

condensing surface.

4.3 Discussion

The modeling results for the high-R walls suggest that vented cladding is essential to ensure 

proper drying of these wall assemblies. In climate zone 7, this is particularly important and has a 

more significant effect on the wall performance than increasing the impermeable to permeable 

insulation (foam versus fiberglass) ratio from 50% to 60%. 

Although this increase in foam does significantly reduce the condensation potential at the interior 

face of the foam from 26% to 1%, it does not eliminate the failures in the other categories. Without the vented cladding, the OSB and total assembly MC still increase over the 3-year period. 

0%

20%

40%

60%

80%

100%

XPS

SPF

SPF+10ACH

XPS

SPF

SPF+10ACH

R40 DBL CELL

R40 DBL CELL+10ACH

XPS

SPF

SPF+10ACH

XPS

SPF

SPF50%+10ACH

R60 DBL CELL

R60 DBL CELL+10ACH

4A 5A 6A 7

Wall ID

Percentage of 3 Year Period that Each Wall Failed the ASHRAE 160 30-

day Moving Average 
33 

5 Brick Walls With Interior Insulation

5.1 Introduction

The thermal resistance provided by uninsulated brick walls (R-3 to R-5) is often insufficient for 

occupant thermal comfort and energy efficiency in most climates. To increase the R-value, the 

wall can be insulated either from the exterior or the interior. But, because of the costs of 

retrofitting a building façade and social pressure to preserve the original exterior brick, retrofits 

to brick buildings are usually conducted from the interior. 

Adding interior insulation significantly affects the thermal and hygric behavior of the masonry 

wall because the masonry is now separated from the conditioned space. In cold climates, this 

causes the temperature to drop within the masonry wall, increasing the potential risk of damage 

due to interstitial condensation and freeze-thaw cycles. Lower masonry wall temperatures also 

affect the drying capacity of the wall. 

Another problematic side effect of adding interior insulation is that it can inhibit drying to the 

inside, whereas an uninsulated wall can dry to the exterior as well as the interior. If a Class I or 

Class II vapor retarder or insulation such as MDSPF or XPS is used, the wall is forced to dry to 

the exterior only. This reduces the drying rate and leads to higher levels of moisture 

accumulation.

To evaluate the effects of interior insulation, CARB evaluated 12-in.-thick brick walls with 

varying hygrothermal properties and levels of insulation. Because the properties of brick can 

vary so drastically, these walls were only evaluated in climate zone 4A. Describing methods for 

evaluating performance is the focus of this section as opposed to providing specific 

recommendations for type and amount of insulation for different climate zones. 

5.2 Wall Construction Details

Twelve-inch-thick walls with solid bricks without any chips, cracks, and voids were assumed. A 

three header section of the wall was modeled with a 1⁄2-in. type-N mortar joint (predominantly 

used in above-grade exterior walls). Figure 15 displays cross sections of the insulation strategies

evaluated, and Table 13 lists the R-values assumed.
34 

Figure 15. Cross sections of the 12-in.-thick brick walls evaluated: (A) uninsulated, interior plaster 

finish; (B) 1-in. air gap, 2 × 4 steel studs with R-13 fiberglass batt insulation and gypsum board 

interior finish; and (C) 1-in. R-5 foam, 2 × 4 steel studs with R-13 fiberglass batt insulation and 

gypsum board interior finish

Table 13. Masonry Wall Assemblies Evaluated

Wall 

Construction

Wall 

Identification

Vapor 

Retarder 

Perm 

Rating

XPS 

R-Value

Fiberglass 

R-Value

 (perm-in) (h∙ft2∙°F/Btu) (h∙ft2∙°F/Btu) 

High density 

face brick + 

2 wythes fill 

bricks

4A-1-

Uninsulated 10.7 0 0 

4A-1-fiberglass 

batt 10.7 0 13 

4A-1-FOAM 10.7 5 13

3 wythes of 

mediumdensity fill 

bricks

4A-2-

Uninsulated 10.7 0 0 

4A-2-fiberglass 

batt 10.7 0 13 

4A-2-FOAM 10.7 5 13
35 

Uninsulated walls were finished with a 1-in. wood furring strip followed by a 3⁄4-in.-thick cement 

plaster on metal lath. For walls with interior insulation, an interior gypsum board with a Class III 

vapor retarder of 10.7 perm (latex paint) was modeled. The short-wave absorptivity of the façade 

was adjusted to 0.68 (red brick). Initial temperature and RH of the materials were set to 68°F and 

80%, respectively. ASHRAE 160-2009 interior boundary conditions (with cooling) were 

assumed. 

West was determined as the worst-case orientation for climate zone 4A. This was based on 

analyzing the annual wind driven rain load for cold year and warm year weather files. 

5.3 Material Properties

Brick characteristics, especially of existing buildings, are often varied and are not easily known. 

Before retrofitting a brick wall, it is suggested that the brick be tested to gather basic material 

properties. If possible, bricks from the outer wythe as well as the inner wythes should be tested 

separately. Typically, the material properties of the face brick differ from the fill bricks. A better 

grade, denser, more durable and freeze-thaw-resistant brick is often used as a face. Because the

inner wythes are not directly subjected to the elements, they can be less weather resistant than 

the outermost wythe. 

Bricks are available in a variety of densities based on the material properties, composition, and 

manufacturing process. According to ASHRAE Fundamentals 2009, fired clay bricks can vary in 

densities between 70 to 150 lb/ft3

 with conductivities ranging from 2.5 to 10.2 Btu∙in./h∙ft2∙°F, 

respectively. 

Based on the materials available in the WUFI database, CARB modeled two variations of the 

masonry wall: 

1. 12-in. masonry wall with one exterior wythe of high-density face brick and two interior 

wythes of medium-density fill bricks.

2. 12-in. masonry wall with three wythes of medium-density fill bricks. 

These variations are meant to mimic a well-preserved exterior wall with higher grade face brick 

and a wall with a lower grade face brick with significant cracks and chips. Modeling these 

variations illustrates the drastic difference in the hygrothermal performance of two different 

brick walls in the same climate.

Material properties were chosen from the materials database included in WUFI Pro 5.1. Table 14

shows properties for the selected brick (in bold) from the WUFI database. Red matt clay brick 

with a density of 120 lb/ft3

 and permeability of 0.93 perm in was used as the face brick. Solid 

brick extruded with a density of 103 lb/ft3 and a permeability of 13.55 perm in was chosen as a 

medium-density fill brick, which is consistent with the tested properties of a typical fill brick

(Straube and Schumacher 2007). 
36 

Table 14. Material Properties for Brick from WUFI Pro 5.1

Name Density

(lb/ft3) 

Thermal 

Conductivity

(Btu/h∙ft∙°F)

Permeability

(perm in) 

Free Saturation

Wf

(lb/ft3)

Wf 

(%)

Brick Old 104.2 0.231 8.05 12.19 12

Buff Matt Clay Brick 107.3 0.248 4.396 0.43 0

Red Matt Clay Brick 120.8 0.286 0.935 3.50 3

Calcium Silicate Brick 123.1 0.358 0.7 8.99 7

Solid Brick Masonry 118.6 0.347 12.88 11.86 10

Solid Brick Extruded 103 0.347 13.55 23.09 22

Solid Brick Historical 112.3 0.347 8.58 14.35 13

Solid Brick Hand Formed 107.68 0.347 7.57 12.48 12

5.4 Additional Failure Criteria (Masonry Walls)

With the exception of OSB MC, all the same failure criteria that were applied to the frame walls

in the previous sections were applied to the brick walls. In addition, the potential for freeze-thaw 

damage and long-term drying were evaluated. 

5.4.1 Freeze-Thaw Damage

Damage can occur when damp masonry wall assemblies are exposed to frequent freeze-thaw 

cycles. The two factors that influence freeze-thaw damage the most are the MC on freezing and

the number of freeze-thaw cycles (Straube and Schumacher 2006). 

Typically, the number of zero crossings (times when the wall’s temperature falls below or climbs 

above freezing, 0°C or 32°F) are calculated at the external face of the brick: the higher the 

number of cycles, the more potential for freeze-thaw damage (Sedlbauer and Kunzel 2000). 

For damage to occur, however, moisture levels in the brick must be above the critical MC. For 

brick, the critical MC is commonly assumed to be 90% of free saturation (Straube and 

Schumacher 2006). The MC of a material at free saturation corresponds to an RH of 100% in the 

material’s pores. Table 15 displays the saturation values for the brick used in this analysis. 

Table 15. Free Saturation and Critical Saturation of Brick Analyzed 

Material Density

(lb/ft3)

Free Saturation (Wf) Critical Saturation (W90)

(lb/ft3) Mass % Vol % (lb/ft3) Mass- Vol %

Red Matt Clay Brick 120.8 3.5 2.9 5.6 3.2 2.6 5.1

Solid Brick Extruded 103 23.09 22.4 37.1 20.8 20.2 33.3

WUFI usually gives the MC as water density (pounds of water per cubic foot of material), but 

the results can also be expressed as volume percent (cubic foot of water per cubic foot of 

building material) or mass percent (pounds of water per pound of dry building material) as 

shown in Table 15 (WUFI). 
37 

The exterior face brick and the interior first fill brick were evaluated for freeze-thaw damage. 

The freezing temperature for the brick was assumed to be lower than 32°F because of the 

dissolved salts in brick pores (Said et al. 2003). A freezing temperature threshold of 23°F and 

thawing threshold of 32°F were used to estimate the number of freeze-thaw cycles within the 

brick wall (Straube and Schumacher 2006). 

5.4.2 Drying of Masonry Walls

Along with assessing freeze-thaw damage, CARB evaluated the drying capacity of masonry 

walls with and without the addition of interior insulation. A masonry wall fitted with interior 

insulation dries slower than an uninsulated wall because the temperature within the masonry wall 

decreases with the addition of interior insulation. 

Drying performance of the walls was evaluated by calculating the number of years it takes for 

the masonry wall to reach the practical MC (W80) of that material. Practical MC corresponds to 

the equilibrium moisture at a RH of 80%. Where unavailable, the practical MC was extrapolated 

from the moisture storage function graphs within WUFI. 

5.5 Results

5.5.1 ASHRAE Criteria

All of the walls fail this criterion when ASHRAE 160-2009 interior conditions with cooling are 

assumed (Figure 16). For this criteria, CARB evaluated surfaces within the assembly that have 

the most potential to condense: the interior side of the third wythe for the uninsulated walls and 

for the walls with a 1-in. gap and R-13 fiberglass cavity insulation. For walls with foam 

insulation, the interface between the foam and the cavity insulation was evaluated. 

Figure 16. Percent of averages that each wall failed the ASHRAE 160-2009 criteria

0%

10%

20%

30%

40%

50%

60%

70%

4A -1-

Uninsulated

4A-1-FGB 4A-1-FOAM 4A -2-

Uninsulated

4A-2-FGB 4A-2-FOAM

High density face brick + 2 wythes fill bricks 3 wythes of med.density fill bricks

Pecent of Number of Averages

Wall ID

Percentage of 3 Year Period that Each Wall Failed the ASHRAE Criteria

ASHRAE 160 Indoor Conditions

30 Days Moving Average 7 Days Moving Average 24 Hrs Moving Average
38 

5.5.2 Condensation Potential

Using the hourly results from WUFI and isotherms from THERM, CARB analyzed the 

condensation potential for each wall. Condensation potential was calculated at different surfaces 

for each wall based on the wall composition. The interior side of the third wythe was determined 

to be the condensing surface for the uninsulated walls and for the walls with a 1-in. gap and R-13 

fiberglass cavity insulation. For walls with foam insulation, the interface between the foam and 

the cavity insulation was chosen as the potential condensing plane. Figure 17 illustrates the 

condensation potential for each case as predicted by WUFI.

Figure 17. Condensation potential for the masonry walls 

Based on WUFI results, walls with the 1-in. gap and fiberglass cavity insulation are at a higher 

risk than the walls with foam. 

Results of the THERM analysis are displayed in Figure 18. Based on the boundary conditions for 

Nashville, and an interior air temperature of 70°F, the temperature gradient across the wall was 

calculated. This gradient was then compared to the dew point temperature of the interior air. The 

pink dotted line indicates the potential condensation surface. Walls [A] and [C] do not show high 

risk of condensation because the temperature at the condensing surface is near or higher than the 

average dew point temperature. Wall [B] is potentially at risk because the temperature at the 

condensing plane lies below the average dew point. Results from THERM concur with the 

results from the WUFI condensation analysis.

0%

5%

10%

15%

20%

25%

30%

4A -1-

Uninsulated

4A-1-FGB 4A-1-FOAM 4A -2-

Uninsulated

4A-2-FGB 4A-2-FOAM

High density face brick + 2 wythes fill bricks 3 wythes of med.density fill bricks

Pecent of Number of Averages

Wall ID

Percent of Modeling Period that the Condensing Surface falls below Inteirior 

Dew Point Temperature

% Over 3 years
39 

Figure 18. Temperature gradient across the wall sections 

(highlighted dotted line indicates potential condensation plane) 

 

5.5.3 Freeze-Thaw Damage

The hygrothermal simulations predict a very low risk of freeze-thaw damage in the walls in 

climate zone 4A. Figure 19 and Figure 20 show the temperature and MC in wythe 1 of the 

uninsulated wall with low- and high-permeability face bricks. The temperature graph shows the 

freezing limit at 23°F and thawing limit at 32°F. For damage to occur, freezing must take place 

when the MC surpasses the critical saturation threshold. 

The exterior face brick encounters freeze-thaw cycles in winter when the temperature falls below 

the freezing limit of 23°F. However, the MC in this wythe lies significantly below the critical 

saturation threshold, thus eliminating the potential risk of freeze-thaw damage.
40 

Figure 19. Freeze-thaw potential of an uninsulated wall with 

high-density, low-permeability face brick

Figure 20. Freeze-thaw potential of uninsulated wall with 

medium-density, high-permeability face brick

0

5

10

15

20

25

Water Content [lb/ft3]

4A-1- Uninsulated Brick Wall - Wythe 1 Face Brick

Critical

Saturation

[90% of free

saturation]

WC @ Wythe 1

0

20

40

60

80

100

120

Oct-12 Oct-13 Oct-14 Oct-15

Temperature [oF]

Time

Freezing

Temp. [23F]

Thawing

Temp. [32F]

Temp. @

Wythe 1

0

5

10

15

20

25

Water Content [lb/ft3]

4A-2-Uninsulated Brick Wall - Wythe 1 Face Brick

Critical

Saturation

[90% of free

saturation]

WC @ Wythe 1

0

20

40

60

80

100

120

Oct-12 Oct-13 Oct-14 Oct-15

Temperature [oF]

Time

Freezing

Temp. [23F]

Thawing

Temp. [32F]

Temp. @

Wythe 1
41 

The third wythe of the 4A-1-FGB and 4A-1-FOAM walls is colder in winter and warmer in 

summer when compared to the temperature of the third wythe of the uninsulated wall. This is 

evident from Figure 21 and Figure 22. A similar trend is observed in walls with low-density, 

high-permeability face brick, but the decrease in temperature during the winter does not result in 

an increased potential for freeze-thaw damage because the MC remains well below the critical 

saturation threshold.

Figure 21. Freeze-thaw potential of an uninsulated wall with 

high-density, low-permeability face brick

0

5

10

15

20

25

Water Content [lb/ft3]

4A-1-Uninsulated Brick Wall - Wythe 3 Fill Brick

Critical

Saturation

[90% of free

saturation]

WC @ Wythe 3

0

20

40

60

80

100

120

Oct-12 Oct-13 Oct-14 Oct-15

Temperature [oF]

Time

Freezing

Temp. [23F]

Thawing

Temp. [32F]

Temp. @

Wythe 3
42 

Figure 22. Freeze-thaw potential of a wall with fiberglass cavity insulation and 

high-density, low-permeability face brick

5.5.4 Maximum Moisture Content

Comparing the maximum MC within all the walls (shown in Figure 23) over a 10-year modeling 

period reveals that the walls with high-density, low-permeability face brick consistently 

experience lower maximum MC levels than walls with medium-density, high-permeability face 

brick. 

In the first case, the MC in the third wythe is consistently higher than that of the outer two 

wythes, and is highest in the wall with the fiberglass cavity insulation and no foam. In the latter 

case, wythe 1 experiences the highest MC levels because of the high permeability of the face 

brick. The MC in wythes 2 and 3 increases with the addition of interior insulation. However, the 

maximum MC of both the high- and the low-permeability bricks are way below their respective

critical MC thresholds. The critical MC for the low-permeability brick is 5.04 vol %, and the 

critical MC for the high-permeability brick is 33.25 vol %. 

0

20

40

60

80

100

120

Oct-12 Oct-13 Oct-14 Oct-15

Temperature [oF]

Time

Freezing

Temp. [23F]

Thawing

Temp. [32F]

Temp. @

Wythe 3

0

5

10

15

20

25

Water Content [lb/ft3]

4A- 1- Brick + 1" Gap + R-13 FG - Wythe 3 Fill Brick

Critical

Saturation

[90% of free

saturation]

WC @ Wythe

3
43 

Figure 23. Comparison of the maximum MC in each of the three wythes of all the walls

5.5.5 Drying of Masonry Walls

Drying potential of the wall is predicted by plotting the mean MC in volume percent over time. It 

is evident from Figure 24 and Figure 25 that all variations of the walls with high-density, lowpermeability face brick dry over time, with the wall with foam on the inside taking the longest to 

reach a steady state of MC. Though the uninsulated wall and the wall with fiberglass insulation 

start with higher initial MC, they reach a steady MC in 2–3 years’ time. The wall with foam 

takes 4–5 years to reach that level.

0.00

5.00

10.00

15.00

20.00

Uninsulated BRK+FGB BRK+FOAM Uninsulated BRK+FGB BRK+FOAM

High density face brick + 2 wythes fill

bricks

3 wythes med. density fill bricks

Maximum Moisture Content [Vol-%]

Maximum Moisture Content in Wythes 

Wythe 1 Wythe 2 Wythe 3
44 

Figure 24. Effect of interior insulation on the drying time of wythe 2 in all the walls with highdensity, low-permeability face brick

Figure 25. Effect of interior insulation on the drying time of wythe 3 in all the walls with highdensity, low-permeability face brick

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Mean Moisture Content [Vol

20122013201420152016201720182019202020212022

-%]

Drying Time [Years]

4A: High Density Face Brick + Fill in Bricks

Effect of Interior Insulation on Brick Drying Time

Wythe 2: 2nd Fill in brick

4A-1-Uninsulated 4A-1-FGB 4A-1-FOAM

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Mean Moisture Content [Vol

20122013201420152016201720182019202020212022

-%]

Drying Time [Years]

4A: High Density Face Brick + Fill in Bricks

Effect of Interior Insulation on Brick Drying Time

Wythe 3: 3rd Fill in brick

4A-1-Uninsulated 4A-1-FGB 4A-1-FOAM
45 

In walls with the medium-density, high-permeability face brick, the MC increases steadily for 

approximately 5 to 7 years in all three wythes and for all variations of interior insulation, with 

the interior foam resulting in the biggest increase, particularly in wythe 2 and wythe 3 (Figure 26

and Figure 27). The foam insulation acts as a vapor barrier restricting drying to the interior. 

Figure 26. Effect of interior insulation on the drying time of wythe 2 in 

all the walls with medium-density, high-permeability face brick

Figure 27. Effect of interior insulation on the drying time of wythe 3 in 

all the walls with medium-density, high-permeability face brick

0

1

2

3

4

5

6

7

8

Mean Moisture Content [Vol

2012

20132014201520162017

201820192020

20212022

-%]

Drying Time [Years]

4A: Med. Density Face Brick + Fill in Bricks

Effect of Interior Insulation on Brick Drying Time

Wythe 2: 2nd Fill in brick

4A-2-Uninsulated 4A-2-FGB 4A-2-FOAM

0

1

2

3

4

5

6

7

8

Mean Moisture Content [Vol

20122013201420152016201720182019202020212022

-%]

Drying Time [Years]

4A: Med. Density Face Brick + Fill in Bricks

Effect of Interior Insulation on Brick Drying Time

Wythe 3: 3rd Fill in brick

4A-2-Uninsulated 4A-2-FGB 4A-2-FOAM
46 

5.5.6 Isopleths

For all masonry walls with interior insulation in climate zone 4A, the ASHRAE 160-2009 indoor 

conditions result in isopleths that indicate the potential for mold on both the interior surface of 

the drywall and the condensing surfaces. 

5.6 Discussion

The analysis conducted on the two types of brick construction supports the hypothesis that 

determining the hygric properties of the brick are imperative in order to make sound 

recommendations for insulation retrofits. In one case, foam between the stud and the brick results 

in the best option. In the other, adding foam exacerbates the predicted moisture accumulation 

over time. The key parameters to evaluate have been defined, but a sound insulation strategy can 

be employed only after the brick properties are known. 
47 

6 General Discussion 

6.1 Condensation Potential

The failure criteria used in this study were based on previous research studies, manuals on 

moisture control in buildings, and available standards. All resources are listed in the References 

section of this report. But this is far from an exact science. For example, condensation potential 

can be calculated more than one way. As was done in this report and in many other works by 

leading moisture specialists, the dew point of the interior air was compared to the temperature of 

the condensing surface. Every work listed in the References section that deals with condensation 

potential in building assemblies uses the interior air as the metric for determining moisture and 

durability risks. This assumes that air leakage from the interior will be the driving force for 

condensation. This method represents a worst-case scenario in the absence of a bulk water leak 

or other major failure in the building. 

Condensation potential can also be evaluated based on vapor drive. Software like WUFI predicts 

the dew point temperature of various surfaces in the wall based on diffusion. This analysis yields 

different, typically much less severe results. Depending on construction quality, materials 

chosen, and occupant behavior, among other factors, the true answer will likely be somewhere in 

between these two methods. There is no recommended maximum threshold for condensation 

potential in the industry at this time. This value needs to be taken into account with all the other 

criteria and assessed on a climate-by-climate and assembly-by-assembly basis.

6.2 ASHRAE 160-2009 Failure Criteria

Another criterion that is open to interpretation is the ASHRAE 160-2009 running average 

criteria. During a December 2011 Building America Standing Technical Committee call on 

vapor impermeable exterior sheathing, it was noted that other research has indicated that the 

ASHRAE 160-2009 30-day criteria might be too stringent and might need to be reevaluated. 

Other experts in the field contend that the method from which these criteria were derived was

meant for the interior surface of the building assembly alone. This modeling supports that 

suspicion, especially in the cases where failures happen in the first few months of the modeling 

period based on initial conditions, but do not occur again over the remaining 3-year period. 

Finally, isopleths were evaluated for all cases examined. In every circumstance, these graphs 

indicated the potential for mold growth on the interior face of the drywall in the moist A and 

marine C climate zones when the ASHRAE 160-2009 interior conditions were used. From years 

of living in these regions and working in hundreds of buildings, it is the opinion of this 

researcher that these results are flawed. Except for under-ventilated bathrooms and perhaps cool 

damp basements, little mold is found inside homes unless there is an unusually high rate of 

moisture introduced into the space such as a bulk water leak from rain, leaking pipes, or 

unusually high occupancy. These isopleths should be used in context with the results of the other 

failure criteria evaluated.
48 

7 General Conclusions

The conclusions in this report are based on extensive modeling and review of existing research. 

To validate the methods and assumptions used in the modeling, CARB is currently installing 

sensors in an R-40, double stud wall insulated with cellulose in climate zone 5A and a brick 

retrofit in zone 4A. Results are expected at the end of 2012. 

Although many failures were predicted in the walls built to minimum code levels, when hybrid 

walls in climate zone 5A were compared to standard, commonly built walls that are insulated 

with fiberglass only, similar failures were predicted. The fiberglass-only wall performed slightly 

better with respect to the ASHRAE 160-2009 criteria, indicating a slightly reduced potential for 

mold growth, but according to these methods of predicting hygrothermal performance, the 

potential still exists. These predictions need to be tempered with the fact that there are millions 

of homes in the United States built with only fiberglass insulation inside the wall cavities which 

are not rotting or experiencing discernible mold growth. Although these methods of evaluating 

the potential for moisture problems inside building assemblies yield valuable feedback when 

comparing different wall constructions to each other and the effects of different operating 

conditions, each on its own should not be taken as an absolute prediction of hygrothermal 

performance. 

High R-value walls constructed with the OSB on the exterior of all the insulation should employ 

a vented cladding to assist in drying of that layer. Drying to the interior is severely limited in 

these walls. Modeling suggests that increasing the wall’s capacity to dry to the exterior reduces 

the risk for moisture-related damage.

High R-value walls constructed with the OSB sandwiched between a permeable insulation on the 

interior and an impermeable insulation on the exterior should maximize the ratio of impermeable 

to permeable insulation to keep the OSB as warm as possible. This reduces the potential for 

condensation and promotes drying to the interior. Based on these modeling results, a minimum 

of 50% of the total cavity wall R-value should be provided by the impermeable insulation in 

climate zones 4 through 6, and 60% is recommended in climate zone 7. 

Before insulation retrofits are undertaken in buildings with structural brick walls, the 

hygrothermal properties of the brick should be determined. Properties vary so widely across the 

country that no generic recommendations for insulation strategies can be given here. Analysis 

shows that the insulation strategies appropriate for one type of brick are not necessarily 

appropriate for another. Criteria that should be analyzed include freeze-thaw cycles in 

conjunction with the critical saturation threshold and drying potential of the masonry. Analysis 

longer than 3 years is recommended when modeling. 

Further research into the appropriateness of the ASHRAE 160-2009 interior conditions in moist 

climates should be conducted. Interior RH levels generated with this method are extremely high, 

reaching 90% in all cases even though air conditioning is assumed. This results in overly 

pessimistic predictions for mold growth on the interior of the assembly. 

Considering almost every wall in this study failed the ASHRAE 30-day criteria, it is 

recommended that industry professionals reevaluate this threshold. 
49 

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