Moisture Durability with 

Vapor-Permeable 

Insulating Sheathing

R. Lepage and J. Lstiburek

Building Science Corporation

September 2013
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iii

Moisture Durability with Vapor-Permeable Insulating Sheathing

Prepared for:

The National Renewable Energy Laboratory

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

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

Building Science Corporation

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

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

September 2013
iv

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v

Contents

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

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

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

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

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

1.1 Introduction..........................................................................................................................1

1.2 Background..........................................................................................................................2

1.3 Relevance to Building America’s Goals..............................................................................2

1.4 Cost-Effectiveness ...............................................................................................................3

1.5 Tradeoffs and Other Benefits...............................................................................................4

2 Experiment ............................................................................................................................................ 4

2.1 Research Question ...............................................................................................................4

2.2 Literature Review.................................................................................................................4

2.3 Technical Approach.............................................................................................................5

2.3.1 Mechanisms of Moisture Damage and Relevant Forces..........................................5

2.3.2 Hygrothermal Simulations.......................................................................................7

2.3.3 BEOpt Modelling..................................................................................................13

3 Results and Analysis ......................................................................................................................... 15

3.1 Hygrothermal Simulations.................................................................................................15

3.1.1 Climatic Effects .....................................................................................................30

3.2 Energy and Cost Analysis..................................................................................................33

4 Summary ............................................................................................................................................. 35

References................................................................................................................................................. 38
vi

List of Figures

Figure 1. Base case wall assembly. .......................................................................................................... 9

Figure 2. Typical wall assembly with exterior insulation...................................................................... 12

Figure 3. Exterior side MC of structural sheathing with no exterior insulation in Chicago.............. 15

Figure 4. Interior side MC of structural sheathing with no exterior insulation in Chicago. .............. 16

Figure 5. Exterior side of structural sheathing MC with 1 in. of exterior insulation. ......................... 16

Figure 6. Interior side of structural sheathing MC with 1 in. of exterior insulation. .......................... 17

Figure 7. Average exterior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 18

Figure 8. Average interior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 18

Figure 9. Peak exterior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 19

Figure 10. Peak interior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 20

Figure 11. Biodegradation risk plot for the exterior side of the sheathing, with 0 in. exterior 

insulation and 50 Perm WRB............................................................................................................. 21

Figure 12. Biodegradation risk plot for the exterior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB............................................................................................................. 21

Figure 13. Biodegradation risk plot for interior side of the sheathing, with 0 in. of exterior 

insulation and 50 Perm WRB............................................................................................................. 22

Figure 14. Biodegradation risk plot for interior side of the sheathing, with 0 in. of exterior 

insulation and 0.1 Perm WRB............................................................................................................ 22

Figure 15. Average exterior side sheathing MC for 1 in., 1 in.+ Poly, and 1 in.+ Kraft at various 

WRB permeances. .............................................................................................................................. 24

Figure 16. Peak exterior side sheathing MC for 1 in., 1 in.+ Poly, and 1in.+ Kraft at various WRB 

permeances......................................................................................................................................... 25

Figure 17. Average interior side sheathing MC for 1 in., 1 in.+ Poly, and 1 in.+ Kraft at various WRB 

permeances......................................................................................................................................... 25

Figure 18. Peak interior side sheathing MC for 1 in., 1 in.+ Poly, and 1 in.+ Kraft at various WRB 

permeances......................................................................................................................................... 26

Figure 19. Biodegradation risk plot for exterior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and polyethylene sheet interior vapor control.............................. 27

Figure 20. Biodegradation risk plot for interior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and polyethylene sheet interior vapor control.............................. 27

Figure 21. Biodegradation risk plot for exterior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and Kraft interior vapor control. ..................................................... 28

Figure 22. Biodegradation risk plot for interior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and Kraft interior vapor control. ..................................................... 28

Figure 23. Relative humidity adjacent to paper-faced GWB, including 7-day and 30-day running 

average. ............................................................................................................................................... 29

Figure 24. Peak exterior side sheathing MC for listed cities with 1 in. of MFI exterior insulation at 

various WRB permeances. ................................................................................................................ 31

Figure 25. Average exterior side sheathing MC for listed cities with 1 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 32

Figure 26. Peak interior side sheathing MC for listed cities with 1 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 32

Figure 27. Average interior side sheathing MC for listed cities with 1 in. of exterior insulation at 

various WRB permeances. ................................................................................................................ 33

Figure 28. BEopt energy use for Chicago, Illinois................................................................................. 34

Figure 29. BEopt results for Chicago, Illinois. ...................................................................................... 34

Unless otherwise noted, all figures were created by Building Science Corporation.
vii

List of Tables

Table 1. ASHRAE 160 Failure Criteria....................................................................................................... 8

Table 2. Boundary Conditions. ................................................................................................................ 10

Table 3. Simulated Cities and HDD and CDD Data (ASHRAE, 2009). .................................................. 10

Table 4. Cladding Material Effects on OSB MC...................................................................................... 11

Table 5. Table of Testing Variables. ........................................................................................................ 13

Table 6. Cost per Square Foot for Wall Options. ................................................................................... 14

Table 7. Airtightness by Water Resistive Barrier................................................................................... 14

Table 8. Reference Cities.......................................................................................................................... 14

Table 9. Hours above 28% MC for Identified Wall Assemblies............................................................. 23

Table 10. Hours above 28% MC for Identified Wall Assemblies with Interior Vapor Control............ 29

Table 11. Peak and Yearly Average MC for Plywood Sheathing.......................................................... 30

Table 12. Peak and Average Moisture for Plywood Sheathing with Polyethylene Sheet.................. 30

Table 13. List of Cities and Associated Climate Zones......................................................................... 31

Unless otherwise noted, all tables were created by Building Science Corporation.
viii

Definitions

ACH Air Changes per Hour

BA Building America Program. More information about BA can 

be found at www.buildingamerica.gov

BSC Building Science Corporation. More information about BSC 

can be found at www.buildingscience.com 

DOE U.S. Department of Energy

GWB Gypsum Wall Board

MC Moisture Content

MFI Mineral Fiber Insulation

OSB Oriented Strand Board

RH Relative Humidity

VWC Vinyl wall covering

WRB Water resistive barrier

WUFI Wärme und Feuchte instationär; hygrothermal simulation 

software; see http://www.wufi-pro.com/
ix

Executive Summary

Exterior sheathing insulation is an effective strategy in increasing the overall R-value of wall 

assemblies; other benefits include decreasing the effects of thermal bridging and increasing the 

moisture durability of the built assembly. Vapor-permeable exterior insulation, such as mineral 

board or expanded polystyrene foam, are one such product that may be used to achieve these 

benefits. However, uncertainty exists on the effects of inward driven moisture and the interaction 

of increased sheathing temperatures on the moisture durability of the edifice. 

Inward driven moisture is only a prominent concern when a wetted moisture-storing cladding is 

exposed to elevated levels of solar radiation. The elevated cladding temperatures create a high 

vapor pressure that drives the moisture into the wall assembly. To mitigate inward flowing 

moisture, it suffices to utilize a low permeance water resistive barrier (WRB). However, this also 

inhibits outward flowing moisture.. The complication arises when, during the heating season, 

outward flowing moisture is inhibited by the WRB, potentially resulting in condensation. The 

alternatives to minimize moisture accumulation are to either increase the surface temperature of 

the sheathing (with the use of exterior insulation), or to enable outward drying of the sheathing. 

To address these concerns, Building Science Corporation (BSC) conducted a series of 

hygrothermal models for cities representing a range of different climate zones (DOE Climate 

Zones 1, 3, 4, 5, and 7). A parametric study was conducted to assess the range of impact of

varying levels of exterior insulation (0 in., 1 in., 2 in., 4 in. of mineral board, R4 per inch) and 

permeances (0.1, 1, 10, 50 perms) of the WRB . Other modulated variables include the presence 

of interior vapor control (polyethylene sheet, Kraft paper), type of structural sheathing (plywood 

or OSB), and the air exchange rates of the gap behind brick cladding (1-4 air changes per hour).

The team found that a WRB permeance in the range of 1 to 10 perms enables sufficient throttling 

of inward driven moisture while still enabling outward drying in all climate zones, from 1 to 7, 

with at least 1 in. of exterior insulation (R4). However, very low permeance WRB (less than 1

perm) should not be utilized unless 1 in. or more of exterior insulation is provided. The team

recommends that at least 2 in. of exterior insultion is utilized in climate zones 6 and 7. High 

permeance WRBs (50 perms), should not be used with vapor-permeable exterior insulation with 

reservoir claddings that are exposed to elevated levels of rain. Low permeance interior vapor 

control results in elevated moisture content (MC) of the sheathing, by capturing the inward 

driven moisture. Low permeance interior vapor control layers should be avoided.

Section 1.0 provides a complete description of the research project and a rationale for the 

investigations as well as the cost basis. Section 2.0 details BSC’s research method, approach, the 

key research questions that were examined, and the procedures utilized to analyse the problems. 

Section 3.0 describes the analysis that will be completed and Section 4.0 summarizes the results.
1

1 Problem Statement

1.1 Introduction

The moisture performance of walls with higher cavity R-values using permeable insulations, 

particularly in homes with lower air exchange rates, is poorly understood. Higher risks of air 

leakage wetting and reduced diffusive drying potentials are theoretically expected to increase 

moisture risk (Straube & Smegal, 2009), but little research is available to better quantify this 

risk. Adding insulating sheathing to existing walls with these characteristics is one way of further 

increasing the thermal resistance of these assemblies. However, for other reasons these exterior 

insulating sheathing products may reduce the outward drying potential of the assembly. 

Insulating sheathing are available in a range of permeances from very high (i.e., more than 70 

US Perms) to very low (i.e., under 0.1 US Perms). There is a need for further research to identify 

the circumstance in which a vapor-permeable insulating sheathing, defined as having a 

permeance greater than 5 perms (IRC, 2012) (such as mineral wool, glass fiber) would be 

preferred over a lower permeable product. This research also includes consideration for the 

impact of several common cladding types, such as vinyl siding, stucco, wood, fiber cement

siding, and brick.

The biggest concern regarding vapor-permeable insulating sheathings is inward driven moisture 

caused by solar radiation hitting a wetted moisture storing cladding, a problem that occurs in all 

climates. When a wetted reservoir cladding is exposed to elevated solar radiation, a high vapor 

pressure is created behind the surface of the cladding. This high vapor pressure results in

outward drying but also creates an inward vapor drive, particularly if the indoor is air 

conditioned to a lower vapor pressure. The vapor permeance of the weather resistive barrier 

(WRB), as well as any structural sheathing (i.e. OSB, plywood, etc) may throttle inward vapor 

drive, but the degree of vapor restriction is unknown and the effects on the moisture durability of 

the wall assembly has not been quantified in such circumstances. A complication arises when a

low vapor permeance WRB is used in cold climates where outward vapor flows may occur. A 

low permeable WRB with no insulating sheathing with vapor-permeable cavity insulation results 

in decreased sheathing temperatures, which may result in condensation. To mitigate this, either 

the sheathing temperature must be elevated, through the use of exterior insulation, or a more 

vapor permeable WRB must be utilized, to enable outward drying. However, the ideal range of 

WRB permeances and exterior insulation is not well defined or understood.

Currently, a range of cost-effective options exists for retrofitting the existing building stock, but 

fall beyond the scope of this project. The addition of exterior insulating sheathing is frequently 

cost prohibitive; however, removal of the cladding is a necessary step to access the sheathing. 

Only select circumstances warrant the addition of exterior insulation: a degraded cladding that 

requires replacement (whereby the addition of a nominal amount of exterior insulation results in 

little additional cost), or a building owner who desires to significantly reduce the energy 

consumption of the structure caused by low thermal resistance of the building enclosure. The 

results from this research project may be equally applied to new construction with similar 

parameters.
2

1.2 Background

Extensive research has been conducted by others into the effects of inward driven vapor in wall 

assemblies. Research has demonstrated that accumulation of moisture in interior finishes caused 

by inward driven moisture may pose moisture durability problems with the wall assembly

(Wilson 1965, TenWolde and Mei 1985, Straube and Burnett 1995, Pressnail et al. 2003, 

Dérome et al. 2010, Dérome and Saneinejad, 2010, Carmeliet and Dérome, 2012). It was found 

that this phenomenon occurs in all climates (from hot and humid to cold and dry climates) and in 

a range of different wall assemblies, all to varying degrees. 

Many strategies have been provided by researchers in mitigating the effects of inward vapor 

diffusion, such as ventilation behind the reservoir cladding or the use of vapor-retarding WRB 

membranes. However, the use of vapor-retarding membranes may result in winter time 

condensation problems in retrofit homes with permeable cavity insulation and no exterior 

insulation. The cavity insulation results in decreased sheathing temperatures that may, depending 

on the interior and exterior climates, cause the sheathing to reach temperatures below the interior 

air dew point. While vapor diffusion may pose condensation problems, the most prominent 

concern is due to air leakage condensation (Quirouette, 1985; CBD 5 A.J. Wilson, 1960; CBD 23 

A.J. Wilson, 1961), which can carry more moisture than vapor diffusion.

Despite the extensive research on inward driven moisture caused by solar radiation, more 

research is still needed. In this research effort. Building Science Corporation (BSC) investigated

the effects of using vapor-permeable insulating sheathing on existing buildings. Maintaining the 

sheathing at higher temperatures will change the temperature dependent vapor permeability, 

sorption isotherm, relative humidity (RH), and drying capacity. Similarly, a higher sheathing 

temperature may also pose increased risk for biodegradation by being more amenable to mold 

and rot growth, should sufficient moisture be available.

1.3 Relevance to Building America’s Goals

Overall, the goal of the U.S. Department of Energy's (DOE) Building America program is to 

“reduce home energy use by 30%-50% (compared to 2009 energy codes for new homes and preretrofit energy use for existing homes).” To this end, we conduct research to “develop marketready energy solutions that improve efficiency of new and existing homes in each U.S. climate 

zone, while increasing comfort, safety, and durability.”1

The addition of exterior insulation enables increased wall R-value more than would otherwise be 

achievable in standard 2×4 in. or 2×6 in. stick frame construction. The addition of an extra 1.25

in. of continuous vapor-permeable exterior insulating sheathing can provide an additional R5 to a 

wall assembly while significantly reducing the effects of thermal bridging. On a 2×4 in. at 16 in.

on center stick frame construction with R13 batt insulation, this results in a 30% energy 

reduction of space conditioning site energy losses through the opaque wall assembly. Increasing 

the thickness of the exterior insulation sheathing only increases the energy savings. Furthermore, 

the addition of exterior insulation decreases the propensity for cold weather condensation 

forming on the back of the structural wall sheathing, as well as providing superior insulation 

performance by minimizing thermal bridging. These factors will reduce embodied energy 

through improved durability and lifespan and greatly reduce energy use for space conditioning

for the life of the enclosure.

 1 http://www1.eere.energy.gov/buildings/building_america/program_goals.html
3

The implementation of outboard insulation wall systems is easily included into any residential 

building retrofits with only minor detailing required around wall penetrations and fenestration. 

Used in conjunction with other recommended high-R-value systems (Straube and Smegal, 2009), 

the use of permeable exterior insulating sheathing can significantly reduce the space conditioning 

energy consumption of residential homes helping to meet the Building America goals of 30%-

50% energy use reduction.

1.4 Cost-Effectiveness

To ensure the cost-effectiveness of the retrofit proposals, a detailed BEopt analysis was

conducted. Each proposed wall assembly will be assigned a cost relative to standard 

construction. These costs will be developed in partnership with BSC’s prototype and community 

builders. 

It is important to note that, should an exterior insulating strategy be adopted for a building 

rehabilitation project, the incremental costs of adding a nominal thickness of exterior insulation 

and application of a WRB with the desired vapor permeability will be very small, as the costs of 

the cladding, sheathing, etc., already included as part of the building rehabilitation. However, if 

the sole intent of the retrofit is to increase the overall wall R-value, then the associated costs of 

the proposed retrofit will be significantly higher.

It cannot be ignored that an initially slightly more expensive system may have to be implemented 

to save a significant amount of energy over the entire life of the structure, which will be much 

longer than a standard mortgage. Research has shown that walls exceeding an R-value of 35 can 

financially pay back during the life of the initial mortgage through energy savings while reducing 

greenhouse gases (Grin, 2008). Because the building enclosure is designed to use less energy, the 

energy and greenhouse gas emissions savings extend for the life of the building, not just the 

duration of the initial mortgage or retrofit loan.

Improving the moisture tolerance and durability of an assembly will also add into the equation of 

life cycle cost analysis, as decreases in expenditures will be required for repairs and remediation 

caused by biodegradation. Furthermore, the longer the assembly lasts, the more energy it will use 

over its lifetime and the more the initial energy efficiency savings will accrue. Proper detailing of 

the assembly is also important to ensure that, over the life of the assembly as components require 

replacement (such as windows and doors), the assembly easily allows these replacements 

without risking damage.
4

1.5 Tradeoffs and Other Benefits

The benefits of the use of properly detailed and installed vapor-permeable exterior insulation in 

retrofit applications with vapor-permeable cavity insulation, as compared to a code wall, are as 

follows:

 Higher R-value

 Lower space conditioning costs

 Enhanced durability and enclosure lifespan

 Increased airtightness

 Increased occupant comfort.

Each of these components are interconnected. The increased R-value and airtightness improve 

energy efficiency and occupant comfort through reducing drafts and improving surface 

temperatures. The added durability of the system reduces maintenance requirements, increases 

the lifespan of the structure, and tolerance to the possible operating conditions within the home.

2 Experiment

2.1 Research Question

The following research questions were answered by this project.

1. What insulating sheathings are available for retrofit applications, what are their material 

characteristics as they relate to thermal resistance and vapor permeance, and what kind of 

cladding attachments can be used?

2. What are the characteristics of existing assemblies? (Note: we will characterize the two 

most common assemblies that are likely to be retrofit in the cold and hot climate zones.)

3. What range of cladding types are common for existing assemblies?

4. What types of interior vapor-control layers are common and what are the permeances of 

these?

5. What is the airtightness of these assemblies?

6. What are the likely retrofit solutions and what guidelines should be established for the 

airtightness, water control, thermal performance and vapor permeance? 

7. What are the recommended solutions? (Note: we will identify how the functions of the 

building enclosure (air control, etc.) are served by the retrofit layers).

2.2 Literature Review

In the literature, an ASHRAE Research Project RP-1235, The Nature, Significance, and Control 

of Solar-Driven Water Vapor Diffusion in Wall Systems, by D. Dérome, A. Karagiozis, and J. 

Carmeliet (2010) was found. In it, it synthesises results from small and large scale laboratory 
5

testing, field testing, and computer simulations. The experimental testing was conducted by 

initially wetting the cladding material and then exposing it to cyclical heating cycles to simulate 

the radiative heating from the sun. Sensors and scales were utilized to quantify the moisture 

accumulation through the depth of the test walls. A total of 18 walls were tested, including 

variations on the interior vapor control (either latex paint of vinyl wall coverings), as well as 

difference in permeability of exterior WRBs.

Some of the key discoveries from this research project include that warm and mixed climates 

pose a risk to vapor-permeable wall assemblies. Further, the occurrence of rain, followed by sun, 

a liquid-absorbing cladding, and the presence of an air cavity, resulted in large inward vapor 

flows.

Similar results were obtained by Wilson 1965, TenWolde and Mei 1985, Straube and Burnett 

1995, Pressnail et al. 2003, Dérome et al. 2010, Dérome and Saneinejad, 2010, Carmeliet and 

Dérome, 2012.

2.3 Technical Approach

A literature review was conducted to assess the relevant properties of moisture related concerns 

of vapor-permeable insulations. Subsequently, hygrothermal modeling software was utilized to 

evaluate the thermal and hygric properties of the proposed assemblies. The hygrothermal 

modelling software chosen was WUFI 5, by the Fraunhofer Institute and Oak Ridge National 

Laboratory. The modeling will be used to determine the most important properties of the wall 

assembly and local climate, which dictate the tendency for inward and outward driven moisture 

potentials.

Due to the focused scope of the research, only wall assemblies with reservoir claddings with 

vapor-permeable cavity insulation (thus necessitating stick-frame construction) were simulated 

and analysed. Unfortunately, this precludes other types of wall assemblies and does not properly 

represent certain climate zones that favour alternative building structures, such as concrete block. 

The goal of the simulations is to determine an optimal range of vapor permeances for the WRBs 

and thickness of the vapor-permeable exterior insulation such that moisture flows are controlled 

and throttled to safe levels. Idealized materials (i.e. a WRB with vapor permeance of 1, 3, 5, 7, 

etc), are utilized to obtain the range of optimal vapor permeances and comparable and readily 

available products on the market are selected based on how closely they comply with the 

suggested vapor permeance.

2.3.1 Mechanisms of Moisture Damage and Relevant Forces

The concerns related to vapor-permeable wall assemblies with moisture sensitive material are the 

inward and outward flows of moisture, primarily by vapor diffusion and convection. Inward 

driven moisture is a result of precipitation being stored in the cladding and water vapor pressure 

differentials, created by high temperature differentials, being forced into the wall. The equation 

defining the vapor flow rate is shown in Equation 1 (Carmeliet & Dérome, 2012).
6

gv = −�δ ∙ pvsat(T)�∇φ − �δφ

∂pvsat(T)

∂T � ∇T (1)

where, 

gv = vapor flow rate [kg·m-2·s

-1

]

δ = vapor diffusion coefficient [kg·Pa-1

·m-2·s

-1

]

T=temperature [K]

pv= vapor pressure [Pa]

pv,sat=saturated vapor pressure [Pa]

φ= pv/pv,sat(T), relative humidity

This equation is a modified form of Fick’s Law that describes isothermal vapor flows with RH

and temperature as the driving force. The first term defines the flow caused by difference in RH, 

and the second term defines the flow caused by temperature gradients.

Various components of the wall assembly restrict the vapor flows. Predominantly, this restriction 

of flow is created by the WRB or the structural sheathing (OSB or plywood). Some wall 

assemblies also feature polyethylene sheet immediately on the exterior of the interior gypsum 

wall board (GWB), which inhibits the ingress of the moisture into the conditioned space.

Depending on the vapor permeance of the WRB, outward moisture flows may also create a 

moisture risk to the wall assembly. Structural sheathing below the dew point may result in the 

formation of condensation from vapor diffusion or air leakage. The use of exterior insulating 

sheathing increases the sheathing temperature and may result in sheathing temperatures above 

the dewpoint of the interstitial air, which reduces condensation risk.

The factors that affect moisture flows through wall assemblies, in approximate order of 

magnitude as discovered through the simulations contained within this report, are:

• Climate

• Micro-climate

• Presence and thickness of exterior insulation

• Vapor permeance of the WRB

• Other factors, including interior RH, material resistance to moisture decay, etc. 

Climate and Microclimate

The climate and microclimate, that is, the climate immediately adjacent to the wall surface,

generate the exterior conditions of the wall assembly. Climates with high moisture loading and 

significant solar radiation pose a greater moisture damage threat to wall assemblies. Contrarily, 

wall assemblies located in dry climates with very little precipitation are much less likely to 

experience moisture decay. The microclimate also significantly modifies the wall’s 

environmental exposure. Areas that are protected from precipitation by items such as large 

overhangs, or are not exposed to solar radiation, are less likely to experience inward driven 

moisture.
7

The climate and microclimate are controlled by the location of the modeled city, the rain 

adherence, exposure, and deposition factors, the surface transfer coefficient, and orientation.

Exterior Insulation and Vapor Permeance of the WRB

The next factors that affect the moisture tolerance of wall assemblies are the presence of exterior 

insulation and the vapor permeance of the WRB. The presence of exterior insulation alters the 

surface temperatures of subsequent inboard wall layers. Higher temperatures result in higher 

Gibbs energy and, as demonstrated through the Arrhenius equation, result in a high capacity for 

drying. The vapor permeability of the WRB restricts moisture flows into the material.

Other Factors

The interior RH affects the MC of the sheathing predominantly during the heating season in cold 

climates. The colder exterior portions of wall assemblies result in lower saturated vapor

pressures and thus a higher propensity for condensation. The higher the interior RH, the higher 

the potential moisture flow, and thus results in higher quantities of condensate. The material 

resistance to moisture decay is also an important factor, but is not as predominant as those

previously listed. A wood substrate is more tolerant to moisture decay than the paper face of 

GWB. A glass-faced GWB is even more tolerant to moisture decay than wood. Depending on the 

substrates, mold and rot may not be as relevant. However, for the purposes of this report, the 

predominant substrates are either plywood, OSB, or paper-faced GWB.

2.3.2 Hygrothermal Simulations

The hygrothermal numerical program chosen to analyse the moisture flows in the wall 

assemblies was WUFI 5, by the Fraunhofer Institute for Building Physics and Oak Ridge 

National Laboratories. Current moisture flow theory has difficulty in properly describing for the 

inhomogeneity, temperature and moisture-dependant properties, and anisotropic properties of

building materials. However, WUFI was programed with the underlying equations being 

calibrated and based upon macroscopic empirical behaviour of organic and inorganic materials 

(Künzel, 1995). This precludes the detailed testing required to generate topological material 

properties (e.g. pore size distribution, frequency of checks and cracks, etc.). The accuracy of the 

WUFI simulations have been verified by the Fraunhofer Institut Bauphysik in Holzkirche, 

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

WUFI possess the capacity to properly account for water vapor adsorption and the 

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

common time step being one hour, considering the effects of sun, rain, temperature, and 

humidity. The quality of the results is extremely dependant on the quality and accuracy of the 

input material and condition data.

2.3.2.1 Metric for Analysis

Great difficulty occurs in determining the moisture durability of building materials. The MC to 

decay mechanism response is not well categorized and defined. WUFI isopleths provide insight 

into the propensity for biological growth but are based on agar substrates, not the various 

materials used in the building wall assemblies. The ASHRAE 160-09 criteria are particularly 

stringent and do not necessarily represent realistic interior RH. Based on the provided equations 

on determining indoor RH (ASHRAE 160-09), Criterion 1 failures (see Table 1) occur on the 

interior surface of the GWB due to elevated humidity during summer months in warm and humid 

climates.
8

To restrict concerns over moisture failure criteria, a relative comparison is utilized instead. This 

eliminates concerns over the absolute accuracy of the wall assembly and is better suited to 

compare the moisture performance of the different wall assemblies. This approach is preferred as 

this is a comparative study of the effects of different vapor permeances and insulation levels of 

wall assemblies.

The MC of a thin slice (4mm) of the OSB on the interior and exterior faces is obtained from the 

simulations on an hourly basis. Only data from the second year of modelling, after verifying that 

first year effects were negligible, is analyzed, to minimize the effects of construction moisture. A 

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

artificially reduce the actual MC with the dryer core. To compare the results, the number of 

hours occurring between 5°C (41°F) to 40°C (104°F) at MC above 28% for wood substrates are 

calculated. Similar calculations are made for MCs above 24% and 20%. However, for GWB, due 

to the sensitivity of the paper facers, the ASHRAE 160 criteria will be utilized (less than 80% 

RH over 30 days).

The failure criteria according to ASHRAE 160-09 are listed in Table 1.

Table 1. ASHRAE 160 Failure Criteria.

Criteria Description

Criterion 1 30-day running average surface RH <80% 

when the 30-day running average surface 

temperature is between 5°C (41°F) and 40°C 

(104°F)

Criterion 2 7-day running average surface RH <98% when 

the 7-day running average surface temperature 

is between 5°C (41°F) and 40°C (104°F)

Criterion 3 24-h running average surface RH <100% when 

the 24-h running average surface temperature 

is between 5°C (41°F) and 40°C (104°F)

The testing will not include extreme values usually associated with major disasters such as 

hurricanes or 1/100 year temperatures.

2.3.2.2 Identification of Topology and Boundary Conditions

A typical wall assembly was first identified by inspecting typical building wall assembly types 

that would be prone to inward vapor drives and that fall within the purview of this research 

project (i.e. in need for recladding, rehabilitation, or improvement of the thermal performance of 

the building). The purpose of this wall assembly is for preliminary modeling to determine 

relevant factors that affect the moisture load on the wall assembly. Such factors include the 

identification of an appropriate cladding and orientation. 

This wall (Figure 1) is made up of a brick cladding, a 3⁄4 in. air space (with 4 ACH venting), two 

layers of #15 felt, 7/16 in. OSB, 2×4 stick framing with R13 fiberglass batt, 1⁄2 in. GWB, and 

latex paint (set to a vapor resistance of 10 perms (Class III)). According to Table R601.3.1 (IRC, 

2009), a class III vapor retarder may be used as the cavity space is ventilated and the location is 

in climate zone 5.
9

Figure 1. Base case wall assembly.

The boundary conditions aid in the generation of the moisture loads and fluxes on the wall 

assembly. A theoretical two story residential home, in a moderately wind exposed area with no 

shading from adjacent structures or objects, was designed. The values for the boundary 

conditions are shown in Table 2.
10

Table 2. Boundary Conditions.

Boundary Conditions Value Notes

Wall Orientation E/SE/S/SW/W Worst case orientation for inward driven 

moisture depends on climate

Wall Inclination 90°

Rain Exposure Factor 0.9 From Straube and Burnett, 2005

Rain Deposition Factor 0.5 From Straube and Burnett, 2005

Rain Adherence Factor 1 Rain adherence accounted for in rain load 

calculation

Exterior Surface Heat Transfer 

Coefficient

0.33 (0.0588) °F·ft2

·h·Btu-1 (m

2

K/W) 

Short Wave Radiation 

Absorptivity

0.68 For red brick

Long-Wave Radiation 

Emissivity

0.9

Interior Surface Heat Transfer 

Coefficient

0.71 (0.125) °F·ft2

·h·Btu-1 (m

2

K/W)

Int. Water Vapor Resistance 0.3 In meters of equivalent still air. Represents 

the vapor resistance of latex paint (10 perms).

To determine the orientation and cladding type, preliminary modeling was conducted with the 

above hypothetical wall in select cities. These cities, seen in Table 3, were representative for a 

range of climate zones. The embedded weather files in WUFI were used for this modeling. The 

interior temperature and RH utilized in the modeling are also included in Table 3. The 

temperature and RH vary sinusoidally over the period of a year, with the amplitude indicated 

with the ± symbol. The temperature peaks on July 20, whereas the RH peaks on August 20.

These values were selected based on BSC experience and recorded data.

Table 3. Simulated Cities and HDD and CDD Data (ASHRAE, 2009).

City DOE 

Climate 

Zone

HDD-65°F 

(18.3°C)

CDD-65°F

(18.3°C)

Indoor 

Temperature

°F (°C)

Indoor RH 

(%)

International Falls, 

MN

7 10487 (5826) 248 (138) 70±2 (21±1) 30±10

Chicago, IL 5A 6311 (3506) 842 (468) 70±2 (21±1) 40±10

St. Louis, MO 4 4504 (2502) 1631 (906) 70±2 (21±1) 40±10

Seattle, WA 4C 4729 (2627) 176 (98) 70±2 (21±1) 45±5

Atlanta, GA 3 2990 (1661) 1667 (926) 72±2 (22±1) 45±5

Miami, FL 1A 130 (72) 4459 (2477) 72±2 (22±1) 55±5

The orientation was determined by running the wall assembly in the above cities at all the 

cardinal orientations from East to West. The yearly average MC of the sheathing was determined 

and compared between each of the orientations. The orientation that resulted in the highest 

yearly average MC was deemed to be the “worst case” orientation for the wall assembly in that 

given climate.
11

Table 4. Cladding Material Effects on OSB MC.

Cladding 

Material

WUFI Material 

Name

WUFI 

Catalog

Max Exterior 

OSB MC

Max Interior 

OSB MC

Brick Solid Brick 

Masonry

FraunhoferIBP 

Holzkirchen; 

Germany

31.3 26.3

Stucco Regular Portland 

Stucco

Generic North 

America 

Database

14.1 14.1

Wood Eastern White 

Pine

Generic North 

America 

Database

14.1 14.0

Cement Board Fiber Cement 

Sheathing Board

Generic North 

America 

Database

14.6 13.6

Lime Plaster 

(Stucco)

Regular Lime 

Stucco

Generic North 

America 

Database

15.4 13.9

From the initial tests, the solid brick masonry, which is an assembly level property, accounting 

for the mortar and the joints between the material interfaces, was selected as it generates 

noticeable inward vapor drives. It is known that the other materials also generate inward vapor

drives in real world conditions, but the material properties provided in the simulation software do 

not account for an assembly level property (i.e. testing the moisture flux across a pristine wood 

sample instead of wood siding, with joints and gaps). 

2.3.2.3 Approach to Simulations

Now that the boundary conditions have been established, the test wall may be modelled with 

slight variations that may impact the moisture behaviour of the wall assembly (see Figure 2). All 

of these tests were conducted in Chicago so as to obtain a relative influence on the wall 

assembly. The discoveries from the Chicago simulations were then extended to the other cities to 

verify that the similar phenomenon occurs. 
12

Figure 2. Typical wall assembly with exterior insulation.

The variables that are to be modulated may be found in Table 5.
13

Table 5. Table of Testing Variables.

Parameters Variable Notes

WRB Permeance 0.1,1,10,50 perms

Exterior Insulation Thickness 0,1,2,4” Roxul Rockboard

Exterior Insulation Permeance 110 perms Measurements from the 

Fraunhofer Institute (IBP)

Interior Vapor Control Polyethylene 0.07 perm

Kraft Paper 1.7 perm

Vinyl 

Wallpaper

0.3 perm Also used to mimic cabinets 

and cupboards. Only modeled 

in St. Louis.

Structural Sheathing 7/16” OSB or 7/16” Plywood Most tests run with OSB, tests 

with plywood were used to 

quantify the difference

Due to all the permutations available, only certain variables are tested in combination with one 

another. The salient testing goals are to identify the optimal range of WRB permeances with 

varying levels of insulation to minimize moisture durability concerns. Other factors listed above 

may modify the results, but these simulations will only be conducted on the wall assembly with

the recommended WRB and insulation levels. In situations where the results were ambiguous, 

parametric testing was conducted to obtain an upper and lower limit to bound the problem.

2.3.3 BEopt Modeling

In most circumstances, the addition of exterior insulation necessitates an exterior retrofit. The 

addition of exterior insulation in retrofit applications for the intent of minimizing energy costs 

will not result in a short-term payback. The incentive behind installing new cladding may be 

from any number of sources, including existing water management problems, comfort or 

durability concerns, end of service life for the cladding, or aesthetic concerns. The need to 

replace the cladding provides an opportunity for the designer or contractor to include exterior 

insulation as a means to increase the energy performance of the building at the same time. The 

cost effectiveness of this from an energy perspective is therefore dependent on the cost of the 

insulation as well as any associated components above and beyond the new cladding installation.

Simulations were run using BEopt simulation software developed by the National Renewable 

Energy Laboratory (NREL). BEopt functions by comparing the annual energy savings of a 

retrofit strategy compared to the annualized cost of the capital expense to install that technology.

However, as the primary concern in this research is moisture, it is difficult to quantify risks of 

moisture damage into a defined monetary value. For the purposes of analysis, it was assumed 

that the existing wall was 15 years away from the end of its useful service life. Thereafter, it 

would have to be replaced. This number is derived from engineering judgment and experience, 

but should be considered carefully when viewing the results of the simulation.

The benchmark home is a 40 ft × 50 ft single story, slab-on-grade house with a 6:12 sloped 

trussed roof. The window to wall ratio is 15% with a glazing distribution of 20% at the front and 

sides, and 40% at the back. It has a vented attic with R30 blown-in cellulose. The parameters that 
14

were controlled were the wall assembly and infiltration. All other parameters had the BEopt 1.3 

default values.

For the comparison in the model, either the existing wall was retrofitted with exterior insulation 

and a new WRB, or the standard wall was replaced after 15 years. Table 6 shows the per unit 

area cost for the improvements (including material and installation costs), whereas Table 7

shows the improved airtightness of the building as a result of standard air sealing techniques in 

combination with a new WRB. These values were selected based on predefined numbers 

included in BEopt and on anticipated achievable tightness levels in retrofit homes. Table 8

indicates the references cities that were used for the modelling.

Table 6. Cost per Square Foot for Wall Options.

Wall Assembly Additional Cost per Ft2

Sheet WRB Premium Sheet WRB Self-Adhering 

Membrane

Replacement Standard 

Wall $3.81 $3.90 $4.34

Replacement Standard 

Wall + 1 in. MFI $4.42 $4.62 $5.29

Existing Wall +1 in. MFI $0.83 $1.03 $1.70

Existing Wall +1.5 in. MFI $0.94 $1.26 $2.27

Exiting Wall + 4 in. MFI $1.74 $0.62 $0.97

Table 7. Airtightness by Water Resistive Barrier.

Water Resistive Barrier Airtightness (ACH50)

Typical Existing 10.3

Typical Sheet WRB 3.7

Premium Sheet WRB 3.7

Self-Adhering Membrane 1.9

Table 8. Reference Cities.

Reference Cities

Chicago, IL

Duluth, MN

Miami, FL

Seattle, WA

Cost data for the various wall components were taken from RS Means Construction Data (2012 

Reed Construction Data). Costs that were not included were inferred based on engineering 

judgment, internal costing analysis, and from input from Building America builder partners.
15

3 Results and Analysis

3.1 Hygrothermal Simulations

Hygrothermal simulations were conducted to identify the relative impact of a multitude of 

boundary conditions, building material properties, and topologies that impact the moisture 

durability of the wall assembly.

The first series of simulations demonstrate thevariations of the MC of a thin exterior and interior 

slice of the OSB sheathing with a 0.1, 1, 10, and 50 perm WRB with 0 in. of exterior insulation 

and 1 in. of exterior insulation, using only latex paint as vapor control (see Figure 3 to Figure 6). 

These figures demonstrate the the annual fluctuation of MCs. From these plots, we may observe 

that the addition of insulation drastically reduces the MC, particularly on the interior side of the 

OSB (outward flowing moisture during the winter months). The resultant sheathing MC in this 

analysis is from vapor diffusion only and does not account for possible air leakage condensation.

The labelling system is first characterized by the cladding type: the thickness of exterior 

insulation; the permeability of the WRB; and finally, a letter indicating any additional properties 

(i.e. P for polyethylene sheet, K for kraft paper, V for vented). An example of a wall assembly 

with brick cladding, with 2 in. of exterior insulation, a 10 perm WRB, and Kraft faced batt 

insulation would be: Brick-2-10-K.

Figure 3. Exterior side MC of structural sheathing with no exterior insulation in Chicago.
16

Figure 4. Interior side MC of structural sheathing with no exterior insulation in Chicago.

Figure 5. Exterior side of structural sheathing MC with 1 in. of exterior insulation.
17

Figure 6. Interior side of structural sheathing MC with 1 in. of exterior insulation.

The figures show that even 1 in. of exterior insulation significantly reduces the MC of the OSB 

sheathing on both the exterior and interior sides of the OSB (Figure 5 and Figure 6 vs Figure 3

and Figure 4). When the yearly average MC of the OSB sheathing (both interior and exterior 

slices) for wall assemblies with 0 in., 1 in., and 2 in. of exterior insulation is plotted against the 

vapor permeance of the WRB, Figure 7 and Figure 8 are produced. The minimum value of the 

curves on these plots indicates the optimal range for the vapor permeance of the WRBs. The 

most important factor to observe is that adding exterior insulation benefits the wall insulation 

more significantly than optimizing the vapor permeance of the WRB (with some slight 

exceptions at high vapor permeances). 
18

Figure 7. Average exterior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances.

Figure 8. Average interior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances.
19

The yearly average numbers all fall well below the 19% MC, a moisture level that is considered 

safe by many industry experts. Investigating the peak MCs may, however, reveal any potentially 

dangerous MC levels in the sheathing. Similarly, plotting the peak MC versus the vapor 

permeance of the WRBs, for both exterior and interior thin slices of the OSB and at varying 

levels of insulation, produces Figure 9 and Figure 10.

Figure 9. Peak exterior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances.
20

Figure 10. Peak interior side sheathing MC for 0 in., 1 in., 2 in., and 4 in. of exterior insulation at 

various WRB permeances.

Figure 7 to Figure 10 demonstrate the yearly average and peak MC experienced over the entire 

year of the OSB for both the interior and exterior faces. Any peak MC exceeding 28% indicates 

that the sheathing may be at risk. From these plots, it can be seen that the best method to 

decrease sheathing MC is the use of exterior insulation. The permeance of the WRB becomes 

secondary, with the exception of highly vapor-permeable WRBs. It can also be seen that a 

pattern emerges with the exterior insulation: it performs better with more vapor tight wall 

assemblies. Vapor-permeable WRBs enable interior moisture to penetrate through the insulation 

during inward driven moisture events. Higher amounts of exterior insulation result in increased

sheathing temperatures. It is hypothesized that this minimizes the effects of solar drying on 

occasions when the cladding is not saturated by keeping the sheathing closer to interior 

conditions.

By identifying critical wall assemblies where the peak MC of the sheathing occurs at dangerous 

moisture levels, further analysis can be conducted to identify the number of hours that are 

experienced at such levels. The following biodegradation risk plots were produced by 

determining the number of hours at which the MC exceeds 20%, 24%, and 28% MC for 

temperature ranges between 5°C to 40°C. These plots function as an hourly histogram with a 5°C 

range. The more hours that the wall experiences above 28% MC, the higher the risk. The number 

of hours below 20% do not pose a concern to the sheathing and are thus not counted. 

A relative impact on the potential for moisture damage can be conducted by comparing the 

number of hours that the sheathing experiences over a year for each different proposed WRB and 

insulation thickness variable. With these simulations, only four such critical wall assemblies

were identified. For the exterior side of the sheathing: 0 in.-50 perms, and 1 in.-50 perms. For the 
21

interior side of the sheathing: 0 in.-50 perm, and 0 in. -0.1 perm. These may be found from 

Figure 11 to Figure 14, respectively.

Figure 11. Biodegradation risk plot for the exterior side of the sheathing, with 0 in. exterior 

insulation and 50 Perm WRB.

Figure 12. Biodegradation risk plot for the exterior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB.
22

Figure 13. Biodegradation risk plot for interior side of the sheathing, with 0 in. of exterior 

insulation and 50 Perm WRB.

Figure 14. Biodegradation risk plot for interior side of the sheathing, with 0 in. of exterior 

insulation and 0.1 Perm WRB.

Table 9 summarizes the number of hours that the OSB experiences above 28% MC at 

temperatures between 5°C and 40°C. On the interior side, the peak MC never exceeded 28% MC 
23

with wall assemblies with even 1 in. of exterior insulation. For assemblies with exterior 

insulation, only highly permeable WRBs result in any duration exhibiting prolonged elevated 

MC. 

Table 9. Hours above 28% MC for Identified Wall Assemblies.

Wall Assembly Location Hours above 28% MC

0”-50 perm Exterior 379

1”-50 perm Exterior 42

1”-10 perm Exterior 0

0”-0.1 perm Interior 37

0”-50perm Interior 250

1”-50 perm Interior 0

Consequently, the results from these simulations suggest that the addition of at least 1 in. of 

vapor-permeable R-4 exterior insulation in the Chicago climate is sufficient to minimize decay 

mechanisms. This is particularly the case on the interior of the OSB. The exception is that highly 

permeable WRBs may still result in high MC on the exterior surface of the OSB, even with 1 in.

of vapor-permeable exterior insulation. However, the duration that the OSB experiences at risk 

of decay with 1 in. of exterior insulation is still significantly less than with no exterior insulation.
24

Variations on a Theme: Interior Vapor Control

As shown in the biodegradation risk plots above, the use of exterior insulation aids in the control 

of the MC of the OSB sheathing. However, restricting the outward flow of moisture could also 

be controlled to minimize elevated MC. The following plots were produced with the use of 

interior vapor control. The vapor control strategies were the use of polyethylene sheet and Kraft 

paper. These simulations were conducted with no cladding ventilation, but still comply with the 

IRC. Vinyl wall coverings (VWC) provide outward vapor control, but significantly inhibit 

inward vapor control. Furthermore, they are not a common method of intentional vapor control 

in wall assemblies and featured more frequently on commercial construction. Consequently, 

VWC will be considered separately.

Figure 15. Average exterior side sheathing MC for 1 in., 1 in.+ Poly, and 1 in.+ Kraft at various 

WRB permeances.
25

Figure 16. Peak exterior side sheathing MC for 1 in., 1 in.+ Poly, and 1in.+ Kraft at various WRB 

permeances.

Figure 17. Average interior side sheathing MC for 1 in., 1 in.+ Poly, and 1 in.+ Kraft at various WRB 

permeances.
26

Figure 18. Peak interior side sheathing MC for 1 in., 1 in.+ Poly, and 1 in.+ Kraft at various WRB 

permeances.

When additional interior vapor control is considered (beyond latex paint), the sheathing MC

increases noticeably at high WRB permeances. In very vapor-permeable WRBs (i.e. 50 perms), 

the increase is significant. The use of polyethylene sheet or Kraft paper effectively traps the 

moisture within the wall cavity. The vapor pressure gradient across the wall assembly when the 

brick is heated by the sun is very high, thus forcing water past the WRB and OSB and into the 

wall assembly. However, once the cladding temperature decreases and drying takes place, there 

is insufficient driving potential for that moisture to fully leave the system. The addition of 

interior vapor control restricted any capacity for the moisture to dry to the interior. Consequently, 

the moisture gets trapped within the wall cavity and results in dangerously high MC. To quantify 

the severity of the MC, a histogram was created, as seen in Figure 19 to Figure 22. The critical 

points identified from the MC plots are 1 in. of exterior insulation with 50 perm WRB, both with 

polyethylene sheet and Kraft paper.
27

Figure 19. Biodegradation risk plot for exterior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and polyethylene sheet interior vapor control.

Figure 20. Biodegradation risk plot for interior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and polyethylene sheet interior vapor control.
28

Figure 21. Biodegradation risk plot for exterior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and Kraft interior vapor control.

Figure 22. Biodegradation risk plot for interior side of the sheathing, with 1 in. of exterior 

insulation and 50 Perm WRB and Kraft interior vapor control.

The results from the use of interior vapor control layers are summarized in Table 10.
29

Table 10. Hours above 28% MC for Identified Wall Assemblies with Interior Vapor Control.

Wall Assembly Location Hours above 28% MC

1”-50 perm w/ Polyethylene Exterior 1312

1”-50 perm w/ Polyethylene Interior 1627

1”-50 perm w/ Kraft Exterior 1229

1”-50 perm w/ Kraft Interior 1202

The results from simulations for the interior vapor control strategies suggest that it actually 

increases the risk to the structural sheathing and that it may do more harm to the structural 

sheathing than good. The numbers of hours above 28% MC with vapor control far exceed the 

number of hours that preclude any form of vapor control. Consequently, the data suggests that 

increased levels of exterior insulation are a better outward vapor control strategy than the use of 

interior vapor-retarding materials.

The city selected for the VWC models was St. Louis, as it has a climate that clearly demonstrates 

the effects of elevated summer humidity while also achieving cooler temperatures in the winter.

The same wall was modeled as in Chicago with the exception that a VWC was inserted on the 

interior layer. The RH on the outside layer of the GWB was monitored throughout the duration 

of the testing and a 7-day and 30-day running average was calculated. Prolonged durations of RH

in excess of 80% may result in mold growth, as per ASHRAE 160-09. Figure 23 demonstrates 

that with VWC, the GWB will likely experience some form of mold growth.

Figure 23. Relative humidity adjacent to paper-faced GWB, including 7-day and 30-day running 

average.
30

According to ASHRAE 160, the use of VWC in a mixed climate will result in Criterion 1 failure, 

with borderline failure of Criterion 2. VWC is not an effective use for outward vapor control and 

possess significant risk to the GWB in the wall assembly.

Variations on a Theme- Plywood Structural Sheathing

Plywood sheathing is a common building material to encounter in retrofit projects. It is more 

vapor permeable than OSB, particularly at higher relative humidities, and thus will allow a 

greater amount of moisture to pass through during periods of elevated moisture within the wall 

cavity. Consequently, it is important to model the recommendations from the simulations with 

plywood, to ensure that the conclusions still result in safe moisture levels in the sheathing.

Table 11. Peak and Yearly Average MC for Plywood Sheathing.

Wall Assembly Location Yearly Average MC (%) Peak MC (%)

1” MFI - 0.1 perm WRB Exterior 10.9 17.9

Interior 11.1 19.3

1” MFI -1 perm WRB Exterior 12.1 17.3

Interior 11.6 19.7

1” MFI -10 perm WRB Exterior 14.2 15.7

Interior 12.9 18.8

1” MFI -50 perm WRB Exterior 17.4 23.6

Interior 13.9 18.2

From Table 11, it can be seen that the peak MC with 1 in. of exterior insulation never exceeds 

23.6% MC. This is contrasted by a peak of 29.3% MC for OSB. Thus, the increased permeability 

of the plywood enables the wall assembly to dry to a greater extent than OSB.

Table 12. Peak and Average Moisture for Plywood Sheathing with Polyethylene Sheet.

Wall Assembly Location Yearly Average MC (%) Peak MC (%)

1” MFI -0.1 perm WRB Exterior 15.1 17.0

Interior 14.7 16.8

1” MFI -1 perm WRB Exterior 17.2 19.5

Interior 17.0 20.5

1” MFI -10 perm WRB Exterior 20.8 28.1

Interior 26.0 48.8

1” MFI -50 perm WRB Exterior 21.9 28.9

Interior 34.1 55.5

The addition of polyethylene sheet in combination with plywood sheathing results in extremely 

high MC with 50 perm WRBs. However, in simulating this model with a non-absorptive 

cladding, the peak MC only achieves 21.5% MC. Thus, the elevated MC is a result of the 

reservoir cladding.

3.1.1 Climatic Effects

The previous series of simulations was only conducted in a Chicago climate. To verify that the 

conclusions obtained from the previous testing equally apply to the other climate zones, 

additional simulations were run for the representative cities for each such climate zone.
31

Table 13. List of Cities and Associated Climate Zones.

City DOE 

Climate 

Zone

HDD-65°F 

(18.3°C)

CDD-65°F

(18.3°C)

International Falls, 

MN

7 10487 (5826) 248 (138)

Chicago, IL 5A 6311 (3506) 842 (468)

St. Louis, MO 4 4504 (2502) 1631 (906)

Seattle, WA 4C 4729 (2627) 176 (98)

Atlanta, GA 3 2990 (1661) 1667 (926)

Miami, FL 1A 130 (72) 4459 (2477)

Plots of peak and yearly average MC on the interior and exterior sliver of OSB sheathing were 

produced for each of these cities. These are shown in Figure 24 to Figure 27.

Figure 24. Peak exterior side sheathing MC for listed cities with 1 in. of MFI exterior insulation at 

various WRB permeances.
32

Figure 25. Average exterior side sheathing MC for listed cities with 1 in. of exterior insulation at 

various WRB permeances.

Figure 26. Peak interior side sheathing MC for listed cities with 1 in. of exterior insulation at 

various WRB permeances.
33

Figure 27. Average interior side sheathing MC for listed cities with 1 in. of exterior insulation at 

various WRB permeances.

From these plots, it can be seen that 1 in. of additional exterior insulation maintains the MC well 

below dangerous levels in nearly every climate zone. The exception is very vapor-permeable 

WRBs, primarily in climate zone 1.

The results of these simulations suggest that the addition of at least 1 in. of exterior insulation 

significantly helps improve the moisture durability of the wall assembly. However, highly vapor

permeable WRBs should be avoided when used with reservoir claddings and mineral fiber 

insulation (MFI) on the exterior. Consequently, WRBs below 10 perms and at least 1 in. of 

exterior insulation are recommended when used in combination with reservoir claddings in 

nearly every climate zone. Certain climate zones, such as 6 to 8, require additional exterior 

insulation. In summation, vapor-impermeable WRBs may be utilized only when at least 1 in. of 

exterior insulation is used in climate zones 1 through 5. A minimum of 2 in. (greater than R7.5)

of exterior insulation should be used in climate zones 6 and 7. Climate zone 8 was not modeled 

in these simulations.

3.2 Energy and Cost Analysis

The BEopt analysis consists of an average production Building America home with standard 

layout and form, average mechanical equipment, ENERGY STAR appliances, and average 

airtightness modeled in Chicago, Phoenix, Duluth, and Miami.

BEopt is a software created to optimize annual energy costs to an annualized capital cost. 

However, as this research project deals specifically with moisture, this parameter is difficult to 

model. The only conclusive method to attempt to categorize the effects on moisture damage on 
34

the structure is to approach the structure from a life cycle analysis—determine the return period 

prior to rehabilitation of the structure as caused by moisture damage. Unfortunately, insufficient 

data exists on the estimated impacts of prolonged moisture exposure to wall assemblies to the 

duration of the building. A range of factors impact whether or not a building that has suffered 

moisture damage undergoes rehabilitation. Small leaks over a long duration do not frequently 

present noticeable symptoms of damage. Further, it is difficult to identify the distribution of 

damage in the existing building stock and to assess the cost for rehabilitation. In an attempt to 

categorize this phenomenon, BSC provided an estimate based on prior experience. However, the 

results from BEopt should only be considered under the assumptions discussed above.

Figure 28 and Figure 29 show the results from the BEopt simulations. 

Figure 28. BEopt energy use for Chicago, Illinois.

Figure 29. BEopt results for Chicago, Illinois.

The results of the BEopt modelling show that in Chicago, the optimized level of exterior 

insulation is 1.5 in. of mineral board insulation. Every single modelled city also found that 1.5 in.
35

of exterior continuous mineral board insulation was optimal. Data relating to the BEopt outputs

may be found in Appendix B.

4 Summary

To comply with initiatives to increase the energy efficiency of homes of the existing building 

stock in the United States, additional insulation and increased airtightness must be included as 

part of the renovation strategy. Many cost-effective approaches exist that result in increased 

insulation levels and improved airtightness. To achieve superior insulation levels, additional 

insulation must be either added to the interior or to the exterior of the existing wall. As the 

addition of exterior insulation is costly, only select circumstances warrant its inclusion: a 

severely deteriorated cladding that requires replacement, or an owner that desires to significantly 

improve the R-value of his building. One such material that could be utilized as exterior 

insulation is mineral board insulation. 

There are concerns on the use of the moisture durability of wall assemblies with vaporpermeable exterior insulation, such as mineral fiber, in retrofit applications with existing walls 

that also possess vapor permeable cavity insulation. Due to the vapor-permeable nature of the 

insulations in use, inward driven moisture poses a problem to wall assemblies under certain 

circumstance.s When used in combination with a reservoir cladding, that has been inundated by 

rain and then exposed to sunlight, inward driven moisture may penetrate through the exterior 

fiber insulation, through the WRB, and into the wall cavity assembly. The natural solution to 

eliminate inward driven moisture is to utilize an impermeable WRB to inhibit the inward flowing 

moisture. The complication arises when, in colder climates, outward flowing moisture is 

similarly inhibited by the same impermeable WRB. This results in elevated MC accumulated in 

the sheathing. The solution is to select an appropriate permeance of the WRB that sufficiently 

throttles inward moisture, but that does not inhibit outward drying of the wall assembly, should it 

get wet. This must also be considered in combination with the addition of the exterior insulation.

To identify the ideal range of permeance and exterior insulation, BSC conducted a series of 

hygrothermal simulations, addressing variables such as amount of exterior insulation, the vapor 

permeance of the WRB, different sheathing materials, and interior vapor-control layers. The 

experiment was conducted to answer research questions that still persist in this area. The 

following questions and answers address this research.

• What insulating sheathings are available for retrofit applications, what are their material 

characteristics as they relate to thermal resistance and vapor permeance, and what kind of 

cladding attachments can be used?

A range of insulating sheathings are available for retrofit applications. As the definition for this 

project entails vapor permeable insulations, the IRC defines “vapor-permeable” as being greater 

than 5 perms. Mineral board and glass insulations fall within this category. They typically have 

an R-value in the range of R4 per inch and come in rigid board format. These products are 

usually woven recycled slag or glass that is spun and adhered to form rigid boards. It typically 

possesses negligible vapor-diffusion resistance (greater than 40 perms). However, as this 

material is vapor permeable, uncertainty exists over the moisture management of walls which 

utilized this product.
36

The insulating sheathing may be attached to the structure with a range of attachment 

mechanisms, from furring strips, to cap-nails.

• What are the characteristics of existing assemblies? (Note: we will characterize the two 

most common assemblies that are likely to be retrofit in the cold and hot climate zones).

What range of cladding types are common for existing assemblies?

The existing assemblies, as defined within this research project, typically consist of 2×4 framing 

spaced 16 in. on center, with R9 to R13 permeable cavity insulation, typically a fibre glass or 

cellulose type insulation. The structural sheathing may consist of either OSB or plywood. This is 

more typical of building stock from the 1980s and 1990s, but may also be from earlier buildings 

that have gone through previous rehabilitation efforts.

The cladding may range from vinyl or aluminum siding, to wood or fiber cement board siding, to 

brick and stucco, depending on local construction preferences. The concerns identified in this 

report are primarily due to inward driven moisture, which necessitates a reservoir style cladding. 

Consequently, this report is focused more on the use of brick or stucco cladding, but the 

conclusions may be extrapolated to non-moisture storing claddings as well. 

• What types of interior vapor-control layers are common and what are the permeances of 

these? 

Typical interior vapor-control layers likely consist of either Kraft facers on the batt insulation

(0.47 perm), polyethylene sheet (0.07perm), or latex paint (10 perm). Vinyl wall coverings

(0.01perm) are sometimes also used, but are much less common.

• What is the airtightness of these assemblies?

The airtightness of these existing assemblies depends greatly on previous rehabilitation or retrofit 

history. A range of values,from 6ACH50 and up may be found. Further research, including field 

blower door testing on existing building stock, is required to properly answer this question.

• What are the likely retrofit solutions and what guidelines should be established for the 

airtightness, water control, thermal performance and vapor permeance? 

Based on the scope of this project, the wall assemblies are already insulated and the intended 

retrofit cladding is of a reservoir type. The thermal performance should strive to achieve the 

relevant building code standards, including considerations for thermal bridging. The vaporpermeance values are outlined within this report. The airtightness and water control require 

specific details that should be discussed in future measure guidelines.

• What are the recommended solutions?

The content of this research assumes that reservoir claddings are exposed to the rain. The most 

severe wall orientation was identified to determine the worst case scenario with regards to 

inward driven moisture. Reservoir claddings that are protected from precipitation do not pose 

any greater threat to the moisture durability of the wall than non-absorptive types of cladding. 
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With the above caveats, BSC recommends using only WRBs that have a permeance of less than 

10 perms, with at least 1 in. of MFI exterior insulation in climate zones 1 through 6, when used 

in combination with reservoir claddings that are exposed to precipitation. 

The team found that WRBs with vapor permeances above 10 perms increase the MC of the 

structural sheathing. The use of vapor-control layers within the wall, such as Kraft or 

polyethylene sheet on the interior, increase the risk for moisture damage within the wall by 

effectively trapping moisture within the wall. Consequently, BSC recommends using latex paint 

to inhibit outward flowing moisture in combination with exterior insulation to reduce any 

moisture damage potential.
38

References

ASHRAE Standard 160. (2009), Criteria for Moisture-Control Design Analysis in Buildings. 

Atlanta, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

ASHRAE (2009), Handbook of Fundamentals, Atlanta, American Society of Heating 

Refrigerating, and Air-Conditioning Engineers, Inc

Carmeliet, J., Dérome, D. (2012). Temperature driven inward vapor diffusion under constant and 

cyclic loading in small-scale wall assemblies: Part 1: experimental investigation/Part 2: heatmoisture transport simulations. Building and Environment 47, pp. 161-169.

Dérome, D., Saneinejad, S. (2010). Inward vapor diffusion due to high temperature gradients in 

experimentally tested large-scale wall assemblies. Building and Environment 45, pp. 2790-2797.

Dérome, D., Karagiozis, A., Carmeliet, J. (2010), The Nature, Significance and Control of SolarDriven Water Vapor Diffusion in Wall Systems – Synthesis of Research Project, ASHRAE 

Research Project RP-1235.

Grin, A. (2008). “Evaluation of High Performance Residential Housing Technology”. University 

of Waterloo, Canada. Chapter 5 pages 77 – 89. 

Pressnail K., J. Timusk, L. Kan, B. Doug, and V.Kan. (2003). In search of a wall for all seasons: 

controlling sun-driven moisture, Proceeding of the 95th Conference on Building Science and 

Technology, Vancouver, B.C.

Quirouette, R.L. (1985), The Difference between a Vapour Barrier and an Air Barrier, Building 

Practice Note No. 54, Division of Building Research, National Research Council of Canada; 

Ottawa, Ontario

Reed Construction Data (2011). RSMeans CostWorks 2011, 15th Annual Edition. Norwell, MA. 

Retrieved May 17, 2011.

Straube, J.F. and E.F.P. Burnett. (1995). Moisture migration in screened wall systems, 

Proceedings of BETEC/ASHRAE/DOE Thermal Performance of Building Envelopes VI, pp. 177-

188.

Straube, J.F., Smegal, J.E. (2009). Building American Special Research Project: High-R Walls 

Case Study Analysis, Research Report – 0903, revised June 8, 2011.

TenWolde, A., and H.T. Mei. (1985). Moisture movement in walls in a warm humid climate. 

Proceeding of ASHRAE/DOE/BTECC Thermal Performance of the Exterior Enevelopes of 

Buildings III, pp. 570-582. Atlanta: American Society of Heating, Refrigeration, and AirConditoning Engineers, Inc.

Wilson, A.G. (1960). Condensation Between Panes of Double Windows. Canadian Building 

Digest-5, Division of Building Research, National Research Council of Canada, Ottawa, Ontario
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Wilson, A.G. (1961), Air Leakage in Buildings, Canadian Building Digest-23, Division of 

Building Research, National Research Council of Canada, Ottawa, Ontario

Wilson, A.G. (1965). Condensation in insulated masonry walls in the summer. Proceedings of 

RILEM/CIB Symposium, Helsinki DBR Tech Paper 226.
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Appendix A – WUFI Material Properties

This appendix includes the material properties used in the hygrothermal simulations. These 

properties characterize the thermal and moisture behavior of the materials in the assembly.

Material properties included are bulk density, porosity, heat capacity, thermal conductivity, 

water vapor diffusion resistance factor, reference water content, free water saturation content, 

and the liquid transport function. For materials that exhibit dependence on external or internal 

input (i.e. ambient relative humidity or moisture content), graphs are included to describe the 

relationship.
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Appendix B – BEo

pt Output Graphs

This appendix includes the output graphs from BEopt. These optimization simulations identify

the ideal level of insulation when analyzed from capital cost to operation and maintenance 

savings perspective over the building’s predetermined lifecycle. The plots for climate zones 1A 

(Miami, FL), 3 (Atlanta, GA), 4C (Seattle, WA), 4 (St. Louis, MO), 5A (Chicago, IL), and 7 

(International Falls, MN). The plots demonstrate the relationship of operation and maintenance 

cost to the level of insulation. A decreasing trend exists in all models, but depending on the 

climate, the optimal level of insulation changes. 
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Chicago, Illinois
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Phoenix, Arizona
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Miami, Florida
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Seattle, Washington
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Duluth, Minnesota
DOE/GO-102013-3910 ▪ September 2013

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