Moisture Risk in Unvented 

Attics Due to Air Leakage 

Paths

D. Prahl and M. Shaffer

IBACOS, Inc.

November 2014
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iii

Moisture Risk in Unvented Attics Due to Air Leakage Paths

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: 

D. Prahl and M. Shaffer 

IBACOS, Inc.

2214 Liberty Avenue 

Pittsburgh, Pennsylvania 15222

NREL Technical Monitor: Stacey Rothgeb 

Prepared under Subcontract No. KNDJ-0-40341-04 

November 2014 
iv

The work presented in this report does not represent 

performance of any product relative to regulated 

minimum efficiency requirements.

The laboratory and/or field sites used for this work are 

not certified rating test facilities. The conditions and 

methods under which products were characterized for 

this work differ from standard rating conditions, as 

described.

Because the methods and conditions differ, the reported 

results are not comparable to rated product performance 

and should only be used to estimate performance under 

the measured conditions.
v 

Contents

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

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

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

Acknowledgments ................................................................................................................................... viii

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

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

2 Project Background ............................................................................................................................. 2

2.1 Vented and Unvented Attics............................................................................................................ 3

2.2 Energy Savings of Unvented Attics................................................................................................. 4

2.3 Attic Moisture-Related Issues.......................................................................................................... 4

2.4 Airflow-Induced Forces................................................................................................................... 5

3 Research Questions............................................................................................................................. 6

4 Technical Approach ............................................................................................................................. 7

4.1 Scope of Study ................................................................................................................................. 7

4.2 Field Data......................................................................................................................................... 8

4.2.1 Attic Airflow Path (Crack) Types....................................................................................... 8

4.2.2 Unvented Attic Air Leakage Rates..................................................................................... 8

4.3 Modeling Effort ............................................................................................................................... 8

5 Results and Discussion..................................................................................................................... 10

5.1 Field Data....................................................................................................................................... 10

5.1.1 Attic Airflow Path (Crack) Types..................................................................................... 10

5.1.2 Discussion of Attic Airflow Path (Crack) Types.............................................................. 13

5.1.3 Unvented Attic Air Leakage Rates................................................................................... 13

5.1.4 Discussion of Field Data................................................................................................... 14

5.2 Computational Fluid Dynamics—Airflow Rates for Airflow Paths.............................................. 15

5.2.1 Discussion of the Computational Fluid Dynamics Analysis ............................................ 15

5.3 CONTAM—Airflow Rates for Unvented Attics........................................................................... 15

5.3.1 Discussion of CONTAM .................................................................................................. 17

5.4 Mass Flow Rate of Water Vapor ................................................................................................... 17

5.4.1 Discussion of Mass Flow Rate of Water Vapor................................................................ 19

5.5 Hygrothermal Analysis—Moisture Content .................................................................................. 19

5.5.1 Discussion of Hygrothermal Analysis.............................................................................. 23

6 Conclusions ........................................................................................................................................ 24

6.1 Future Research ............................................................................................................................. 24

References................................................................................................................................................. 26
vi 

List of Figures

Figure 1. Airflow paths for (left) a vented (conventional) attic and (right) an unvented 

(cathedralized) attic.............................................................................................................................. 4

Figure 2. The study workflow..................................................................................................................... 7

Figure 3. Plumbing vent penetrations..................................................................................................... 10

Figure 4. Delaminating ccSPF airflow path ............................................................................................ 10

Figure 5. Ridge vent opening................................................................................................................... 11

Figure 6. Surface area openings at the roof sheathing......................................................................... 11

Figure 7. Plumbing, bath fan venting, or flue vent airflow path........................................................... 12

Figure 8. Surface area airflow path at the face of a roof rafter............................................................. 12

Figure 9. Ridge vent airflow path with delamination at the face of a roof rafter and ridge............... 13

Figure 10. Pre-retrofit (vented) and post-retrofit (ccSPF-insulated unvented) attic airflow and 

pressure curves.................................................................................................................................. 14

Figure 11. Location of attic airflow paths on the CONTAM model....................................................... 16

Figure 12. Airflow pressure relationship (between attic interior and exterior) for the family of 

airflow paths input into CONTAM ..................................................................................................... 16

Figure 13. Histogram of CONTAM simulated hourly exfiltration and infiltration airflow rates 

through a ccSPF-insulated unvented attic in Minneapolis, Minnesota. Negative values 

indicate outdoor air flowing into the attic space............................................................................. 17

Figure 14. CONTAM attic airflow rates determined for Minneapolis, Minnesota. The shaded 

regions indicate instances when the outdoor temperature was lower than the dew point 

of the airflow moving through the attic cracks. .............................................................................. 18

Figure 15. Histogram of the number of continuous hours that airflow is moving through attic 

airflow paths and condensation could occur.................................................................................. 19

Figure 16. Schematic view of the WUFI 2D model for the plumbing airflow path.............................. 20

Figure 17. WUFI representation of the airflow path............................................................................... 20

Figure 18. Hygrothermal results indicating the OSB moisture content at various distances 

from the edge of the airflow path...................................................................................................... 21

Figure 19. Hygrothermal results for plumbing airflow path with 0 and 0.25 CFM airflow ................. 22

Figure 20. Hygrothermal results for plumbing airflow path with 2.5 and 12.5 CFM airflow .............. 22

Unless otherwise noted, all figures were created by IBACOS.

List of Tables

Table 1. Data Collected for Three Attics*................................................................................................ 14

Table 2. Airflow Characteristics Relating Airflow Rate to Pressure .................................................... 15

Table 3. Airflow Path Type, Number, and Airflow at 4 Pa for Each Airflow Path and the Total ........ 15

Unless otherwise noted, all tables were created by IBACOS.
vii 

Definitions

2D Two-Dimensional

3D Three-Dimensional

ACH Air Changes per Hour

BEoptTM Building Energy Optimization (software)

ccSPF Closed-Cell Spray Polyurethane Foam

CFD Computational Fluid Dynamics

OSB Oriented Strand Board

TMY3 Typical Meteorological Year 3

WUFI Wärme und Feuchte instationär (software)
viii

Acknowledgments

This report was prepared for the U.S. Department of Energy Building America program. Funding 

for this work was provided by the National Renewable Energy Laboratory under Subcontract No.

KNDJ-0-40341-04. 

The authors acknowledge Building Science Laboratories, Building Science Corporation, and 

Steven Winter Associates for their technical assistance with this research.
ix

Executive Summary

IBACOS completed an initial analysis of moisture damage potential in an unvented attic 

insulated with closed-cell spray polyurethane foam. To complete this analysis, the research team 

collected field data, used computational fluid dynamics to quantify the airflow rates through 

individual airflow (crack) paths, simulated hourly flow rates through the leakage paths with

CONTAM software, correlated the CONTAM flow rates with indoor humidity ratios from 

Building Energy Optimization software, and used Wärme und Feuchte instationär Pro twodimensional modeling to determine the moisture content of the building materials surrounding 

the cracks. 

Given the number of simplifying assumptions and numerical models associated with this 

analysis, the results indicate that localized damage due to high moisture content of the roof 

sheathing is possible under very low airflow rates. Reducing the number of assumptions and 

approximations through field studies and laboratory experiments would be valuable to 

understand the real-world moisture damage potential in unvented attics. 
1 

1 Introduction

This report investigates moisture risk in unvented (cathedralized) attics in cold climates resulting 

from airflow paths (cracks) from the unvented attic to the outdoors. The analysis uses attic 

leakage rates for existing houses, although the results are applicable to new construction if the 

leakage in the new construction is similar (i.e., similar airflow path geometry, airflow magnitude, 

location of leaks) to that discussed in this report. Specifically, this report investigates, through 

numerical simulations calibrated against measured data, the moisture potential of a two-story, 

2,050-ft2 house located in Minneapolis, Minnesota, in an unvented attic insulated with closedcell spray polyurethane foam (ccSPF) at the roof deck. In this house, the attic temperature and 

relative humidity are equal to those of the living space (this was modeled via an open attic 

hatch), and the enclosure leakage is 5 air changes per hour at 50 Pa (ACH50). Note that in this 

report, the attic leakage was included in the envelope leakage; however, the volume of the attic 

was not used to determine the air changes per hour due to the fact that the attic will not be used 

as a living space. This report also presents field-recorded attic leakage rates of three existing 

houses where ccSPF was applied at the roof deck to houses retrofitted from vented to unvented 

attics. At two of those three houses, the attic leakage was recorded before the vented attic was 

retrofitted to an unvented attic.

The purpose of this study is to investigate the hypothesis that ccSPF unvented attics have some 

moisture damage risk due to airflow paths between the ccSPF and attic building materials, at 

connections between building materials (e.g., wood-to-wood connections), or at other typical 

roof penetrations (e.g., plumbing or flue stack). To investigate this question, the IBACOS 

research team first worked to understand the airflow paths through ccSPF-insulated unvented 

attics and then to evaluate the moisture risks associated with the airflow paths. Because the intent 

was to understand the risks, the scope of this report was limited to worst-case scenarios; 

conservative assumptions were made that maximized the water vapor available for condensation 

and the environment for condensation. 

The focus of this study is heating climates (climate zones 6 and 7) where condensation will occur 

on the roof sheathing due to low outdoor temperatures. Although moisture deposition may also 

occur during space cooling periods (where condensation is most likely to occur on cold space 

conditioning equipment surfaces), it is not the primary risk in a heating-dominant environment. 

In a heating-dominant climate, the indoor air is warmer and has more moisture than the outdoor 

air and thus has lower density. Lower density air moves to the highest point (i.e., the attic) and 

travels through attic cracks to the outdoors. 
2 

2 Project Background

In 2011, a spray foam manufacturer approached IBACOS to discuss moisture-related risks 

associated with retrofitting vented to unvented attics with ccSPF. The manufacturer’s primary 

concern with unvented attics was the potential risk of condensation at the interior surface of the 

ccSPF at the roof deck when fiberglass batt insulation was installed on the attic floor. The 

research team made two primary assumptions:

1. The temperature and humidity ratio of the living space and the attic air were not in 

equilibrium. A small amount of living space air leaked into the attic, and no air leaked 

from the attic to the exterior.

2. The attic floor fiberglass batt insulation (i.e., thermal barrier) restricted the heat flow into 

the attic space, resulting in an intermediate thermal zone between indoor and outdoor 

temperatures. This condition reduces the temperature of the downward-facing ccSPF at 

the roof deck, creating a scenario where the dew point of the attic air could more easily 

be achieved on the ccSPF surface. 

Results of this work indicated that if the spray foam at the underside of the roof deck was an air 

barrier preventing leakage to the outside, if there was series leakage between the attic and the 

conditioned space below, and if there was insulation at the attic floor, then there can be a risk of 

condensation on the underside of the foam. Although many other factors (e.g., daily solar 

loading, night sky radiation, snow accumulation on the roof, condition of the existing attic floor 

insulation) are involved, this risk is conservatively dependent on inside temperature and relative 

humidity, average annual winter outdoor temperature, and, most importantly, the relative Rvalue of the insulation on the attic floor. In effect, insulation at the attic floor prevents the surface 

materials in the attic space from becoming warm enough to prevent condensation when air from 

the living space leaks into the attic.

The analysis discussed above did not consider the air leakage from the attic to the exterior, 

although it is known that some leakage does occur, because the complexity of the problem makes 

solutions highly uncertain. The analysis in this report focuses on airflow paths and condensation 

that can result in the airflow paths. 

To begin this project, the research team completed a “back-of-the-envelope” calculation 

(Equation 1) to evaluate the mass of water vapor moving through airflow paths of an attic:

, yr

day 365

day

hr 24

hr

min 60

yr

lb

%30&70@ %30&70@ 2 vaporOH CFMavg ∗= ω F RH ∗ r F RH ∗∗∗ (1) 

where 

CFM = airflow rate (cubic feet of air per minute)

ω = humidity ratio (pounds of moisture per pound of dry air) 

ρ = density of air (pound of dry air per cubic foot of air) 
3 

70F&30%RH = 70°F at 30% relative humidity 

Assuming 140 CFM, an air density of 0.0741 lb of dry air per cubic foot, and 0.0046 lb of 

moisture per pound of dry air, Equation 1 becomes 

yr

lb

25,000 

yr

day 365

day

hr 24

hr

min 60

ft

lb

0741.0

lb

lb

0046.0

min

ft 140 

yr

lb

 vaporOH

3

dry air

dry air

OH 3 vaporOH

2

2 2

=

∗= ∗ ∗∗∗

The result of the back-of-the-envelope calculation indicates that approximately 25,000 lb of 

water vapor flow through attic airflow paths during a year. This also assumes that the air 

pressure in the attic is greater than the outdoor air pressure all year, which is not actually the 

case, but is a steady-state assumption. 

Next, the research team determined the number of hours that the temperature of the outdoor air 

was at or below the dew point of the attic air; for Minneapolis, there are 3,500 h when the 

outdoor temperature is below the dew point of air at 

70°F and 30% RH (approximately 40% of the year). 

Multiplying the mass flow rate (approximated with 

Equation 1) by the percentage results in approximately 

9,900 lb of water vapor flowing through attic cracks 

when condensation conditions are present. In reality, only a minor amount of the air that flows 

through attic cracks will come into contact with the building materials due to air films at the 

crack edges. Even if 15% of the mass flow rate is used, approximately 1,500 lb of water vapor 

potentially could condense in the cracks. This simple calculation indicates that an environment 

exists for liquid water deposition or very high moisture content in moisture-sensitive attic 

building materials.

Although many homes are being built or retrofitted with unvented attics, the field performance of 

ccSPF installed as an air barrier in unvented attics is not well documented. This study is a first 

attempt at developing an understanding of airflow paths in ccSPF-insulated unvented attics and 

investigates the moisture risk associated with these airflow paths. 

The following subsections provide both a conceptual background and a summary of previously 

completed work related to this study. 

2.1 Vented and Unvented Attics

A vented (conventional) attic has insulation placed on the attic floor and has intentional air 

venting to the exterior. An unvented (cathedralized) attic has insulation placed at the roof deck, 

with no intentional venting to the exterior. Also, in most houses that are retrofitted from vented 

to unvented attics, the pre-retrofitted attic insulation is allowed to remain on the attic floor. 

As Figure 1 illustrates, a vented attic allows air to move through the attic space. One 

disadvantage of a vented attic space that also houses space conditioning equipment and ducts is 

that the temperature of the attic can become hot during summer and correspondingly cold in 

winter. If an air handler unit and/or ducts are located within that vented attic, the hot or cold attic 

air will increase the space conditioning load on the house. 

Dew point: The temperature at 

which water vapor in air 

condenses into liquid water.
4 

Figure 1. Airflow paths for (left) a vented (conventional) attic and

(right) an unvented (cathedralized) attic

2.2 Energy Savings of Unvented Attics

Unvented attics are considered semi-conditioned spaces. Studies have shown that, although an 

unvented attic is not designed to distribute conditioned air into the attic space, a natural air 

exchange occurs between the living space and the unvented attic. For example, Rudd (2005) 

compared the envelope pressurization test results of 10 newly built houses with unvented attics 

in Banning, California. Envelope pressure tests were completed with the attic hatch open and 

closed. The results indicate that the CFM50 airflow was reduced an average of 15% when the 

attic hatch was closed. This indicates that the living space and the unvented attic are 

communicating. The study also monitored the temperatures of the attic and the living space. The 

monitoring results indicated that there was little temperature difference between the attic and the 

living space. 

Because the temperature of an unvented attic approaches the temperature of the living space, air 

handling equipment and ducts placed in an unvented attic are conceptually located in conditioned 

space. A 9% cooling energy savings was measured by Parker et al. (2001) in a side-by-side roof 

research test in Florida. Rudd et al. (2000) completed side-by-side research of vented and 

unvented attics in homes in Las Vegas, Nevada, and Tucson, Arizona, and they found a 5% 

cooling energy savings. These savings are influenced by the leakiness of the ducts; the leakier 

the ducts, the greater the potential energy savings because the conditioned air leakage is lost into 

the vented attic as opposed to the unvented attic. Research indicates that unvented attics offer 

energy savings in both heating and cooling climates. 

2.3 Attic Moisture-Related Issues 

Moisture in an attic space related to attic ventilation was first studied by Rowley et al. (1939). 

Several studies of vented attics indicated that in heating climates, attic ventilation reduced the 

risk of condensation, whereas ventilation contributed to the risk of condensation in southern 

humid climates (Rudd and Lstiburek 1998; Lstiburek 1993; TenWolde and Rose 1999). 

Studies of moisture damage in unvented attics have been completed for dense pack cellulose by 

Fitzgerald (2010) and indicate that 20% of incorrectly installed (i.e., no air rafter ventilation built 

into the assembly to dry materials) dense pack cathedralized attics failed within the first 10 years 

of installation. Schumacher and Lepage (2012) authored Guideline for Moisture Control in 
5 

Dense-Packed Assemblies, which provides guidance for the use of dense pack cellulose in 

cathedralized unvented attics. Four strategies are recommended in the guideline, including the 

use of ccSPF on the underside of the roof deck with cellulose below it. The report indicates that 

unvented attics that have correctly installed ccSPF offer the following benefits for both heating 

and cooling climates: 

• In heating climates, the ccSPF insulation creates a thermal barrier that prevents the living 

space air, which is warm and moist, from contacting building materials that are at a 

temperature equal to or lower than the dew point of the living space air. 

• In cooling climates, the ccSPF insulation creates an air barrier that prevents outdoor air, 

which is warm and humid, from entering the attic space. 

Work by Straube et al. (2010) used Wärme und Feuchte instationär (WUFI) Pro Version 5.01

software to analyze various combinations of air-impermeable insulation (spray foam) and airpermeable insulations (fiberglass and cellulose), and they drew conclusions about appropriate Rvalues and vapor control strategies. In particular, Straube et al. (2010, p. 8) state:

For the roofs insulated with partial-depth spray foam fills, air leakage through the 

insulation is effectively stopped, as the foam seals all the joints and cracks in the 

roof deck. It cannot seal the airflow through wood-to-wood joints: these require 

alternate air sealing treatment and are not considered by the analysis in this paper.

Grin et al. (2013) studied the unvented attic moisture risk resulting from bulk water transport due 

to precipitation through damaged roofs. They concluded that an unvented attic can adequately 

withstand a water leakage event from rain without damage to the building materials. 

2.4 Airflow-Induced Forces

The two methods that create a pressure differential that results in airflow from the building 

interior to the exterior through attic cracks are (1) stack effect and (2) wind-induced pressure 

differentials (Walker 1989). In this study, the research team used CONTAM2

 Version 3.1 

software to simulate the dynamic nature of stack effect and wind-induced pressure effect in an 

unvented attic. CONTAM has been used in numerous studies because of its ability to model the 

multizone relationship of airflow in houses (Emmerich et al. 2003; Emmerich 2001; Nirvan et al. 

2012). Although studies using CONTAM often are related to contaminant transport, the physics 

of air movement is the same. 

 

1 Fraunhofer Institute for Building Physics, Stuttgart, Germany. www.wufi.de/index_e.html. 2 www.nist.gov/el/building_environment/contam_software.cfm. 
6 

3 Research Questions

This project sought to answer the following research questions for existing houses in cold 

climates: 

1. Does air leakage between an attic and the outdoors occur after the application of ccSPF? 

2. For Question 1, what is the magnitude of air leakage rates? 

3. If there is air leakage in ccSPF attic assemblies, what is the potential moisture 

accumulation on the roof sheathing at possible air leakage pathways?
7 

4 Technical Approach

To investigate the moisture impact of air leakage through attic airflow paths found in ccSPFinsulated unvented attics, the research team gathered field data to understand leakage rates and 

used numerical models to simulate physical phenomena. Figure 2 illustrates the project 

workflow, showing that the study workflow combined field measurements with multiple 

numerical models to assess moisture-related risk in ccSPF-insulated unvented attics. 

Figure 2. The study workflow3

The following sections detail the technical approach used in this project, beginning with the 

documentation of the scope of the study. 

4.1 Scope of Study

This study is intended to be a first-pass analysis of the moisture potential in a ccSPF-insulated 

attic due to airflow paths through the unvented attic of an existing home. The study relies on 

numerical models and field data to gain a better understanding of how moisture moves through 

attic airflow paths and the potential risk of moisture accumulation in building materials. Future 

work in unvented roof assemblies should be targeted at collecting field data to more accurately 

document gross attic air leakage, commonly found individual airflow paths, and moisture content 

of building materials at known airflow paths. 

The first step of the project was to identify airflow paths. To accomplish this, the project team 

reviewed photos of air leakage paths and made 3D models of the airflow paths to be used in the 

CFD analysis. The next step was to use the CFD analysis to characterize the pressure/airflow 

relationship and to enter that into CONTAM. The CONTAM model was trued up using field data 

of attic leakage rates. Then the CONTAM data were merged with BEopt-produced indoor 

temperature and relative humidity data to determine the mass flow rate of water vapor through 

the individual airflow paths. Finally, the team used WUFI to determine the moisture content of 

the building materials using the water vapor flow rate through the crack and the Typical 

Meteorological Year 3 (TMY3) weather file for Minneapolis, Minnesota. 

 

3 3D is three-dimensional. CFD is computational fluid dynamics. BEoptTM is Building Energy Optimization 

software, Version 2.1.0.2, http://beopt.nrel.gov/. 
8 

4.2 Field Data

To simulate the leakage rate of ccSPF insulation on the roof decks of unvented attics, the 

research team needed first to characterize the airflow paths and then to quantify the gross 

unvented attic air leakage rates. The following two sections describe the approach the team used 

to gather and analyze the field data.

4.2.1 Attic Airflow Path (Crack) Types

The researchers reviewed in-house photographs collected from past building quality inspections 

done in-house and by others. After reviewing the photographs, the team determined that several 

types of attic airflow paths might be found in a ccSPF-insulated unvented attic. The categories 

were determined to be as follows: (1) plumbing penetrations, (2) spray foam delamination, 

(3) framing intersections, and (4) ridge vent sealing. Another category that exists but was not 

used in this report is poor sealing of soffit vents in existing houses. These vents are sealed during 

the application of the ccSPF when a house is retrofitted from a vented to unvented attic. These 

types of airflow paths have a large amount of variation, and further work is needed to develop a 

characterization of these airflow paths. 

Once the airflow paths were identified, the research team used Rhinoceros4 Version 5 software 

to create 3D solid models of the cracks. 

4.2.2 Unvented Attic Air Leakage Rates

Air leakage rates for unvented attics are not well documented; thus, the research team completed 

three field tests to gain a better understanding of gross leakage rates of unvented attics. The 

method used was a multipoint guarded blower door test. A guarded blower door test uses two 

blower door assemblies. One blower door pressurizes the house, and a second blower door is 

located in the attic access opening, where it is brought to the same pressure as the house to 

isolate the house leakage from the attic leakage. Once the multipoint leakage rates were 

recorded, the team used regression analysis via ASTM E779 methods (ASTM 2010) to 

determine a mathematical relationship between the gross airflow out of the attic and the pressure 

differential between the interior and exterior.

4.3 Modeling Effort

As illustrated in Figure 2, the modeling effort required the use of four different modeling 

programs: ANSYS CFD Version 15.05

, CONTAM Version 3.1, BEopt Version 2.1.0.2, and 

WUFI Pro Version 5.0. First, 3D models of the cracks, as discussed in Section 4.2.1, were input 

into ANSYS. ANSYS then was driven at steady state with approximately five different mass 

flow rates, and the pressure differentials between the interior and exterior of the crack were 

recorded. The research team used Equation 2 to develop the mathematical description of each 

airflow path (ASHRAE 2005, Chapter 27.12, Equation 32). 

 

4 www.rhino3d.com/. 5 ANSYS, Inc., Canonsburg, PA: www.ansys.com/Products/Simulation+Technology/Fluid+Dynamics. 
9 

, n ∆= PCq (2) 

where 

q = airflow rate (CFM)

C = leakage flow coefficient (CFM/(Pa)n) 

ΔP = pressure difference across the leak (Pa)

n = flow exponent (dimensionless) 

C and n were determined using ASTM E779 methods (ASTM 2010). The power law equation 

(Equation 2) is a continuous representation of the airflow rate as a function of pressure across a 

crack. 

Once the flow rate for each crack type was characterized mathematically, the flow parameters 

(C and n) for each crack type were entered into the CONTAM model to determine the hourly 

flow rate through each of the attic leakage paths. The CONTAM model was driven with TMY3 

weather data. The building enclosure leakage, including the attic leakage, of the CONTAM 

model was 5 ACH50, where the air change per hour does not include the attic volume. 

Next, a BEopt Version 2.1.0.2 model, which was equivalent to the CONTAM model 

geometrically and in enclosure tightness, was used to simulate the hourly indoor temperature and 

humidity ratio. Note that the BEopt model does not account for leakage from the house into the 

attic space. The hourly CONTAM airflow rates were matched with the hourly temperature and 

humidity ratio from the BEopt model to determine the hourly mass flow rate of water vapor 

through the attic cracks. 

The hourly mass flow rate of water vapor through the plumbing airflow path (the largest crack) 

was input into WUFI (numerical hygrothermal analysis software) to determine the moisture 

absorption and desorption in the attic materials. Hygrothermal analysis is used to study the 

durability of building enclosure components. Generally, hygrothermal analysis quantifies the 

wetting and drying of building materials based on moisture available to the material at the 

current time step as a function of the moisture content of the material at the previous time step, 

plus the addition or removal of moisture due to the conditions of the materials and air 

surrounding the crack. The strength of hygrothermal analysis is the ability to look at the wetting 

and drying cycles of building materials to generalize the moisture content of building materials 

based on indoor, outdoor, and initial conditions. The disadvantage of hygrothermal analysis is 

that it is highly subject to the assumptions and parameters input into the model and, when used to 

investigate a specific condition, can lead to wrong answers. It is a useful tool to develop an 

understanding of physical phenomena, but the results should be considered in light of the 

capabilities of the model.
10 

5 Results and Discussion

The following sections describe the analysis and include a short discussion of the results. 

5.1 Field Data

The field data recorded for this project are presented in the following four sections. 

5.1.1 Attic Airflow Path (Crack) Types

IBACOS has extensive experience with quality assurance and construction defects in residential 

housing and, for this project, produced a portfolio of potential airflow path photos. Figure 3

through Figure 6 show some of these possible pathways.

Figure 3. Plumbing vent penetrations

Figure 4. Delaminating ccSPF airflow path
11 

Figure 5. Ridge vent opening

Figure 6. Surface area openings at the roof sheathing

The research team then used the photos to make 3D models of the airflow paths. The arrows in 

Figure 7 through Figure 9 illustrate the direction of airflow out of the attic space. The assumption 

at both the surface and ridge vent cracks was that the ccSPF would delaminate from the wood 

framing (as illustrated in Figure 4) and that the preexisting opening was not completely sealed 

with foam. Delamination of ccSPF can occur due to improper ccSPF installation practices. 
12 

Figure 7. Plumbing, bath fan venting, or flue vent airflow path

Figure 8. Surface area airflow path at the face of a roof rafter
13 

Figure 9. Ridge vent airflow path with delamination at the face of a roof rafter and ridge

5.1.2 Discussion of Attic Airflow Path (Crack) Types

Figure 3 through Figure 9 illustrate that some of the building materials (e.g., shingles) are not 

present in the model. The research team chose to complete the analysis without some of the 

building materials because the restriction of a properly installed roof system would have very 

little air leakage, yet the field measurements presented in this report indicate otherwise. This 

assumption provides a starting point to help understand the potential airflow from a family of 

defined airflow paths. A more accurate method would be to quantify the actual air leakage at 

identified leakage pathways in a statistically significant number of houses in multiple climate 

zones, which is beyond the scope of this project. 

5.1.3 Unvented Attic Air Leakage Rates

The research team identified two homeowners who were retrofitting their homes from vented to 

unvented attics with ccSPF insulation during the time span of this study. The envelope and attic 

leakage for each of these two homes (House 1 and House 2) were recorded prior to each retrofit, 

and once the ccSPF was installed, both houses were tested again. An additional homeowner was 

found who had already upgraded their attic from a vented to unvented attic (House 3), and the 

leakage in that home was recorded. The research team analyzed the test results using TecTite 4.0 

software6 to determine airflow rates as a function of pressure. 

Table 1 presents the results of the analysis. Figure 10 represents the graphical form of the values 

presented in Table 1. 

 

6 The Energy Conservatory, www.energyconservatory.com/blower-door-test-software. 
14 

Table 1. Data Collected for Three Attics*

Leakage 

Metric

House 1 – Attic House 2 – Attic House 3 – Attic

PreRetrofitPostRetrofitPreRetrofitPostRetrofitPreRetrofitPostRetrofit

C (CFM/(Pa)n) 196 ±

6.0%

91 ±

5.6%

186 ±

14.7%

53 ±

5.7%

Not 

available

126 ±

33.9%

n 

(Dimensionless)

0.632 ±

0.016

0.493 ±

0.015

0.690 ±

0.045

0.660 ±

0.015

0.650 ±

0.097

Leakage

@ 4 Pa

471 ±

39 CFM

180 ±

14 CFM

484 ±

107 CFM

132 ±

10 CFM

310 ±

165 CFM

R-Squared 0.998 0.997 0.996 0.995 0.999

*At House 1 and House 2, the attic airflow was recorded before and after the ccSPF-insulated unvented attic was 

installed. C represents the flow coefficient (CFM/(Pa)n

), and n (dimensionless) represents the flow exponent. 

Figure 10. Pre-retrofit (vented) and post-retrofit (ccSPF-insulated unvented)

attic airflow and pressure curves

5.1.4 Discussion of Field Data

The results shown in Table 1 are interesting because the attic leakage was reduced approximately 

65% for House 1 and House 2. The ccSPF was installed by a competent installation team. 

Although there are only two data points, the results indicate that the retrofit unvented attics are 

still reasonably leaky. The calculated total attic leakage at 4 Pa in Table 1 was used to assist in 

the development of CFD and CONTAM models as discussed in Sections 5.2 and 5.3.

0

500

1,000

1,500

2,000

2,500

3,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Airflow (CFM)

Pressure Differential Between Attic and Exterior (Pa)

House 1-pre House 1-post House 2-pre

House 2-post House 3-post
15 

5.2 Computational Fluid Dynamics—Airflow Rates for Airflow Paths

The research team used the ANSYS CFX7 computer code to determine the airflow rate for dry 

air at standard temperature at various pressure differentials across the airflow paths based on 

pressure. The team also completed a mesh study to ensure that the results were not biased due to 

the mesh network of the airflow path. The results of the mesh study indicated that the results 

were not biased by the mesh network. Using regression analysis, the team determined the flow 

coefficients and the flow exponent for the different types of airflow paths. Table 2 presents the 

airflow path regression analysis. 

Table 2. Airflow Characteristics Relating Airflow Rate to Pressure

Airflow Path

Type

CFM

@ 4 Pa

C

(CFM/(Pa)n)

n

(Dimensionless)

Plumbing 6.3800 3.1900 0.5050

Surface 0.0405 0.0196 0.5228

Gable 2.0E-5 0.00001 0.5014

Ridge 1.4300 0.694 0.5230

C represents the flow coefficient; n represents the flow exponent.

5.2.1 Discussion of the Computational Fluid Dynamics Analysis

The results of the CFD analysis indicate that very low airflow rates will be achieved with surface 

and gable end airflow paths. As mentioned earlier, the 3D models were created from photos 

rather than from physical measurements of actual airflow paths. Physical measurements of 

airflow rates of individual crack types would be more accurate and would avoid these limitations 

but were beyond the scope of this study. The project team chose this assumption because the 

variability of moisture moving through the airflow path would be determined using the BEopt 

model. 

5.3 CONTAM—Airflow Rates for Unvented Attics

The CONTAM model was created with the airflow path characteristics determined by the CFD 

analysis. Several iterations were necessary to adjust the CONTAM model to have similar gross 

attic airflow rates as found with the field measurements. The research team reused the ridge vent 

airflow path as a soffit airflow path to achieve total attic airflows that were similar to the field 

measurements. Table 3 indicates the number of airflow paths used in the CONTAM model. 

Table 3. Airflow Path Type, Number, and Airflow at 4 Pa for Each Airflow Path and the Total

Airflow Path Type Number of Airflow Paths Used 

in the CONTAM Model

Plumbing 2

Surface 120

Gable 16

Attic Ridge 57

Eave 57

Total for Airflow Path Family 252

 

7 ANSYS, Inc. Canonsburg, PA: 

www.ansys.com/Products/Simulation+Technology/Fluid+Dynamics/Fluid+Dynamics+Products/ANSYS+CFX. 
16 

The number of cracks indicated in Table 3 represents a surface crack at every roof sheathing 

intersection and rafter interface. Attic ridge and eave cracks were added at every rafter. Two 

plumbing cracks were included, and 16 gable cracks were included. Figure 11 illustrates the 

general crack locations for a few of the airflow paths. 

Figure 11. Location of attic airflow paths on the CONTAM model 

Once the cracks were added to the CONTAM model, the research team evaluated the airflow rate 

and pressure relationship curve by driving CONTAM with a range of interior pressures inside the 

house to simulate a multipoint blower door test without wind. Figure 12 illustrates the resulting 

airflow as a function of pressure curve for the family of airflow paths added to the CONTAM 

model. The flow coefficient and exponent are 90.3 CFM/Pan

 and 0.5214 dimensionless. 

Figure 12. Airflow pressure relationship (between attic interior and exterior) 

for the family of airflow paths input into CONTAM 
17 

Once IBACOS was comfortable with the air leakage rates of the family of airflow paths input 

into the CONTAM model, the team simulated the air leakage of the attic using a TMY3 weather 

file for Minneapolis, Minnesota, and a constant indoor temperature of 70°F and no airborne 

water vapor to produce hourly air leakage rates. Figure 13 presents the airflow rates through the 

family of airflow paths simulated with the CONTAM model and their frequency of occurrence. 

Figure 13. Histogram of CONTAM simulated hourly exfiltration and infiltration 

airflow rates through a ccSPF-insulated unvented attic in Minneapolis, Minnesota. 

Negative values indicate outdoor air flowing into the attic space. 

5.3.1 Discussion of CONTAM 

The histogram shown in Figure 13 indicates that the cumulative airflow entering the attic (the 

sum of all negative airflows) occurs less than 2% of the year. This is due to the dominant stack 

effect found in a two-story house in a cold climate that essentially produces a constant airflow 

out of the attic. The average airflow rate is 185 CFM, which is approximately equal to the family 

of the airflow paths when experiencing 4 Pa pressure differential. The upper tail of the 

histogram, approximately the 95th percentile, has an airflow rate of 310 CFM; using the airflow 

pressure relationship presented in Figure 12, the differential pressure is approximately 11 Pa. 

Based on this analysis, the CONTAM model produces results that are comparable to the field 

data. 

5.4 Mass Flow Rate of Water Vapor 

The research team then combined the hourly CONTAM airflow rates with the indoor 

temperature and humidity ratio from a BEopt model. The BEopt model was geometrically 

identical to the CONTAM model and used Building America House Simulation Protocols 

(Hendron and Engebrecht 2010) for occupancy, latent loads, and temperature set points (68°F 

heating and 71°F cooling). Each airflow rate was converted to a mass flow rate of water vapor 

using an hourly version of Equation 1. The mass flow rate of water vapor passing through the 

attic airflow paths was calculated to be approximately 36,000 lb of water vapor per year. 

0%

2%

4%

6%

8%

10%

12%

-120

-90-70-50-30-10

20406080

110130150170200220240260290310330350380400420440470490510530

Frequency of Occurrence

Airflow Between Attic and Outdoors (CFM)
18 

As an exercise to better understand the condensation events that might occur, the team calculated 

the dew point temperature of the air flowing out of the attic space for each hour. Next, the team 

compared the outdoor temperature to the dew point temperature of the air flowing through the 

crack; the results are presented in Figure 14. This figure illustrates the airflow rate for each hour 

using round black dots and gray vertical bars to indicate that a condensation event is possible 

(i.e., when the outdoor temperature is equal to or lower than the dew point temperature of the 

indoor air). 

Figure 14. CONTAM attic airflow rates determined for Minneapolis, Minnesota.

The shaded regions indicate instances when the outdoor temperature

was lower than the dew point of the airflow moving through the attic cracks. 

Although one of the necessary environmental conditions is present for a condensation event (i.e., 

the dew point temperature is reached), condensation may not occur due to solar radiation and/or 

heat transfer of the warm air moving through the crack to the attic materials and/or condensation 

warming the building materials. WUFI results that include solar and night sky radiation are 

presented later in this report. 

To illustrate the impact that latent heat transfer can have with the building materials, the project 

team completed the following example. The estimate is included here solely for the purpose of 

example. Assume that an airflow of 300 CFM of air enters an airflow path at 70°F and 50% 

relative humidity and leaves the airflow path at 30°F and 100% relative humidity. The mass of 

condensed water would be approximately 6 lb in 1 h. The latent heat of condensation is 

approximately 970 Btu/lb water, and this results in approximately 5,800 Btu/h of heat added to 

the airflow path due to condensation. Again, WUFI results presented later in this report include 

the heat loss and addition due to evaporation, condensation, night sky radiation, and solar 

radiation.

The gross flow rate of water vapor through the attic cracks shown in Figure 14 was then screened 

to remove the occurrences when condensation events were not possible (the outdoor temperature 

was greater than the dew point temperature). The results were summed, resulting in 

-150

-100

-50

0

50

100

150

200

250

300

350

400

450

500

550

600

1/12/13/14/15/16/17/18/19/1

10/111/112/1

Time

Infiltration and Exfiltration Attic 

Airflow (CFM)

Outdoor Air Temperature Below Indoor Air Dew Point Attic Airflow (CFM)
19 

approximately 9,230 lb of water vapor flowing through the attic airflow paths when condensation 

events were possible. For this spreadsheet analysis, there were 2,364 h when a condensation 

event was possible, approximately 27% of the year. 

The project team analyzed the data presented in Figure 14 to determine the number of continuous 

hours that a condensation event might occur. Figure 15 presents the results of that analysis. 

Figure 15. Histogram of the number of continuous hours that airflow is

moving through attic airflow paths and condensation could occur 

5.4.1 Discussion of Mass Flow Rate of Water Vapor

The spreadsheet analysis presented above indicates that condensation events are possible. 

Figure 15 indicates that 16% of the time, a continuous moisture event potentially occurred 

between 4 and 8 h. The result of this analysis is that the potential exists for a condensation event 

to occur and that the duration is long enough that there could be continuous wetting. This 

analysis does not consider the heat transfer that occurs as air flows through the airflow path, 

potentially warming the surface, or the condensation warming. However, it does provide insight 

into the environmental conditions needed for a moisture event to occur in an attic airflow path. 

5.5 Hygrothermal Analysis—Moisture Content

The research team used WUFI modeling to calculate the moisture content of the building 

materials using the surface area of the cracks, the water vapor flowing through the cracks, and 

the TMY3 outdoor weather file. The condition of the air flowing through the airflow path was 

determined, as stated above, by merging the CONTAM mass flow rate with the BEopt-simulated 

indoor air conditions. 

WUFI two-dimensional (2D) modeling was used to simulate the moisture content of the building 

material surrounding the airflow path. The airflow path selected for analysis was the plumbing 

vent. The plumbing vent was presented above and included a small airflow path along the round 

plumbing vent that allowed air to flow over the building materials (the foam and roof sheathing). 

Figure 16 is an image of the 2D model and indicates the materials used in the simulation. 

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

22%

24%

48

12162024283236404448525660646872768084889296

100104108112116

Frequency of Occurrence 

Continuous Condensation Event (h)
20 

Figure 16. Schematic view of the WUFI 2D model for the plumbing airflow path8 

Figure 17 is an image of the model that was entered into WUFI 2D. 

Figure 17. WUFI representation of the airflow path

The project team used WUFI 2D to determine the moisture content of the building materials

surrounding a crack for four airflow rates. Using the model, the research team could adjust the 

airflow rates to paint a more useful picture of the building material moisture absorption and 

desorption than running one instance of the model. Additionally, one-dimensional WUFI 

modeling was used but produced unrealistic results because the drying potential of the airflow 

was not included. 

The hygrothermal analysis was completed by the following process: (1) holding the airflow rate 

through the plumbing airflow path constant at 200%, 40%, 4%, and 0% (no airflow) of the 

CONTAM-predicted airflow at 4 Pa; (2) merging the airflow conditions with the indoor air 

conditions from BEopt; and (3) using TMY3 outdoor conditions for temperature, relative 

humidity, and solar and night-sky radiation.

 

8

 OSB is oriented strand board.
21 

Figure 18 presents the hygrothermal results.

Figure 18. Hygrothermal results indicating the OSB moisture content 

at various distances from the edge of the airflow path
22 

A spatial/temporal representation of the moisture content in the OSB sheathing was made and is 

provided in Figure 19 and Figure 20. In these figures, the red shaded areas show moisture 

content in the OSB above 20%; the axis moving toward the reader from the page indicates the 

time of year, and the horizontal axis indicates the distance away from the edge of the air leakage 

pathway. 

Figure 19. Hygrothermal results for plumbing airflow path with 0 and 0.25 CFM airflow

Figure 20. Hygrothermal results for plumbing airflow path with 2.5 and 12.5 CFM airflow
23 

5.5.1 Discussion of Hygrothermal Analysis

The 2D WUFI results indicate that drying occurs at higher airflow rates and that high moisture 

content in the sheathing is localized at the crack location. Based on the 2D analysis of a very low 

airflow rate, the area surrounding the plumbing penetration would have enough moisture to cause 

damage at a distance of 1.5 in. into the OSB sheathing. At higher airflow rates, the moisture risk 

lessens due to the drying nature of the heat energy in air moving through the crack. The results 

indicate that small-magnitude airflow paths may have higher risk than large-magnitude airflow 

paths. The results for the scenario with no airflow indicate that moisture risk occurs, although 

this may be unrealistic because WUFI is seeing the air moisture of a dynamic airflow although 

the condition is static; the air in the crack will be stagnant and will not have the same properties 

as the dynamic airflows calculated with CONTAM and BEopt.
24 

6 Conclusions

Two recent trends in the residential housing industry are to install unvented attics in new homes 

and to retrofit existing homes from vented to unvented attics. The purpose of this study was to 

investigate the moisture risk due to airflow paths out of unvented attics that use ccSPF at the roof 

deck. The results of the study and answers to the research questions are detailed below.

1. Does air leakage between an attic and the outdoors occur after the application of ccSPF? 

The results of this study indicate that there is air leakage from an unvented attic to the 

outdoors. The data were collected at three houses that were being retrofitted from vented 

to unvented attics. 

2. For Question 1, what is the magnitude of air leakage rates? 

The average leakage of the three houses retrofitted from vented to unvented attics was 

207 CFM at 4 Pa of pressure. 

3. If there is air leakage in ccSPF attic assemblies, what is the potential moisture 

accumulation on the roof sheathing at possible air leakage pathways? 

There is potential moisture accumulation in building materials surrounding air leakage 

paths that have low airflow. The results from the hygrothermal analysis indicate that 

moisture accumulation is localized to an area 5 in. from the crack and dries before the 

next winter. 

The modeling performed by the research team indicates that for a typical meteorological 

climate year in Minneapolis, Minnesota, airflow paths with low airflow rates will result 

in sheathing moisture contents above 20% for most of the winter and spring. This could 

result in localized damage of building materials. Higher airflow rates do not have as 

much moisture accumulation during winter due to the drying potential of the airflow 

across the crack actually warming the sheathing and eliminating it as a condensing 

surface. Because ccSPF reduces the airflow from an attic, converting a vented attic to an 

unvented attic may result in reduced airflow through leakage pathways and could result 

in localized damage. 

6.1 Future Research

Results in this report are based on many simplifying assumptions, and future work should be 

undertaken to improve on this analysis and to expand the industry’s knowledge of actual air 

leakage characteristics in unvented roof assemblies. Future analyses could include the following: 

• Collection of more field data to identify gross attic leakage rates in new and retrofitted

homes

• Collection of field data to understand the types of leaks (airflow paths) that are present

• Laboratory measurements to characterize the flow and pressure relationship at attic 

leakage pathways
25 

• Laboratory measurements to provide an understanding of the transport and accumulation 

of moisture in ccSPF roof systems where steady-state or hourly predicted airflow with 

water vapor is present and the outdoor conditions are simulated in several climate zones

• The use of accelerated weathering to determine how moisture is transported in ccSPF 

assemblies when there is a constant airflow and humidity ratio. 

Finally, the authors would like to stress that the results presented in this report are based on 

numerical models that should be used only to guide decision making. These results indicate that 

moisture risk could happen in unvented attics, given very small leakage paths. To better 

understand the moisture risk in unvented attics, decision makers should collect actual field data. 
26 

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DOE/GO-102014-4526 ▪ November 2014 

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