Application of Spray Foam 

Insulation Under Plywood 

and Oriented Strand Board 

Roof Sheathing

A. Grin, J. Smegal, and J. Lstiburek

Building Science Corporation

October 2013
NOTICE

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iii

Application of Spray Foam Insulation Under Plywood and 

Oriented Strand Board Roof 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:

A. Grin, J. Smegal, and J. Lstiburek

Building Science Corporation

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

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

October 2013
iv

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v

Contents

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

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

1 Background and Significance for Building America ........................................................................ 1

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

1.2 Project Background..............................................................................................................1

1.3 Relevance to Building America’s Goals..............................................................................1

1.4 Cost Effectiveness................................................................................................................2

1.5 Tradeoffs and Other Benefits...............................................................................................2

1.6 Integration Opportunities.....................................................................................................2

1.7 Contact Information.............................................................................................................2

2 Experiment ............................................................................................................................................ 3

2.1 Research Questions..............................................................................................................3

2.2 Technical Approach.............................................................................................................3

3 Analysis Background........................................................................................................................... 4

3.1 Unvented Roof Systems.......................................................................................................4

3.2 Code Requirements for Roofs..............................................................................................5

3.3 Hygrothermal Fundamentals................................................................................................8

4 Hygrothermal Analysis ...................................................................................................................... 11

4.1 Analysis Program...............................................................................................................11

4.2 Boundary Conditions.........................................................................................................11

4.3 Roof Systems Modeled......................................................................................................11

4.4 Wetting—Rainwater Leakage............................................................................................12

4.5 Minneapolis Hygrothermal Results ...................................................................................13

4.6 Seattle and Miami Hygrothermal Results..........................................................................21

5 In-Service Roof Explorations ............................................................................................................ 27

5.1 House 1: New Orleans, Climate Zone 2, June 2012..........................................................28

5.2 House 2: New Orleans, Climate Zone 2, July 2012...........................................................29

5.3 House 3: Coquitlam, Climate Zone 4c, July 2012.............................................................29

5.4 House 4: Coquitlam, Climate Zone 4c, July 2012.............................................................29

5.5 House 5: Westerville, Climate Zone 5, June 2012.............................................................30

5.6 House 6: Pontiac, Climate Zone 5, July 2012....................................................................31

5.7 House 7: Minneapolis, Climate Zone 6, July 2012............................................................32

5.8 House 8: Minneapolis, Climate Zone 6, July 2012............................................................33

5.9 House 9: Juneau, Climate Zone 7, July 2012.....................................................................33

5.10 House 10: Juneau, Climate Zone 7, July 2012.......................................................34

5.11 House 11: Juneau, Climate Zone 7, July 2012.......................................................34

6 Conclusions ........................................................................................................................................ 35

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

List of Figures

Figure 1. Cathedralized or unvented attics .............................................................................................. 5

Figure 2. Example high-R hybrid unvented cathedralized ceiling/attic................................................. 5

Figure 3. ccSPF unvented roof with condensation ................................................................................. 8

Figure 4. Moisture balance of wetting and drying ................................................................................... 9

Figure 5. Minneapolis plywood sheathing moisture content of ccSPF unvented roof...................... 14

Figure 6. Minneapolis sheathing MC—north versus south .................................................................. 15

Figure 7. Minneapolis sheathing MC—OSB versus plywood............................................................... 16

Figure 8. Minneapolis plywood sheathing MC based on standard and high interior RH.................. 17

Figure 9. Minneapolis plywood sheathing MC based on interior RH—larger leak............................. 18

Figure 10. Minneapolis plywood sheathing with ccSPF and ocSPF ................................................... 19

Figure 11. Minneapolis plywood sheathing MC ocSPF and high RH .................................................. 20

Figure 12. Minneapolis sheathing MC—ocSPF, 2 perm coating, and 5 perm coating....................... 21

Figure 13. Seattle sheathing moisture content north 2 in. ccSPF + 5.25 in. cellulose ...................... 22

Figure 14. Seattle sheathing moisture content north 2 in. ccSPF + 5.25 in. fiberglass..................... 23

Figure 15. Seattle sheathing moisture content north 8 in. ocSPF ....................................................... 24

Figure 16. Miami sheathing moisture content north 2 in. ccSPF + 5.25 in. cellulose........................ 25

Figure 17. Miami sheathing moisture content north 2 in. ccSPF +5.25 in. fiberglass ....................... 25

Figure 18. Miami sheathing moisture content north 8 in. ocSPF......................................................... 26

Figure 19. Miami sheathing moisture content north 8 in. ocSPF high stress .................................... 26

Unless otherwise noted, all figures were created by BSC.

List of Tables

Table 1. Industry Team Member Contact Information............................................................................. 2

Table 2. 2012 IRC Table R806.5 and Table N1102.1.1 R-Values ............................................................. 7

Table 3. Basic Modeled Roof Assembly ................................................................................................. 12

Table 4. Minneapolis TMY2 and U.S. Climate Normals Calculated Rainfall........................................ 13

Table 5. Miami WUFI (Cold Year) Calculated Rainfall............................................................................ 13

Table 6. Seattle WUFI Calculated Rainfall .............................................................................................. 13

Table 7. Minneapolis Annual Rainfall Volume Calculations at a Roof Ridge ..................................... 15

Table 8. In-Service Exploration House List ............................................................................................ 27

Unless otherwise noted, all figures were created by BSC.
vii

Definitions

BSC Building Science Corporation

ccSPF Closed-cell spray polyurethane foam

IRC International Residential Code

MC Moisture content, weight basis, usually in %

ocSPF Open-cell spray polyurethane foam

OSB Oriented strand board

RH Relative humidity, reported in %

SPF Spray polyurethane foam

TMY2 Typical Meteorological Year 2
viii

Executive Summary

Unvented roof strategies with open cell and closed cell spray polyurethane foam (ocSPF and 

ccSPF) insulation sprayed to the underside of roof sheathing have been used since the mid-1990s 

to provide durable and efficient building enclosures. However, there have been isolated 

moisture-related incidents reported anecdotally that raise potential concerns about the overall 

hygrothermal performance of these systems. The incidents related to rainwater leakage and 

condensation concerns. Condensation concerns have been extensively studied by others and are 

not further discussed in this report (Straube et al. 2010).

This project involved hygrothermal modeling of a range of rainwater leakage and field 

evaluations of in-service residential roofs using spray polyurethane foam (SPF) insulation. All of 

the roof assemblies modeled exhibited drying capacity to handle minor rainwater leakage. All 

field evaluation locations of in-service residential roofs had moisture contents (MCs) well within 

the safe range for wood-based sheathing.

The quantity of water passing through a roof system is difficult to quantify, but hygrothermal 

modeling is possible using ASHRAE 160, Minneapolis Typical Meteorologial Year 2 and U.S.

Climate Normals weather data, and WUFI weather data. WUFI 5 was used to determine the 

effect of 0.01% to 1.00% of the rainfall entering the unvented roof system as a leak and coming 

in contact with the wood-based structural roof sheathing. The 2012 International Residential 

Code-compliant roofing system using ccSPF on plywood sheathing with cellulose insulation on 

the interior has the capability according to WUFI to safely dry a leak up to 0.6% of the rainfall in 

Minneapolis. In Seattle the roof systems modeled were able to accommodate up to 0.6% for 

ocSPF and 1.0% for ccSPF and in Miami up to 1.5% could be dried out when using ocSPF.

Assuming the recommended fully adhered membrane is properly designed, detailed, and 

installed water should have very little likelihood of ever entering the system through leaks.

ocSPF dries more readily than ccSPF, but ocSPF allows more wetting of the sheathing during the 

winter months and accordingly requires a Class II vapor retarder coating directly applied to its 

interior surface as specified by International Residential Code. Interior relative humidity can 

directly affect the sheathing MC in all scenarios and hence wintertime relative humidity in a 

climate zone 6 home should be < 40%. Orientation and sheathing materials created variations 

within the system, but these variations were relatively small compared to the type of SPF and 

vapor permeance of coatings used. Oriented strand board (OSB) sheathing, ocSPF, and roofs 

facing north maintain the highest MCs, but all systems modeled were within the safe range for 

wood-based sheathing. Damage could occur where the volume of the leak, frequency of the 

leak(s), or quantity of interior moisture driven into the system are more than modeled in this 

study.

Explorations of 11 in-service roof systems were completed. The exploration involved taking a 

sample of SPF from the underside of the roof sheathing, exposing the sheathing, then taking a 

moisture content reading. All locations had MCs well within the safe range for wood-based

sheathing. One full-roof failure was reviewed, as an industry partner was involved with replacing 

structurally failed roof sheathing. In this case the manufacturer’s investigation report concluded 

that the SPF was installed on wet OSB, based on the observation that the SPF did not adhere well 

to the substrate and the pore structure of the ccSPF at the ccSPF/OSB interface was indicative of 

a wet substrate.
1

1 Background and Significance for Building America

1.1 Introduction

Open cell spray polyurethane foam (ocSPF) and closed cell spray polyurethane foam (ccSPF)

insulation sprayed to the underside of roof sheathing is a popular strategy for increasing roof 

insulation levels in all climate zones. Unvented roof strategies with spray polyurethane foam 

(SPF) have been used since the mid-1990s to provide durable and efficient building enclosures.

However, there have been isolated incidents of failures (either sheathing rot or SPF 

delamination) reported anecdotally that raise some potential concerns about the hygrothermal 

performance and durability of these systems. The incidents were related to rainwater leakage and 

condensation concerns.

Condensation concerns have been extensively studied by others (Straube et al. 2010) and are not 

further discussed in this report.

It is unclear whether the rainwater leakage issues are a material issue, an application issue, both,

or neither—or even whether issues actually exist in sufficient numbers to be of concern. The 

2011 Standing Technical Committee on Enclosures has identified this as an important topic for 

additional research work (Lstiburek 2011).

The primary risks for roof systems are:

• Rainwater leaks

• Condensation from diffusion and air leakage

• Built-in construction moisture. 

This report deals directly with rain and indirectly with built-in construction moisture. The 

technical approach used in this project combined hygrothermal modeling of a range of rainwater 

leakage scenarios, and field evaluations of residential roofs using SPF insulation.

1.2 Project Background

Spray foams have advantages over alternative methods with respect to providing air sealing in 

complex assemblies—particularly roofs. SPF can provide the thermal, air, and vapor control 

layers in both new and retrofit construction. In cases where mechanical systems are located in 

attics, moving the air control layer and thermal control layer to the underside of the roof deck has 

particularly large advantages compared to sealing and insulating attic ceilings and ductwork. In 

addition, it might not be desirable (in hurricane or wildfire areas) or practical (in retrofits) to add

roof vents at soffit locations. Accordingly, there may not be any practical alternative to moving 

the air control layer and thermal control layer to the underside of the roof deck.

1.3 Relevance to Building America’s Goals

The energy savings goals set by the U.S. Department of Energy’s Building America program for 

both new and existing homes are 30%–50% relative to a home built based on the 2009 

International Energy Conservation Code.
2

The insulation methods used to achieve the energy use reduction goals set out by the U.S. 

Department of Energy should not result in moisture-related durability risks or failures. Research 

into the performance of current recommended assemblies that use SPF should be completed,

through theoretical modeling and field review of installed roof insulation methods, to determine 

if the recommended assemblies have long-term associated durability risks.

1.4 Cost Effectiveness

Durable assemblies exist for a long time and make the best use of construction resources.

Because the assemblies last for a long time, their energy resource use should be considered over 

the life of the assembly, and should therefore be substantially reduced during the initial design.

Improving the moisture tolerance and durability of an assembly is also necessary to determine 

the whole-house life cost.

The roof system’s lifespan may be significantly reduced from durability flaws in the design.

Replacement of a roof system due to a design flaw is costly and could be avoided with a 

thorough understanding of the system interactions. The cost effectiveness of this investigation is 

in the savings of never having to replace a roof system (framing, insulation, and sheathing) over 

the intended life of the house. According to Building Science Corporation’s (BSC) experience it 

is estimated that the cost associated with replacing the roof framing, insulation, and sheathing for 

a typical home is on the order of $30,000–$50,000. Even in the case of localized bulk water 

leakage failures (approximately 2–10 ft2

) the costs to repair the roofing system (and the damage 

below it) can be very significant. These costs should be avoided with a long-term, durable, 

energy-efficient design and installation of the original roofing system.

1.5 Tradeoffs and Other Benefits

The increased R-value and airtightness of SPF roof systems improve energy efficiency and 

occupant comfort by reducing drafts and improving surface temperatures. The durability of these

systems and their maintenance requirements and tolerance to the possible operating conditions 

within the home should be investigated.

1.6 Integration Opportunities

The information developed from this research will help enable the safe implementation of high 

R-value, airtight SPF roof systems on both prototype homes and homes in production-built 

communities.

1.7 Contact Information

The following are the BSC Industry Team members involved in this project:

Table 1. Industry Team Member Contact Information

Company Name Team Member

Dow Gary Parsons

BASF Paul Campbell

Honeywell Xuaco Pascual

Icynene Paul Duffy
3

2 Experiment

2.1 Research Questions

The following research questions are answered by this project:

• Are there risks associated with installing SPF under plywood and oriented strand board 

(OSB) roof decks, specifically moisture and durability issues?

• Are roof leaks a serious problem with SPF roof assemblies?

• What is happening in these systems with high measured sheathing moisture content (MC)

(i.e., 20+) but with no evidence of damage to the sheathing?

• Are there moisture durability risks associated with installing SPF under OSB roof decks 

in climates with high rainfall?

2.2 Technical Approach

This project hygrothermally modeled a range of rainwater leaks in code-compliant residential 

unvented roofs that used SPF insulation. Three climate zones were used in this analysis. In 

addition, field explorations were conducted of in-service residential roofs using SPF insulation.

WUFI 5 hygrothermal modeling software was used for the analysis.

The hygrothermal modeling was used to determine drying potentials and assess the relative risks 

of rainwater leakage in SPF roof systems. Past field work and published work (Straube et al. 

2010) has already addressed the condensation risks of various systems in various climate zones.

Hygrothermal modeling was conducted on roof assemblies located in the principal climate zones 

of interest defining building performance in the lower 48 states—a hot climate with significant 

rain (Miami), a cold climate (Minneapolis), and a marine climate with significant rain (Seattle).

These locations “bracket” the expected in-service conditions of concern for unvented roof 

assemblies.

A number of roofs were reviewed to visually inspect the sheathing of in-service residential roofs 

using SPF against the underside of the roof sheathing. This enabled the correlation of modeled 

low sheathing MCs to in-service roofs with measured low sheathing MCs.

The field explorations were designed to provide information about the actual performance of 

roofs using SPF insulation. The isolated failures of SPF roofs have led some practitioners to 

believe that these systems trap moisture in the sheathing. The intent of the explorations of inservice roofs was to measure the sheathing MC and verify that it is within a safe level. Field

explorations of installed ocSPF and ccSPF roof assemblies were conducted via visual inspection 

and core samples. Industry partners were approached to source specific installations of SPF 

under roof sheathing that could be investigated. The evaluations of the assemblies were based on 

visual examination of the materials, supported by quantitative moisture meter readings, and 

product sampling where necessary.

The field evaluation locations were selected based on availability and timing. All locations that 

were made available by the industry partners were evaluated.
4

3 Analysis Background

3.1 Unvented Roof Systems

A successful roof (or roof-ceiling assembly) will perform the following tasks:

• Provide a water management system to keep precipitation out.

• Provide an air barrier system between the indoors and outdoors.

• Provide a thermal control system to keep the heat out during the summer and retain heat 

during the winter.

• Provide a vapor control system to maintain a durable environment that does not allow 

condensation and does not promote mold growth.

BSC experience suggests that when failures occur in wood-frame roofs insulated with SPF at the 

deck, it is typically due to leakage of bulk water (precipitation), or vapor diffusion condensation.

Vapor diffusion condensation can occur as an outward drive or as an inward drive. Proper roof 

enclosure system design can avoid the majority of failures. SPF insulated roofs are common in 

retrofit work. In retrofit work, the order of work to be considered is important. Health and safety 

issues must be addressed first and are more important than durability issues. Durability issues are 

in turn more important than saving energy. Lstiburek (2010) provides the background and 

approach for the preparatory work necessary prior to insulating an attic. The guide focuses on 

combustion safety, ventilation for indoor air quality, and attic ventilation for durability. The 

guide provides a scope of work and specification for the air sealing of many points of air leakage 

in common attic spaces.

Unvented attic assemblies, or cathedralized attics that move the insulation and airtightness planes 

to the slope, have been developed to overcome two major problems with vented attics (Figure 1). 

These problems are:

• Locating ducts/air handling units in the attic space causes major air leaks of conditioned 

air (and thus forced infiltration/exfiltration), and heat/loss gain through the ductwork.

• Designs with complex coffered ceiling planes, numerous penetrations by lights, speakers, 

vents, etc. make it practically difficult to achieve the airtightness required just below the 

insulation layer.
5

Figure 1. Cathedralized or unvented attics

All unvented attic and cathedral ceiling designs must provide for either a very high degree of 

airtightness or avoidance of condensation by warming sensitive surfaces. To meet durability 

goals in most applications, the airtightness must be provided by a continuous membrane—

preferably adhered to the top surface of the structural roof deck and under rigid insulation that 

provides condensation control. In designs where the airtightness is provided between framing 

elements, SPF has been found to be a practical solution. However, all wood-to-wood joints in the 

framing must still be sealed. Figure 2 shows the application of SPF to form an air barrier in a 

hybrid roof system.

Figure 2. Example high-R hybrid unvented cathedralized ceiling/attic

3.2 Code Requirements for Roofs

The 2012 International Residential Code (IRC) defines vapor retarder class information. A vapor 

retarder is defined as: A measure of the ability of a material or assembly to limit the amount of 

moisture that passes through that material or assembly. Vapor retarder class shall be defined 

using the desiccant method with Procedure A of ASTM E96 as follows:
6

• Class I: 0.1 perm or less

• Class II: 0.1 perm to 1.0 perm

• Class III: 1 perm to 10 perms.

The IRC has had information on unvented attics for a few editions. The 2012 edition contains 

the following requirements, most notably the addition of the vapor retarder classes to R806.5 (4).

The unvented attic and unvented enclosed rafter assemblies section R806.5 is as follows

(ICC 2012).

R806.5 Unvented attic and unvented enclosed rafter assemblies. Unvented attic assemblies 

(spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed 

rafter assemblies (spaces between ceilings that are applied directly to the underside of the roof 

framing members/rafters and the structural roof sheathing at the top of the roof framing 

members/rafters) shall be permitted if all the following conditions are met:

1. The unvented attic space is completely contained within the building thermal envelope.

2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the 

unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.

3. Where wood shingles or shakes are used, a minimum 1⁄4 in. (6mm) vented air space 

separates the shingles or shakes and the roofing underlayment above the structural 

sheathing.

4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor 

retarder, or shall have a Class II vapor retarder coating or covering in direct contact with 

the underside of the insulation.

5. Either items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the 

insulation directly under the structural roof sheathing.

5.1 Air-impermeable insulation only. Insulation shall be applied in direct contact with 

the underside of the structural roof sheathing.

5.2 Air-permeable insulation only. In addition to the air-permeable insulation 

installed directly below the structural sheathing, rigid board or sheet insulation 

shall be installed directly above the structural roof sheathing as specified in Table 

R806.5 for condensation control.

5.3 Air-impermeable and air-permeable insulation. The air-impermeable insulation

shall be applied in direct contact with the underside of the structural roof 

sheathing as specified in Table R806.5 for condensation control. The airpermeable insulation shall be installed directly under the air-impermeable 

insulation.

5.4 Where preformed insulation board is used as the air impermeable insulation layer, 

it shall be sealed at the perimeter of each individual sheet interior surface to form 

a continuous layer.
7

The IRC for climate zones 1, 2, 3, and 4 requires that a Class I vapor control layer not be 

installed on the interior side of the assembly. This is to prevent inward driven moisture from 

being trapped in an assembly. Installing a low permeance vapor control layer on the interior in a 

cooling-dominated climate can quickly deteriorate the assembly.

Table N1102.1.1 of the 2012 IRC lists the thermal insulation requirements for each assembly. A 

summary of the requirements combining Table R806.5 and Table N1102.1.1 is shown in Table 2.

Table 2. 2012 IRC Table R806.5 and Table N1102.1.1 R-Values

Climate Zone

Minimum Rigid Board or

Air-Impermeable Insulation

R-Value

Total Required 

Installed R-Value

2B and 3B Tile Roof Only 0 (none required) 30

1, 2A, 2B, 3A, 3B, 3C R-5 30

4C R-10 30

4A, 4B R-15 38

5 R-20 38

6 R-25 49

7 R-30 49

8 R-35 49

In cold climates it is important to note the ratio of vapor-impermeable to vapor-permeable Rvalues in Table 2. For cold climates the air-impermeable insulation is maintained at 50% or more 

of the total R-value of the roof system. This is for condensation control. When building high Rvalue roof systems, BSC recommends that this ratio be maintained or exceeded. If an R-80 

cathedral ceiling or cathedralized attic is to be constructed in a cold climate, it is recommended 

that a minimum of R-40 (50%) be air-impermeable insulation installed and layered according to 

section R806.5 of the 2012 IRC.

For climate zone 1 (Miami hygrothermal analysis), R-5 air-impermeable insulation is required as 

a part of the R-30 roof. For climate zone 4 (Seattle hygrothermal analysis), R-10 airimpermeable insulation is required as a part of the R-30 roof. Because the roofs analyzed are 

intended to be as cost effective as possible, and because a normal pass of ccSPF is approximately 

2 in., R-12 was used for ccSPF as the air-impermeable layer for Miami and Seattle. Due to the 

low incremental cost of ocSPF, an R-30 pass of ocSPF is used as the air-impermeable layer in 

lieu of a hybrid system in Miami and Seattle. For colder climates, alternate layers will be used to 

meet the IRC and optimize costs. These systems are discussed within the hygrothermal modeling 

section.

A roof that does not meet the IRC has the potential to have condensation within the assembly. As 

an example, the roof shown in Figure 3 was designed as an unvented roof assembly using ccSPF

against the underside of the sheathing and batt insulation inboard of the SPF. The ccSPF in some 

locations was only 1 in. thick and R-28 batt was installed to the interior. The IRC requirement 

for climate zone 5 is R-25 for the SPF in this case and only R-6 to R-10 was present in this 

home. This resulted in condensation forming on the face of the ccSPF as the face of the SPF was 

colder than the dew point of the interior air. The quantity of condensation was further increased 

as the relative humidity (RH) in the house was quite high because drywall mudding and painting 
8

were underway. It is important for condensation control to maintain possible condensation 

surface temperatures above the dew point of the interior air.

Figure 3. ccSPF unvented roof with condensation

3.3 Hygrothermal Fundamentals 

Assessing moisture-related durability risks involves three different moisture processes: wetting, 

drying, and moisture storage/redistribution. These three processes in combination with the safe 

storage capacity of each component/material will determine the risk of moisture damage to an 

assembly. This report includes only a brief overview of the wetting mechanisms: they are 

covered in more detail by Lstiburek (2006) and Straube and Burnett (2005).

There are three main wetting mechanisms generally acting on the roof system. They are:

• Bulk water penetration from the exterior

• Vapor diffusion (from exterior or interior)

• Air-transported moisture (air leakage carrying moisture).

The first source of wetting is bulk water from the exterior. This can cause the greatest amount of 

damage in the shortest amount of time. The best strategy to avoid water ingress into the roof or 

living space from the exterior is to properly layer flashings, properly detail roof penetrations, and 

provide a properly applied water management system below the exterior water shedding layer.

For SPF assemblies, a fully adhered membrane is recommended to be applied to the full area of 

roof structural sheathing for bulk water management before the roof cladding or shingles are

installed. Vapor diffusion can be handled with a properly implemented vapor control system. For 

SPF assemblies, the vapor diffusion control is either provided by the SPF itself or through a 

vapor control layer in direct contact with the SPF. Air leakage and the associated possible 

condensation can be limited with a properly designed and installed air barrier system, and by 
9

maintaining possible condensation surface temperatures above the dew point of the interior air 

with proper enclosure design. For SPF assemblies, the SPF insulation serves as the air barrier.

Drying is important since nearly all building enclosures will experience wetting at some point. In 

roof systems, there is drying potential to both the interior and exterior if the enclosure design 

allows it. In the roofs modeled in this study the drying potential is inward as a fully adhered 

membrane is recommended beneath the roof shingles for bulk water management reasons and 

most such membranes are vapor impermeable.

The safe storage capacity (balance of wetting and drying) of an individual material or enclosure 

system is fundamental to good building design (Figure 4). It is rarely economical to build an 

enclosure with no risk of wetting; therefore, managing the risk is important. In any building 

enclosure, building materials should be chosen based on moisture tolerance that correlates to the 

risk of moisture accumulation in the enclosure.

Figure 4. Moisture balance of wetting and drying

Hygrothermal modeling predicts the moisture-related risk associated with each roof assembly.

During hygrothermal modeling, a key value monitored is the MC of the wood-based structural 

sheathing. Wood MCs of plywood or other wood-based materials such as OSB should not be 

judged on pass/fail criteria. When MC measurements are analyzed in the laboratory or in the 

field, the reading should be kept in context and good building science judgment is required to 

determine the moisture risk to the plywood or other wood-based material. For example, elevated 

wood MCs in the cold winter months are much safer from a mold growth perspective than 

similar MCs in the summer, when mold will grow more quickly. Also, high MC for a short 

period followed by drying is not necessarily risky, as wood-based materials are able to manage 

high MCs for short periods without exceeding the safe storage capacity of the assembly. In 

general, decay will not occur unless the MC is greater than fiber saturation for a prolonged 

wetting

drying

safe storage 

capacity
10

period of time (Steffen 2000). Fiber saturation is commonly reached at an MC of approximately 

25%–30% (Baker 1969).

The risk to the wood structural sheathing was assessed based on the following criteria based on 

Forest Products Laboratory (2010):

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

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

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

successful, but conservative assessments usually require corrective action

• Peak sheathing MC > 28%—Moisture-related problems are expected and this level of 

leakage for this design and location is not recommended.
11

4 Hygrothermal Analysis

4.1 Analysis Program

The analysis was completed using WUFI 5 hygrothermal modeling software. WUFI 5 is one of 

the most advanced commercially available hygrothermal moisture programs in use today. Its 

accuracy has been verified against numerous full-scale field studies of enclosure performance 

(roofs, walls, foundations, parking garage decks, etc.) over a number of years. It is one of the few 

models in the public domain that can properly account for adsorption of water vapor, 

absorption/redistribution of liquid water, and night sky radiation. Given the appropriate inputs, 

WUFI calculates heat and moisture flow every hour under the influence of sun, rain, 

temperature, and humidity. The material properties from WUFI’s Generic North American 

Materials database were utilized for the simulation of the proposed roofs. The testing will not 

include extreme values usually associated with major disasters but it will include a portion of 

rainfall injected into the system to simulate a small leak past the rainwater management layers.

4.2 Boundary Conditions

Typical Meteorological Year 2 (TMY2), U.S. Climate Normals, and WUFI weather files were 

used for this analysis. The Minneapolis location (climate zone 6) was analyzed because colder 

climates can stress the roof sheathing more in terms of condensation potential as well as interior 

to exterior vapor drives. The Minneapolis systems were modeled with dark shingles on both 

north and south orientations on a 6/12 pitch. The north orientation would represent a worst case 

scenario with limited solar heating and drying. The Seattle (climate zone 4) and Miami (climate 

zone 1) roofs were modeled on the north orientation only and were modeled with OSB sheathing 

only, as these were found to be the most stressed material and orientation from the Minneapolis 

modeling.

The indoor conditions varied on a sinusoidal curve with a period of one year. The temperature 

was 68°F ± 2°F (20°C ± 1°C). For the Minneapolis modeling, the RH ranged from 30% in the 

winter to 60% in the summer for the standard humidity case. The RH ranged from 40% in the 

winter to 60% in the summer for the high humidity case. The 40% RH wintertime condition is 

generally considered beyond the upper level of recommended wintertime RH for cold climates in 

houses in general and creates a high-stress environment for all standard wall assemblies and is 

typically not seen in houses in cold climates. For Miami the RH ranged from 60% in the winter 

to 70% in the summer. For Seattle the RH ranged from 40% in the winter to 60% in the summer.

This modeling assumes good construction practices such as the materials are installed to the 

manufacturers’ recommendations, and that interior RH is maintained at a reasonable level during 

the winter. Variations on the interior RH, and associated implications, are discussed. Each 

assembly starts with the sheathing MC at 18%, which is the maximum allowed by typical SPF

manufacturers.

4.3 Roof Systems Modeled

The scope of this report includes only unvented cathedralized roof assemblies using SPF

insulation. Two roofs have been analyzed—one using ccSPF and one using ocSPF. In BSC’s 

experience these two roofs represent a significant portion of the production house roof 

construction options when using SPF under the roof sheathing and are regularly used in the deep

retrofit home market. Both roofs incorporate the minimum required R-value of SPF (R806.5) and 
12

complete the R-value with netted cellulose insulation or fiberglass (N1102.1.1). BSC 

recommends that unvented roofs incorporating SPF insulation beneath the roof deck have a fully 

adhered membrane above the roof deck and below the roof cladding. This membrane is a 

secondary layer of drainage protection, but it is also good protection during construction before 

the roof cladding is installed. Keeping the roof dry before SPF installation is required by most 

insulation manufacturers. The basic modeled roof assembly is shown in Table 3:

Table 3. Basic Modeled Roof Assembly

Minneapolis 

Roof A

Minneapolis 

Roof B

Miami and Seattle 

Roof A

Miami and Seattle 

Roof B

Exterior air Exterior air Exterior air Exterior air

Asphalt shingles Asphalt shingles Asphalt shingles Asphalt shingles

1⁄2-in. plywood or 

OSB structural roof 

sheathing

1⁄2-in. plywood or 

OSB structural roof 

sheathing

1⁄2-in. OSB structural 

roof sheathing

1⁄2-in. OSB structural 

roof sheathing

R-25 ccSPF R-25 ocSPF + 5 perm R-12 ccSPF R-30 ocSPF

R-24 fibrous air and 

vapor permeable 

insulation

R-24 fibrous air and 

vapor permeable 

insulation

R-19 fibrous air and 

vapor permeable 

insulation

Interior air Interior air Interior air Interior air

*Most Class II vapor retarder coatings when installed on ocSPF have an effective value of 5 perm.

4.4 Wetting—Rainwater Leakage

ASHRAE 160 (ASHRAE 2008) was used as a basis for choosing the quantity of rainwater 

leakage. Section 4.6.1 of the Standard discusses rain penetration:

“In the absence of specific full scale test methods and data for the as-built exterior wall system 

being considered, the default value for water penetration through the exterior surface is 1% of the 

water reaching that exterior surface. The deposit site for the water shall be the exterior surface of 

the water-resistive barrier. If a water-resistive barrier is not provided then the deposit site shall be 

described and a technical rationale for its selection shall be provided.”

ASHRAE 160 states that “This standard deals with rain penetration in walls only. Roof systems 

are to be designed and built such that there is no rainwater penetration.” However, this 

information was used as a starting point for the roof system hygrothermal modeling. WUFI has 

the capability to inject a percentage of the rainfall into the assembly in a specific location.

ASHRAE 160 suggests that 1% of the rainfall that hits the surface passes the wall primary water 

shedding layer and is deposited on the drainage plane. For this modeling, parts of the 1% rainfall

that pass the primary drainage plane are considered to leak past the fully adhered membrane on 

the roof sheathing and are hence deposited into the wood-based structural sheathing. A properly 

designed, detailed, and installed system should not be subject to any of the leakage discussed in 

this section, but it is important to understand the drying capability of the systems. The drying 
13

capability of the modeled roofs is shown based on the injection of water stemming from rainfall 

events in TMY2, U.S. Climate Normals, and WUFI weather data.

Table 4 shows the total rainfall for the year in Minneapolis and the average rainfall event

according to TMY2 and U.S. Climate Normals data. Typically annual rainfall is reported in 

inches, but this information does not help many people understand the hands-on quantity of 

rainfall. Rainfall on a 4 ft2 sloping area of roof (6/12 pitch) near the ridge was calculated. Four 

square feet was chosen, as in BSC’s experience this is a typical area that is damaged by a roof 

leak. The annual rainfall on a 4 ft

2 roof area is almost 70 gal (260 L). This does not account for 

an area such as a valley or roof edge where there are concentrations from water run-down from 

the rest of the roof. The total rainfall in Minneapolis from these data is 27.5 in. and the average 

hourly rainfall is 0.22 in.

Table 4. Minneapolis TMY2 and U.S. Climate Normals Calculated Rainfall

Total Annual 

Rainfall

1% of Annual 

Rainfall

Average Hourly 

Rainfall

1% of Average 

Hourly Rainfall

Gal/4 ft2 69 1 0.5 0.01

oz/4 ft2 8782 88 69.0 0.7

L/4 ft2 260 2.6 2.0 0.02

mL/4 ft2 259,943 2,599 2,044 20

Similar data to Table 4 were developed from WUFI for Miami and Seattle. Table 5 shows data 

developed from the Miami cold year weather file, which has significantly more rainfall than the 

warm year file. This was done to ensure a high-risk environment was modeled. Table 6 shows 

the data developed from WUFI for Seattle. Although the total annual rainfall increases for Miami 

and Seattle, the average rainfall decreases significantly from Minneapolis.

Table 5. Miami WUFI (Cold Year) Calculated Rainfall

Total Annual 

Rainfall

1% of Annual 

Rainfall

Average Hourly 

Rainfall

1% of Average 

Hourly Rainfall

Gal/4 ft2 209 2 0.2 0.002

oz/4 ft2 26678 267 23.9 0.2

L/4 ft2 790 7.9 0.7 0.007

mL/4 ft2 789,676 7,897 706 7

Table 6. Seattle WUFI Calculated Rainfall

Total Annual 

Rainfall

1% of Annual 

Rainfall

Average Hourly 

Rainfall

1% of Average 

Hourly Rainfall

Gal/4ft2 112 1 0.1 0.001

oz/4 ft2 14249 142 8.5 0.1

L/4 ft2 422 4.2 0.3 0.003

mL/4 ft2 421,780 4,218 253 3

4.5 Minneapolis Hygrothermal Results

One percent of an average rainfall event in Minneapolis is 0.7 oz (20 mL) per 4 ft2 area and 

ASHRAE 160 assumes that this quantity passes the exterior cladding and lands on the drainage 
14

plane. For the roof systems analyzed, fractions of the ASHRAE 1% rainfall (ranging from 0.01% 

to 1.00% of total rainfall) is able to pass the fully adhered membrane and come in contact with 

the wood-based sheathing. This is the amount of water that is modeled and has to dry through the 

roofing system or accumulate and possibly cause damage. Figure 5 shows the correlated 

plywood sheathing MCs for 0.01%–1.00% rainfall leaks for the roof system described with 

ccSPF and cellulose insulation. Over the three-year analysis period, all of the injection scenarios 

are within the safe range and are not maintained above 25% MC. For the 0.80% and 1.00% cases 

there appears to be an increasing trend long term, which could likely lead to damage; the rest of 

the assemblies demonstrated a decreasing trend in MC.

Figure 5. Minneapolis plywood sheathing MC of ccSPF unvented roof

To quantify the amount of water injected at the leak per year in each scenario Table 7 was 

developed. The 0.60% case, and cases less than that, have an overall trend of getting drier each 

year—i.e., the roof can manage the imposed water loads from a 0.6% rainwater leak. The roofing 

system using ccSPF and cellulose insulation on plywood sheathing has the capability, according 

to WUFI, to dry 53 oz (1.6 L) of water through a 4 ft2 area of plywood per year. The graph also 

shows that the regular wettings from rainfall do not lead to accumulated moisture in the 

sheathing if the leak is < 0.6% of the rainfall. Assuming the fully adhered membrane is properly 

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

1.00% North Mid MC 0.80% North Mid MC 0.60% North Mid MC 0.40% North Mid MC

0.20% North Mid MC 0.10% North Mid MC 0.05% North Mid MC 0.01% North Mid MC
15

designed, detailed, and installed, these quantities of water have little likelihood of ever entering 

the system through rainwater leaks.

Table 7. Minneapolis Annual Rainfall Volume Calculations at a Roof Ridge

0.01%

of Rain

0.05%

of Rain

0.10%

of Rain

0.20%

of Rain

0.40% 

of Rain

0.60%

of Rain

0.80%

of Rain

1.00%

of Rain

Gal/4 ft2 0.01 0.03 0.07 0.1 0.3 0.4 0.6 0.7

oz/4 ft2 0.9 4.4 8.8 18 35 53 70 88

L/4 ft2 0.03 0.13 0.26 0.5 1.0 1.6 2.1 2.6

mL/4 ft2 26 130 260 520 1,040 1,560 2,080 2,599

Considering the 0.10% case, which is assuming 10% of the rain that passes the cladding can get 

past the fully adhered membrane, a sensitivity analysis was completed to determine the effects of 

roof orientation, OSB versus plywood sheathing, interior RH, and finally replacing the R-value 

of ccSPF with an equivalent R-value of ocSPF and two levels of class III vapor retarder.

Table 7 shows plywood sheathing MCs at 0.10% rainfall contacting the plywood on a north 

orientation and a south orientation with ccSPF and cellulose insulation. The graph shows that the 

north orientation increases the peak MC of the sheathing by up to 3% but that the sheathing MCs

all remain well below any level of risk.

Figure 6. Minneapolis sheathing MC—north versus south

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.10% South Mid MC 0.10% North Mid MC
16

Figure 7 shows sheathing MCs at 0.10% rainfall contacting the sheathing considering OSB and 

plywood sheathing at 1⁄2-in. thick on the north orientation with ccSPF and cellulose insulation.

The graph shows that OSB sheathing maintains 1%–1.5% higher MC than plywood, but also that 

the sheathing MCs all remain well below any level of risk.

Figure 7. Minneapolis sheathing MC—OSB versus plywood

Peak plywood sheathing MC was affected by up to 1% when considering two interior RH

scenarios (Figure 8). The first scenario allowed wintertime interior RH to fall to 30% while the 

second only fell to 40%. Both scenarios had summertime RH of 60%. In both scenarios the 

sheathing MCs all remain well below any level of risk.

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.10% North OSB MC 0.10% North Ply MC
17

Figure 8. Minneapolis plywood sheathing MC based on standard and high interior RH

To determine if the interior RH affects a larger leak in a different way, the 0.60% leak was 

analyzed at the standard and high RH scenarios. As shown in Figure 9, the MC of the sheathing 

is affected only slightly more by the change in interior RH. The sheathing MCs all remain well 

below any level of risk.

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.10% North MC High RH 0.10% North MC Std RH
18

Figure 9. Minneapolis plywood sheathing MC based on interior RH—larger leak

The type of SPF was analyzed in Figure 10. The ocSPF in this case requires a Class II vapor 

retarder installed on the interior face of the insulation. A 5 perm coating was modeled, as this 

represents the actual effective achieved perm value based on BSC experience with spray-applied 

Class II vapor retarder coatings. Figure 10 shows sheathing MCs at 0.10% rainfall contacting the 

plywood sheathing on the north orientation. The ocSPF MC varies significantly more than the 

ccSPF, but both remain within safe wood MCs. The ocSPF dries much faster than the ccSPF

during warm weather, but also has the ability to pass more moisture from the interior to the 

exterior during cold weather and hence the MC of the plywood increases significantly more 

during those periods.

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.60% North MC High RH 0.60% North MC Std RH
19

Figure 10. Minneapolis plywood sheathing with ccSPF and ocSPF

Because the ocSPF more easily tracks interior RH, a comparison using only ocSPF was 

completed. Figure 11 shows the effect of increased RH on a system using ocSPF and a 5-perm 

coating with a 0.10% rainfall leak. With elevated interior wintertime RH of 40%, the plywood 

sheathing MCs rise above 25%, but are not sustained for a significant period of time. If interior 

RH is kept below 35% in this climate zone, this system can operate safely.

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.10% North MC ocSPF w/ 5 Perm 0.10% North MC ccSPF
20

Figure 11. Minneapolis plywood sheathing MC ocSPF and high RH

The coating on the interior of the ocSPF plays a significant role in the vapor transmission from 

the interior to the exterior in a cold climate. The ocSPF system in Figure 11 was altered to 

include an effective 2 perm coating instead of the 5 perm coating previously considered. This 

was modeled and compared to the ccSPF system. With a 0.10% rain leak, the MC of the 

sheathing is shown in Figure 12. The 2 perm coating with standard RHs reduces the sheathing 

MC by almost 5%, while using ccSPF decreases it another 5%. Both the 2 perm coating and 5 

perm coating on ocSPF dry the sheathing faster than ccSPF, but both get the sheathing wetter 

during cold weather than ccSPF.

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.10% North MC ocSPF w/ 5 Perm High RH 0.10% North MC ocSPF w/ 5 Perm Std RH
21

Figure 12. Minneapolis sheathing MC—ocSPF, 2 perm coating, and 5 perm coating

4.6 Seattle and Miami Hygrothermal Results

The modeling from Minneapolis showed that OSB sheathing on the north orientation was at the 

highest risk for deterioration. Based on this information, the roof systems in Miami and Seattle 

were modeled using OSB on the north orientation to evaluate the level of highest risk associated 

with rainwater leakage and its effect on the sheathing MC.

ASHRAE 160 assumes that 1% of an average rainfall event passes the exterior wall cladding and 

lands on the drainage plane. Rainfall sourced volumes up to that 1% are modeled assuming they 

pass the fully adhered backup drainage plane created by the adhered roof membrane and then

come in contact with the wood-based sheathing. This is the amount of water that then has to dry 

through the roofing system or accumulate and possibly cause damage. Within climate zones 1 

and 4 the vapor barrier requirements are such that no interior vapor control is required. This has 

positives and slight negatives, as is shown in the following figures.

Similar to the data shown in Figure 5, 0.1%–1% of rainfall was injected into the sheathing and 

the resultant wood MCs were compared. The Seattle modeling shows that a roofing system using 

ccSPF insulation against the underside of the roof sheathing and cellulose insulation was able to 

0%

5%

10%

15%

20%

25%

December June December June December June December

Plywood Sheathing Moisture Content

Date

0.10% North MC ocSPF 2 Perm 0.10% North MC ocSPF 5 Perm 0.10% North MC ccSPF
22

cycle 8%–24% MC with the influences of both indoor RH and up to the full 1% rainwater leak 

(Figure 13). The three-year analysis shows a drying trend of the system, although it is not 

recommended to allow MCs to regularly attain more than 25% for long durations, as occurs with 

the full 1% leakage. When the cellulose is replaced with fiberglass insulation (Figure 14) a very 

similar model is produced. In this system, cellulose and fiberglass perform nearly identically.

Figure 13. Seattle sheathing MC north 2 in. ccSPF + 5.25 in. cellulose
23

Figure 14. Seattle sheathing MC north 2 in. ccSPF + 5.25 in. fiberglass

Climate zone 4 is a difficult climate in that there is relatively continuous rain-based wetting as 

well as inward and outward vapor drives due to the heating and cooling of the homes during the 

alternating seasons. Figure 15 shows the sheathing MC as modeled between 0.1% and 1% of the 

rainwater. The modeling shows an increasing trend to the sheathing MC if more than 0.6% of 

rainwater leaks into the assembly. This correlates with the data analyzed for Minneapolis. In the 

Seattle case, both rainwater leakage year round and outward vapor drive in cold weather caused 

increased MCs in the sheathing while drying abruptly occurred during warm weather with the 

relatively high vapor ocSPF.
24

Figure 15. Seattle sheathing MC north 8 in. ocSPF

The Miami climate modeling shows that although the systems will be exposed to significant 

wetting throughout the year, there is also an equally large vapor drive produced by the heat and 

solar exposure, which forces the incidental moisture through the assembly and into the interior of 

the home. Figure 16 and Figure 17 are very similar graphs showing the sheathing MC of ccSPFinsulated unvented cathedralized roofs using cellulose and fiberglass insulation. Both systems

safely dry out the full 1% of the rainwater leakage into the interior space. Figure 18 shows the 

sheathing MC for an ocSPF insulated roof in Miami and indicates that the full 1% rainwater leak 

is easily managed by the roof with significant inward vapor drives due to temperature and solar 

incidence with very limited outward vapor drives usually initiated by cooler outdoor weather.

The Miami modeling was then further stressed to determine its upper limit of wetting. Using the 

ocSPF case, the rainwater percentage was increased from 1% to 1.5%, 2.0%, and then 5%.

Figure 19 shows the sheathing MCs for the high leak rates in Miami using ocSPF. At the 2% step 

the roof began to have extended periods of time with the exterior layer of the sheathing 

experiencing MCs above 25% and as such the dryable limit was chosen as 1.5%.
25

Figure 16. Miami sheathing MC north 2 in. ccSPF + 5.25 in. cellulose

Figure 17. Miami sheathing MC north 2 in. ccSPF +5.25 in. fiberglass
26

Figure 18. Miami sheathing MC north 8 in. ocSPF

Figure 19. Miami sheathing MC north 8 in. ocSPF high stress
27

5 In-Service Roof Explorations

A number of roofs were reviewed to visually inspect the sheathing of in-service residential roofs 

using SPF against the underside of the roof sheathing. This enabled the correlation of modeled 

low sheathing MC to in-service roofs with measured low sheathing MC. The exploration

consisted of the following:

• Attaining permission from a homeowner with an existing unvented roof using SPF

insulation in contact with the roof sheathing

• Completing an invasive investigation on the roof from the interior

• Removing an area of interior drywall (if present)

• Removing fibrous insulation (if present)

• Removing a core of the SPF

• Inspecting the sheathing

• Taking an MC reading of the sheathing

• Taking photo documentation

• Repairing the area

• Documenting the results and roof system.

Eleven locations were reviewed. These locations span climate zones 2 to 7. Table 8 shows the 

list of houses and their locations. 

Table 8. In-Service Exploration House List

House Location Climate Zone Date of Site Visit

House 1 New Orleans, Louisiana 2 June 2012

House 2 New Orleans, Louisiana 2 July 2012

House 3 Coquitlam, British Columbia 4C July 2012

House 4 Coquitlam, British Columbia 4C July 2012

House 5 Westerville, Ohio 5 June 2012

House 6 Pontiac, Michigan 5 July 2012

House 7 Minneapolis, Minnesota 6 July 2012

House 8 Minneapolis, Minnesota 6 July 2012

House 9 Juneau, Alaska 7 July 2012

House 10 Juneau, Alaska 7 July 2012

House 11 Juneau, Alaska 7 July 2012

The following sections discuss each exploration, show photos where possible, and list the MC

readings for each location. All sheathing readings, in all locations reviewed to date, are within 

the safe range for wood-based sheathing materials. Section 5.6 shows a failure of a SPF roof 

insulation system. In this case the manufacturer’s investigation report concluded that the SPF

was installed on wet OSB based on the observation that the SPF did not adhere well to the 
28

substrate and the pore structure of the ccSPF at the ccSPF/OSB interface was indicative of a wet 

substrate.

5.1 House 1: New Orleans, Climate Zone 2, June 2012

Description

• Built 2009

• Cathedralized attic

• R-21—~3.5 in. ccSPF below OSB roof 

sheathing

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

Exploration Location 1—North Lower

• 6% moisture content reading

• No visible signs of moisture damage

Exploration Location 2—West Upper

• 7.5% MC reading

• No visible signs of moisture damage

Exploration Location 3—East Upper

• 6.5% MC reading

• No visible signs of moisture damage

Exploration Location 4—West Lower

• 7.0% MC reading

• No visible signs of moisture damage

This information correlates well to modeling of 

warm locations with drives that enhance drying and 

have limited wetting.
29

5.2 House 2: New Orleans, Climate Zone 2, July 2012

Description

• Built 2009

• Cathedralized attic

• R-21—~3.5 in. ccSPF below OSB roof 

sheathing

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

Exploration Location 3—East Upper

• 6% MC reading

• No visible signs of moisture damage

Exploration Location 4—West Lower

• 7% MC reading

• No visible signs of moisture damage

This information correlates well to modeling of 

warm locations with drives that enhance drying and 

have limited wetting.

5.3 House 3: Coquitlam, Climate Zone 4c, July 2012

No photos available.

Description—North Upper Center Attic Bay

• R-20—~5.5 in. ocSPF with 2 coats latex 

paint below 1⁄2-in. OSB sheathing 

Exploration Findings

• 8.7% OSB MC reading

• 8.1% OSB MC reading

• 7.5% OSB MC reading

• No visible signs of moisture damage

• OSB appeared clean and new looking

5.4 House 4: Coquitlam, Climate Zone 4c, July 2012

No photos available.

Description—North Upper End Attic Bay

• R-19—~5 in. ocSPF below 1⁄2-in. OSB 

sheathing

Exploration Findings

• 8.5% OSB MC reading

• 8.4% OSB MC reading

• No visible signs of moisture damage

• OSB appeared clean and new looking

• 9.2%, 10.1% framing bottom chord 

• 12.4% framing mid-height

• 13.5% framing upper 
30

5.5 House 5: Westerville, Climate Zone 5, June 2012

Description

• Built 2004

• Unvented cathedralized attic

• R-12—~2 in. ccSPF below roof sheathing

• R-28 fiberglass batt

• 1⁄2-in. drywall with latex paint

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

This information correlates well to modeling of cold 

climates with appropriate levels of vapor control.

Exploration Location 1—North Lower

• 13.5% MC reading

• This location had a roof leak in the 6 

months before the MC reading

Exploration Location 2—North Upper

• 7% MC reading

• No visible signs of moisture damage
31

Exploration Location 3—South Lower

• 8% MC reading

• No visible signs of moisture damage

Exploration Location 4—South Upper

• 7% MC reading

• No visible signs of moisture damage

5.6 House 6: Pontiac, Climate Zone 5, July 2012

Description

• New Home January 2012

• Cathedralized attic

• R-40—~6 in. ccSPF below OSB sheathing

Exploration Findings

• Roof sheathing dry in all locations

• Roof sheathing structural failure

Exploration Locations

• OSB swelled and deteriorated

• During July exploration OSB was dry

• OSB was able to dry to the interior

• Roof sheathing MC < 6% in all locations

when tested in July

• Roof sheathing was removed and replaced

• Manufacturer’s analysis report concluded 

that the SPF was installed over very wet 

OSB, which sealed in the liquid water

causing the deterioration
32

5.7 House 7: Minneapolis, Climate Zone 6, July 2012

Description

• 1941, retrofit 2012

• Cathedralized attic

• R-21—~3.5 in. ccSPF below 1x board roof

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

This information correlates well to modeling of cold 

climates with appropriate levels of vapor control.

Exploration Location 1—Northwest Lower

• 9.2% MC reading

• No visible signs of moisture damage

Exploration Location 2—Southwest Lower

• 6.9% MC reading

• No visible signs of moisture damage
33

5.8 House 8: Minneapolis, Climate Zone 6, July 2012

Description

• Built 1914, retrofit January 2009

• Cathedralized attic

• R-21—~3.5 in. ccSPF below 1x board roof

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

This information correlates well to modeling of cold 

climates with appropriate levels of vapor control.

Exploration Location 1—North Upper

• 12.9% MC reading

• No visible signs of moisture damage

Exploration Location 2—North Upper

• 6.7% MC reading

• No visible signs of moisture damage

5.9 House 9: Juneau, Climate Zone 7, July 2012

Description

• Under construction 2012

• Cathedralized attic in heated garage

• R-15—~4 in. ocSPF below sheathing

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

Exploration Location 1—Southeast Lower

• 15.1% MC reading

Exploration Location 2—Southeast Upper

• 12.7% MC reading

Exploration Location 2—Northwest Mid

• 12.9% MC reading
34

5.10 House 10: Juneau, Climate Zone 7, July 2012

Description

• New home 2012

• Cathedralized attic

• R-19—~3 in. ccSPF below sheathing

• R-28—~7.5 in. ocSPF below ccSPF

Exploration Findings

• All sheathing locations investigated are 

within safe MC readings

Exploration Location 1—East Lower

• 16.9% MC reading (sheathing)

• No visible signs of moisture damage

Exploration Location 2—West Upper

• 15.1% MC reading (sheathing)

• 14.1% MC reading (framing)

• No visible signs of moisture damage

5.11 House 11: Juneau, Climate Zone 7, July 2012

Description

• Built in 2009

• Apartment above commercial paint supply 

store

• Cathedralized attic

• R-19—~3 in. ccSPF below sheathing

• R-20—~5.5 in. ocSPF below ccSPF

Exploration Findings

• All sheathing locations investigated are 

elevated. Due to the unique situation of the 

interior conditions, it is recommended that 

this location and its interior conditions be 

monitored further.

Exploration Location 1—South Mid

• 22.9% MC reading

• No visible signs of moisture damage

Exploration Location 2—North Mid

• 21.2% MC reading

• No visible signs of moisture damage
35

6 Conclusions

Unvented roof strategies with ocSPF and ccSPF insulation sprayed to the underside of roof 

sheathing have been used since the mid-1990s to provide durable and efficient building 

enclosures. However, there have been isolated moisture-related incidents reported anecdotally 

that raise potential concerns about the overall hygrothermal performance of these systems. The 

incidents related to rainwater leakage and condensation concerns. Condensation concerns have 

been extensively studied by others and were not discussed in this report (Straube et al. 2010).

This project involved hygrothermal modeling of a range of rainwater leakage and field 

evaluations of in-service residential roofs using spray foam insulation. All of the roof assemblies 

modeled exhibited drying capacity to handle minor rainwater leakage. All field evaluation 

locations of in-service residential roofs had MCs well within the safe range for wood-based 

sheathing.

Hygrothermal modeling was conducted on roof assemblies located in the principal climate zones 

of interest defining building performance in the lower 48 states—a hot climate with significant 

rain (Miami), a cold climate (Minneapolis), and a marine climate with significant rain (Seattle).

These locations “bracket” the expected in-service conditions of concern for unvented roof 

assemblies.

The field evaluation locations were selected based on availability and timing. All locations that 

were made available by the industry partners were evaluated.

The quantity of water passing through a roof system is difficult to quantify, but hygrothermal 

modeling was possible using ASHRAE 160, TMY2 and U.S. Climate Normals weather data, and 

WUFI weather data. WUFI 5 was used to determine the effect of 0.01%–1.00% of rainfall

entering the unvented roof system as a leak and coming in contact with the wood-based roof 

sheathing in Minneapolis, Seattle, and Miami. The 2012 IRC-compliant roofing system in 

Minneapolis using ccSPF on plywood sheathing with cellulose insulation on the interior has the 

capability according to the modeling to safely dry 53 oz (1.6 L) of water through a 4-ft2 area of 

plywood per year. MCs > 20% were seen during the modeling, but the systems were typically 

able to dry during the summer and return to < 8% MC. Within the Seattle analysis the ccSPF 

insulated OSB-sheathed roofs were able to handle up to 1% rainwater leakage, while the ocSPF 

roof experienced elevated MC when more than 0.6% rainwater leakage was introduced into the 

system. This is due to both rainwater leakage and outward vapor drives during the heating 

season. The ocSPF roofs dried out much more readily than the ccSPF roofs. The Miami analysis 

showed that that both ccSPF and ocSPF roofs dried, even up to 1.5% rainwater leakage, although 

both experienced more short-term fluctuation than similar roofs in the Seattle climate.

Assuming the fully adhered membrane is properly designed, detailed, and installed, these 

quantities of water have little likelihood of ever entering the system through rainwater leaks. The 

modeling showed ocSPF dries more readily than ccSPF, but ocSPF with a 5 perm coating or a 2 

perm coating allows more wetting of the sheathing during the winter months.

Interior RH can directly affect the sheathing MC in all scenarios and BSC recommends that

wintertime RH in climate zone 6 homes should be maintained at < 40%—a limit that is typical 
36

for standard houses in this climate zone. Wintertime RHs higher than this typically result in 

window condensation, wall and foundation assembly hygrothermal performance issues even in 

high performance houses.

Orientation and sheathing materials create variations within the system, but these variations are 

relatively small compare to the type of SPF and vapor permeance coatings used. The modeling 

showed that OSB sheathing, ocSPF with 5 perm coating, and roofs facing north maintain the 

highest MCs, but all systems were within the safe range for wood-based sheathing.

Explorations of 11 in-service roof systems were completed. The exploration involved taking a 

sample of SPF from the underside of the roof sheathing, exposing the sheathing, then taking an 

MC reading. All locations investigated had MCs well within the safe range for wood-based

sheathing. One failure was reviewed, as an industry partner was involved with replacing 

structurally failed roof sheathing. In this case the SPF manufacturer’s investigation report 

concluded that the SPF was installed on wet OSB based on the observation that the SPF did not 

adhere well to the substrate and the pore structure of the ccSPF at the ccSPF/OSB interface was 

indicative of a wet substrate. The other investigations were important to verify there have not 

been any hidden issues. Where a failure has occurred due to a rain leak, the volume of the leak, 

frequency of the leak(s), or quantity of interior moisture driven into the system, were likely 

significantly more than modeled in this study.

The following summarizes the findings of this research and how it relates to the questions.

Are there risks associated with installing spray foam under plywood and OSB roof decks 

specifically moisture and durability issues?

Based on this modeling there are no known risks with using SPF insulation under plywood and 

OSB roof decks if the following requirements are met:

• The installation complies with the 2012 IRC.

• A fully adhered leak-free roof membrane is installed.

• The roof sheathing is and framing dry below 18% before SPF installation.

• And when using ocSPF a low-perm Class II vapor retarder is installed where required.

Are roof leaks a serious problem?

Roof leaks can accumulate and be a problem if they allow > 1% of the rainfall to pass the 

drainage plane and deposit on the wood sheathing. Using the ASHRAE 160P criteria of a 1% 

rainfall leak past the exterior primary rain shedding layer, and assuming all of that 1% is able to 

pass the adhered roofing membrane to the sheathing, on the north orientation the leak can cause 

increased MC of the sheathing and may eventually lead to deterioration. If the leak is repaired, 

the systems analyzed in this report were capable of drying to the interior. If the leak is < 1%,

most of the systems analyzed were able to repeatedly dry over time (Figure 5). Proper design and 

careful installation can limit if not eliminate roof leaks. Note that this is based on a climate zone 

with a moderate rainfall.
37

What is happening in these systems with high measured sheathing MC (i.e., 20+) but with 

no evidence of damage to the sheathing?

The ocSPF analysis section showed that depending on when the system was measured there is a 

significant fluctuation in the sheathing MC. There was a six-month swing from 4% MC to 25% 

MC repeatedly as the interior RH drove moisture into the assembly followed by drying of the 

assembly (Figure 11). The ccSPF did not experience such a fluctuation (Figure 8) and both 

systems dried to acceptable levels seasonally. Sheathing MCs > 20% can occur and if they have 

the ability to dry relatively quickly, the system will not experience damage. If the system 

maintains high MC, damage is likely to occur. 

Are there moisture durability risks associated with installing spray foam under OSB roof 

decks in climates with high rainfall?

All of the roof assemblies modeled exhibited drying capacity to handle minor rainwater leakage.

Based on this modeling there are no known risks with using SPF insulation under OSB roof 

decks if the following requirements are met: 

• The installation complies with the 2012 IRC.

• A fully adhered leak-free roof membrane is installed.

• The roof sheathing and framing is dry below 18% before SPF installation.

• And when using ocSPF a low-perm Class II vapor retarder is installed where required.
38

References

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

Standard 160, Criteria for Moisture-Control Design Analysis in Buildings, ASHRAE Standard 

160P, Sept. 2008

Baker, M., Decay of Wood. Canadian Building Digest 111. National Research Council 

Canada.1969.

Forest Products Laboratory, Wood Handbook, U.S. Department of Agricultrure, Forest Service, 

2010.

International Code Council, 2012 International Residential Code, 2012.

Lstiburek, J., Guide to Attic Air Sealing, Building Science Press, MA, 2010.

Lstiburek. J., and Grin, A. Building America Special Research Project: Advanced Framing 

Deployment, RR-1004. www.BuildingScience.com, Nov., 2010.

Lstiburek. J. Enclosures Standing Technical Committee Strategic Plan, v2011a, Building 

Science Corporation, Somerville, MA, Dec. 2011

Steffen, M., Moisture and Wood-Frame Buildings. Canadian Wood Council, Building 

Performance Series No. 1, 2000.

Straube, J., Ice Dams. Building Science Digest 135, www.buildingscience.com, Oct., 2006.

Straube, J.F., and Smegal, J. Building America Special Research Project—High-R Walls Case 

Study Analysis. RR-0903. www.BuildingScience.com, Mar. 2009

Straube, J.F., Smegal, J. and Smith, J. Moisture-Safe Unvented Wood Roof Systems, Building 

Science Corporation Research Report –RR-1001, www.BuildingScience.com, April 2010

Straube,J., High R-value Enclosures for High Performance Residential Buildings in All Climate 

Zone, Building Science Press, Somerville, MA, 2010.

Straube, J., and Burnett, E., Building Science for Building Enclosures. Westford, MA, Building 

Science Press, 2005.
buildingamerica.gov

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