Interaction of Unvented 

Attics With Living Space in 

Three Northeast Homes

Srikanth Puttagunta and Omari Faakye

Consortium for Advanced Residential Buildings

February 2015
ii

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iii

Interaction of Unvented Attics With Living Space in

Three Northeast Homes

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: 

Srikanth Puttagunta and Omari Faakye

Steven Winter Associates, Inc.

of the 

Consortium for Advanced Residential Buildings (CARB)

61 Washington Street

Norwalk, CT 06854

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40342-05

February 2015
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

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

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

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

1.2 Modeling Interior and Attic Temperature............................................................................1

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

2.1 Research Questions..............................................................................................................4

2.2 Methodology........................................................................................................................4

2.2.1 Sites..........................................................................................................................4

2.2.2 Sensor Placements ...................................................................................................6

2.2.3 Characterization of Test Sites - Building Leakage Testing ...................................10

2.2.4 Characterization of Test Sites - Duct Leakage Testing .........................................13

2.2.5 Characterization of Test Sites – Solar Irradiance ..................................................13

3 Results................................................................................................................................................. 15

3.1 Performance Testing ..........................................................................................................15

3.1.1 House 1 ..................................................................................................................15

3.1.2 House 2 ..................................................................................................................15

3.1.3 House 3 ..................................................................................................................16

3.2 Field Monitoring ................................................................................................................17

3.2.1 House 1 ..................................................................................................................17

3.2.2 House 2 ..................................................................................................................20

3.2.3 House 3 ..................................................................................................................22

4 Discussion........................................................................................................................................... 26

5 Conclusion .......................................................................................................................................... 28

References................................................................................................................................................. 30
vi 

List of Figures

Figure 1. Average indoor and attic temperature for vented and unvented attics ................................ 2

Figure 2. Average indoor and attic humidity ratio for vented and unvented attics ............................. 3

Figure 3. Location of test sites .................................................................................................................. 5

Figure 4. Sketch of Unvented Attic 1 ........................................................................................................ 6

Figure 5. Sketch of Unvented Attic 2 ........................................................................................................ 6

Figure 6. Temperature differences between sensor at CM and sensors at other positions in 

Unvented Attic 1 ................................................................................................................................... 7

Figure 7. Humidity ratio differences between sensor at CM and sensors at other positions in 

Unvented Attic 1 ................................................................................................................................... 8

Figure 8. Temperature differences between sensor at CM and sensors at other positions in 

Unvented Attic 2 ................................................................................................................................... 9

Figure 9. Humidity ratio differences between sensor at CM and sensors at other positions in 

Unvented Attic 2 ................................................................................................................................... 9

Figure 10. Humidity ratio at the CM and LT sensors, in the living space, and outdoors at House 2 

for a typical summer week................................................................................................................. 10

Figure 11. Depressurization test with unvented attic hatch open. ...................................................... 11

Figure 12. Depressurization test with unvented attic hatch closed. ................................................... 11

Figure 13. Depressurization test with unvented attic hatch carefully sealed..................................... 12

Figure 14. Depressurization test of house and attic. The attic depressurization flow rate minus the 

difference between blower door cfm50 with the attic hatch open and closed is the leakage of 

just the unvented attic. ...................................................................................................................... 12

Figure 15. Global horizontal irradiance .................................................................................................. 13

Figure 16. Summer impact of solar irradiance on Unvented Attic 1.................................................... 14

Figure 17. Temperature conditions at House 1...................................................................................... 18

Figure 18. House 1 temperature differential with respect to outdoor temperature............................ 18

Figure 19. Humidity conditions at House 1 ............................................................................................ 19

Figure 20. House 1 humidity ratio differential with respect to outdoor humidity ratio ..................... 19

Figure 21. Temperature conditions at House 2...................................................................................... 20

Figure 22. House 2 temperature differential with respect to outdoor temperature............................ 21

Figure 23. Humidity conditions at House 2 ............................................................................................ 21

Figure 24. House 2 humidity ratio differential with respect to outdoor humidity ratio ..................... 22

Figure 25. Sketch of House 3 attic. ......................................................................................................... 22

Figure 26. Temperature conditions at House 3...................................................................................... 23

Figure 27. House 3 temperature differential with respect to outdoor temperature............................ 24

Figure 28. Humidity conditions at House 3 ............................................................................................ 24

Figure 29. House 3 humidity ratio differential with respect to outdoor humidity ratio ..................... 25

Figure 30. Temperature trend lines for the three test homes and a BEopt model............................. 26

Figure 31. Humidity trend lines for the three test homes and a BEopt model ................................... 26

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

List of Tables 

Table 1. Temperature and Humidity Ratio Trends for Various House Types ...................................... ix

Table 2. Model Home Specifications......................................................................................................... 2

Table 3. Description of Each Test Site...................................................................................................... 5

Table 4. Summary of Unvented Attic Configurations.............................................................................. 5

Table 5. Average Temperature and Humidity Ratio Difference Between Centrally Placed Sensor 

and Other Sensors Placed All Around the Unvented Attic Space Over Winter and Spring ......... 7

Table 6. Whole-Building Depressurization Test Results at House 1 ................................................... 15

Table 7. Whole-Building Depressurization Test Results at House 2 ................................................... 16

Table 8. Duct Leakage Test Results at House 2 .................................................................................... 16

Table 9. Whole-Building Depressurization Test Results at House 3 ................................................... 17

Table 10. Temperature and Humidity Ratio Trends for Various House Types ................................... 27

Table 11. Temperature and Humidity Ratio Trends for Various House Types ................................... 28

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

Definitions

ACH50 Air Changes per Hour at 50 Pascal

BEoptTM Building Energy Optimization software

CARB Consortium for Advanced Residential Buildings

ccSPF Closed-cell spray polyurethane foam

cfm Cubic feet per minute

ocSPF Open-cell spray polyurethane foam

ORNL Oak Ridge National Laboratory

RH Relative humidity
ix

Executive Summary

Unvented roof assemblies are becoming common in North American construction. It is estimated 

that more than 100,000 have been constructed since 1995 (Schumacher 2007). According to 

Residential Energy Consumption Survey 2009 data, more than 30% of homes with attics (about 

10.4 million housing units) are located in the very cold/cold climate areas. The magnitude of 

these numbers for the potential of unvented attic retrofits emphasizes the significance of properly 

understanding how the attic conditions vary with respect to the conditioned space below. 

Oak Ridge National Laboratory (ORNL) is being funded by the U.S. Department of Energy to 

conduct hygrothermal envelope modeling to predict the sheathing moisture for various unvented 

attic conditions in different climate zones. This analysis will help to identify risks associated 

with installing spray foam under roof decks. Presently, only living space temperature and relative 

humidity conditions are used in the modeling tool; these are assumed to be the same for the 

living and attic spaces. An initial study by ORNL in the mixed-humid climate suggested this is 

not a correct assumption, so ORNL requested field measurements of attic conditions and various 

other parameters that may influence the attic conditions in three varying unvented attic 

configurations in the cold climate region. These data will be used to validate ORNL’s modeling 

methodologies and results. 

To support this research, the Consortium for Advanced Residential Buildings (CARB), a DOE 

Building America research team led by Steven Winter Associates, Inc., gathered cold climate 

field measurement data for model validation and in support of design guidelines for constructing 

efficient, durable unvented roof assemblies. Data analysis of three test homes suggests that duct 

leakage and building infiltration have the greatest impact on the difference between the unvented 

attic and the conditioned living space conditions. If the unvented attic received indirect space 

conditioning from supply-side duct leakage, the attic temperature conditions tracked closer to the 

living space. Trends in the humidity ratio didn’t seem to be significantly impacted by supply-side 

duct leakage. Also, the building leakage of the living space (all unvented attics were reasonably 

tight) resulted in variations in the humidity ratio trends; the leakier test home had much higher 

humidity ratios starting in the spring months. The other two tighter homes did not show signs of 

higher humidity ratios in the unvented attics, with respect to the living space, until the summer 

months. While this is only a small sample, based on this initial field monitoring, the equations in 

Table 1 are suggested for estimating attic conditions in the Northeast. 

Table 1. Temperature and Humidity Ratio Trends for Various House Types

House Type Unvented Attic Temperature

(°F)

Unvented Attic Humidity 

Ratio (lbw/lbda)

Energy-Efficient Home With Ducts in 

Unvented Attic (Supply-Side Leakage)

TL + 0.0009(TOA)

2 – 0.0508(TOA) 

– 0.1281 HRL + 14.606(HROA)

2

 – 

Energy-Efficient Home Without Ducts in 0.1424(HROA) + 0.00005

Unvented Attic

TL + 0.0007(TOA)

2 + 0.0751(TOA) 

– 7.5243

Existing Cape Style Home With Only an 

Unvented Attic Upgrade and No Ducts in 

Attic

TL + 0.0026(TOA)

2

 – 0.005(TOA) 

– 7.2737

HRL + 12.062(HROA)

2 + 

0.1192(HROA) – 0.0002

* variables: T = temperature, HR = humidity ratio

* subscripts: L = living space, OA = outdoor air
1

1 Introduction

Oak Ridge National Laboratory (ORNL) is being funded by the U.S. Department of Energy 

(DOE) to conduct hygrothermal envelope modeling to predict the sheathing moisture for various 

unvented attic conditions in different climate zones. This analysis will help to identify risks 

associated with installing spray foam under roof decks. To generalize this approach for all U.S.

climate zones, an enhanced model to predict attic conditions depending on climate zone; heating, 

ventilation, and air-conditioning (HVAC) system type; duct leakage; air and vapor tightness of 

the attic floor; etc. must be developed and validated. Presently, only living space temperature and 

relative humidity (RH) conditions are used in the modeling tools. The software assumes that 

conditions are the same for the attic space, but that assumption is likely not accurate. Thus, to 

verify or disprove this assumption, field measurements of attic conditions and various other

parameters that may influence the attic conditions are needed. 

The potential for condensation is of particular concern with unvented attic roof systems in 

International Energy Conservation Code climate regions 5A and 6A. As such, the Consortium 

for Advanced Residential Buildings (CARB), a DOE Building America research team led by 

Steven Winter Associates, Inc., focused its research efforts on the cold climate field 

measurement data needed by ORNL for model validation and in support of design guidelines for 

constructing efficient, durable unvented roof assemblies.

1.1 Relevance to Building America’s Goals

Unvented roof assemblies are becoming common in North American construction. It is estimated 

that more than 100,000 have been constructed since 1995 (Schumacher 2007). According to 

Residential Energy Consumption Survey 2009 data, more than 30% of homes with attics (about 

10.4 million housing units) are located in the very cold/cold climate areas. The magnitude of 

these numbers for the potential of unvented attic retrofits emphasizes the significance of properly 

modeling unvented attics. Understanding how the attic conditions vary throughout a heating 

season, and what they are relative to the conditioned space below, is important to verify 

modeling assumptions about attic conditions and improve the predictions of hygrothermal 

modeling studies. This ongoing research effort should have a significant impact in accurately 

modeling the potential energy savings of converting to an unvented attic for the millions of 

housing units that have vented attics in the very cold/cold climate areas.

1.2 Modeling Interior and Attic Temperature

To investigate the difference between vented and unvented attic conditions, the hourly 

temperature and RH estimates were plotted for a typical, moderately efficient 2,400-ft2 house 

(building specifications provided in Table 2). The model was done in BEoptTM (Building Energy 

Optimization) version 2.1.0.0, software that provides capabilities to evaluate residential building 

designs and identify cost-optimal efficiency packages at various levels of whole-house energy 

savings along the path to zero net energy.
2

Table 2. Model Home Specifications

Options Category Options Description

Walls R-13 blown cellulose, Gr-1, 2 × 4, 16 in o.c.

Wall Sheathing Oriented strand board, R-5 extruded polystyrene

Foundation R10 perimeter

Windows Double-pane, low-e, insulated frame, argon filled

U = 0.34, solar heat gain coefficient = 0.30

Roof/Ceiling R-49

Air Leakages 5.0 ACH50, 0.5 shelter coefficient

Ventilation Exhaust only, 100% ASHRAE 62.2

Appliances Benchmark*

Lighting Benchmark*

Cooling System Seasonal energy efficiency ratio 14

Heating System Gas furnace, 92.5% annual fuel utilization efficiency

Ducts 7.5% leakage, R-8

Water Heater 0.82 energy factor, tankless

* See Metzger et al. (2012) for details of the benchmark for each category.

Figure 1 shows the difference in the temperature estimates for each type of attic, as well as 

indoor temperature. The indoor temperatures in each house were very similar; therefore, the 

average was calculated. Figure 2 shows estimates of indoor and attic humidity ratios. Despite the 

difference between vented and unvented attic humidity ratio values, they follow a similar path. 

While the primary focus of this field monitoring is to provide data in support of ORNL’s 

modeling efforts on unvented attics, CARB provides a comparison of these temperature and 

humidity ratio estimates to trends in the measured data in Section 4. 

Figure 1. Average indoor and attic temperature for vented and unvented attics
3

Figure 2. Average indoor and attic humidity ratio for vented and unvented attics
4

2 Experiment 

2.1 Research Questions

The research proposed here does not directly answer research questions concerning condensation 

potential and the integrity of unvented attic roof systems. Rather, it provides data to support the 

validation of an ORNL modeling effort that will attempt to answer those questions. The key 

research questions for this data collection effort were:

• Is it appropriate to assume that the unvented attic conditions are the same as the fully 

conditioned living space below?

• What interior boundary conditions should be used when modeling unvented roof 

assemblies; i.e., what are the appropriate attic space temperature and humidity ratio

conditions?

• What variables have the greatest impact on the difference between the unvented attic and 

the conditioned space conditions?

2.2 Methodology

CARB’s technical research efforts entailed:

• Developing the measurement plan to quantify the characteristics of each test site with 

respect to whole-house building tightness, attic leakage, living space leakage, and, if 

applicable, duct system leakage.

• Performing the house characterization testing and installing remote temperature and RH 

loggers in three selected test homes. Field monitoring was conducted from the November 

2013 through August 2014.

• Collecting and scrubbing the data for model validation by ORNL. In addition, CARB 

analyzed the field data from the three test sites to determine if any trends could be 

identified. 

2.2.1 Sites

Three unvented attics were investigated in this experiment and were all located in Connecticut 

(Figure 3), which is within DOE climate zone 5A. Basic characterizations of the three test sites 

are provided in Table 3 and Table 4. Each unvented attic was insulated with closed-cell spray 

polyurethane foam (ccSPF) at the roof deck, but two of the sites used alternative insulations to 

reach the final assembly R-values. The volume of the unvented attic in House 2 was the largest 

and it housed all the second-floor mechanicals, which included an energy recovery ventilator, air 

handler unit, and ductwork. Houses 1 and 3 had no mechanicals in the unvented attics. 
5

Figure 3. Locations of test sites

Table 3. Description of Each Test Site

Site House Type Attic Type

House 1 Energy-efficient home without ducts in 

unvented attic Full unfinished attic

House 2 Energy-efficient home with ducts in unvented 

attic (supply side leakage) Full unfinished attic

House 3 Existing cape style home with only an unvented 

attic upgrade and no ducts in attic.

Upper attic of a finished 

Cape Cod attic*

* Refer to Figure 25.

Table 4. Summary of Unvented Attic Configurations

House 1

(Westport)

House 2

(Old Greenwich)

House 3

(Fairfield)

Surface Area (ft2) 

(exposed/total) 580/1,075 1,239/2,285 153/257

Volume (ft3) 1,488 4,704 91

Insulation 6.5-in. ccSPF (R-39)

4-in. ccSPF (R-27) +

3-in. mineral wool batt 

(R-12) + foil-faced 

fiber glass batt (R-13)

2-in. ccSPF (R-13) + 

6-in. open-cell spray 

polyurethane foam

(ocSPF) (R-22)
6

2.2.2 Sensor Placements

Houses 1 and 2 had large spaces in the unvented attics (1,488 ft3 and 4,704 ft3 in volume,

respectively); thus, multiple sensors were used to determine if there were variations in the 

temperature and humidity conditions throughout those two attics. As sketched in Figure 4 and 

Figure 5, 15 sensors measuring temperature and RH were strategically placed in the unvented 

attics to determine the distribution of conditions in the attic. For the unvented attic in House 3, 

only one centrally located temperature/RH sensor was used because the unvented attic was only

91 ft3 in volume.

Figure 4. Sketch of Unvented Attic 1

Figure 5. Sketch of Unvented Attic 2

Temperature 

and RH

Sensors

East

South

East

West

North
7

Analyzing the differential between the centrally placed temperature/RH sensor, CM, and other 

sensors in the unvented attic resulted in minimal differences. Looking at Table 5, average 

temperature and humidity ratio differences were all below ±1°F and ±0.00002 lbw/lbda (±0.5% in 

terms of RH), respectively, and the middle 80% range for temperature and humidity ratio

differences in each unvented attic was less than ±1°F and ±0.00019 lbw/lbda (±1.6% in terms of 

RH), respectively. The temperature and humidity ratio distribution in Attic 1 is provided in 

Figure 6 and Figure 7, respectively. Other than some summertime variation in the humidity ratio 

at location RB3 and LB1 (as noted in Figure 4), which are likely due to summertime solar heat 

gain in the morning and afternoon, the CM sensor is a good representation of the overall 

conditions in Unvented Attic 1. It was thereafter assumed that the CM sensor is a suitable single 

point measurement of the overall unvented attic conditions when evaluating with respect to the 

living space for House 1. 

Table 5. Average Temperature and Humidity Ratio Difference Between Centrally Placed Sensor 

and Other Sensors Placed All Around the Unvented Attic Space Over Winter and Spring

Attic

80% Range (Min-Max) Average

Temperature 

Difference

(°F)

Humidity Ratio 

Difference

(lbw/lbda)

Temperature 

Difference

(°F)

Humidity Ratio 

Difference

(lbw/lbda)

1 –0.9 to 0.7 –0.00011 to 0.00019 –0.12 0.00002

2 –1.7 to 0.2 –0.00039 to 0.00011 –0.60 –0.00009

Figure 6. Temperature differences between sensor at CM and sensors

at other positions in Unvented Attic 1
8

Figure 7. Humidity ratio differences between sensor at CM and sensors

at other positions in Unvented Attic 1

Unvented Attic 2 had similar trends in the temperature measurements of all attic sensors (Figure 

8), with most measurements within a range of ±4°F. Unvented Attic 2 shows is an anomaly with

the humidity measurements along the attic ridge versus the central measurement point during the 

summer months (Figure 9), even though the ridge temperature measurements are within a range 

of –0.5°F to 1.5°F of the CM temperature measurement. As the LT humidity ratio measurement 

seems to lag the outdoor conditions (Figure 10), this anomaly may be a result of air leakage at 

the ridge beam that is impacting the upper measurement sensors. For the purposes of the analysis 

of the unvented attic with respect to the living space for House 2, the CM sensor will still be used 

as a representative measurement of the overall attic conditions.
9

Figure 8. Temperature differences between sensor at CM and sensors

at other positions in Unvented Attic 2

Figure 9. Humidity ratio differences between sensor at CM and sensors

at other positions in Unvented Attic 2
10

Figure 10. Humidity ratio at the CM and LT sensors, in the living space,

and outdoors at House 2 for a typical summer week

2.2.3 Characterization of Test Sites—Building Leakage Testing

Depressurization tests were performed to determine how connected the three test attics were to 

the conditioned spaces. This connectivity was determined by zonal pressure diagnostics. The 

building envelope air leakage area was determined using a multipoint depressurization test at 10, 

20, 30, 40, 50, and 60 Pa to enable a linear regression analysis following ASTM E779 test 

procedures. The specific test configurations that were used to quantify the connectivity of the

unvented attics to living spaces are illustrated in Figure 11 through Figure 14. 

Initially, standalone depressurization tests were performed with the houses in the following 

configurations:

• Figure 11: Attic hatch open

• Figure 12: Attic hatch closed (natural state)

• Figure 13: Attic hatch closed (sealed with tape).
11

The pressure differential between the attic and the living space was recorded at each test 

pressure.

Afterward, an additional test fan was located in the attic hatch to perform guarded pressure 

testing (Figure 14). At each pressure, the attic test fan was used to bring the unvented attic 

pressure to the same as the living space. 

Figure 11. Depressurization test with unvented attic hatch open

Figure 12. Depressurization test with unvented attic hatch closed
12

Figure 13. Depressurization test with unvented attic hatch carefully sealed

Figure 14. Depressurization test of house and attic. The attic depressurization flow rate minus the 

difference between blower door cfm50 with the attic hatch open and closed is the leakage of just 

the unvented attic.

The attic-to-living space “coupling factor” was also measured. This was defined as the ratio of 

the attic-to-outdoor differential pressure to the home-to-outdoor differential pressure at –50 
13

Pascal. A coupling factor of zero indicates no air transport between the attic and living space, 

while a value of one means that these two spaces are completely connected.

2.2.4 Characterization of Test Sites—Duct Leakage Testing

A duct pressurization test was used to measure total duct leakage, as well as supply and return 

duct leakage, in House 2. The other two sites did not have ductwork. It is important to know 

whether leakage is on the supply or return side to accurately analyze the impact of the duct 

leakage on the unvented attic’s interaction with the living space below.

2.2.5 Characterization of Test Sites—Solar Irradiance

Because all three sites are within 20 miles of each other, solar irradiance was monitored at House 

1 only. Though not a part of the original test plan, this field monitoring was included for the 

period of March 2014 to July 2014 to compare measured values to the assumptions in a BEopt 

simulation and to assist the modeling validation work that ORNL will perform. 

Figure 15. Global horizontal irradiance

When taking a closer look at the solar data (Figure 16), the attic temperature profile slightly lags 

the outdoor temperature profile, which slightly lags the solar irradiance profile, as anticipated. 
14

Figure 16. Summer impact of solar irradiance on Unvented Attic 1
15

3 Results

3.1 Performance Testing

The following are results of the house characterization performance testing for each test home.

For all three homes, the attic-to-living space “coupling factor” was in the 0.84–0.89 range, 

meaning that the unvented attics were well connected to the living space.

3.1.1 House 1

In Table 6, the attic depressurization airflow, 163 cfm at –50 Pa, is the leakage between the attic 

and the outside. While an unvented attic, nearly a quarter of the whole-house leakage is still 

through this building component. The total building leakage of 658 cfm50 translates to an ACH50

of ~1.9 for the home.

Table 6. Whole-Building Depressurization Test Results at House 1

House Depressurization

(–1.6 Pa Baseline) CFM10 CFM20 CFM30 CFM40 CFM50 CFM60

House Depressurization Airflow (CFM)

Open Attic Hatch 203 352 459 563 659 736

Closed Attic Hatch 219 337 456 553 628 738

Sealed Attic Hatch 214 332 448 546 640 728

Pressure Differential of Attic With Respect to House (Pa)

Open Attic Hatch 0 0 0 0 0 0

Closed Attic Hatch 0.5 1.3 2.1 2.9 3.6 4.9

Sealed Attic Hatch 0.9 2 3.2 5.4 5.7 7

Attic Depressurization 

Airflow for Neutral Pressure 

to House (CFM)

41 82 120 135 163 187

House Depressurization 

Airflow (CFM) 224 347 477 558 658 753

The various test scenarios indicate that all the leakage to the outside (both from outside to the 

attic and from the house below the ceiling) goes through the blower door and that the total 

leakage of 658 cfm at –50 Pa is nearly identical to the first test with the attic access open (659 

cfm at –50 Pa), which results in no pressure difference between the house and the attic. This is a 

good, quick check that testing results make sense. Therefore, the leakage from outside into the 

house below the attic at 50 Pa is 658 – 163 = 495 cfm.

3.1.2 House 2

The results of the whole-building depressurization testing at House 2 were fairly similar to 

House 1. Building leakage at –50 Pa was 724 cfm50, which translates to an ACH50 of ~1.2 for 

the home. Of that total building leakage, 161 cfm50 was through the attic to outdoors.
16

Table 7. Whole-Building Depressurization Test Results at House 2

House Depressurization

(–1.5 Pa Baseline) CFM10 CFM20 CFM30 CFM40 CFM50 CFM60

House Depressurization Airflow (CFM)

Open Attic Hatch 177 344 497 605 724 787

Closed/Sealed Attic Hatch 180 338 455 561 669 766

Pressure Differential of Attic With Respect to House (Pa)

Open Attic Hatch 0 0 0 0 0 0

Closed/Sealed Attic Hatch 1.3 2.6 3.8 4.8 5.8 7.1

Attic Depressurization 

Airflow for Neutral Pressure 

to House (CFM)

53 88 113 140 161 184

House Depressurization 

Airflow (CFM) 194 342 465 576 676 779

This test site was the only one with ductwork in the unvented attic. Duct leakage test results

(Table 8) suggest that the overall duct system is fairly well air sealed with a small percentage of 

leakage on the supply side. While the total duct leakage was measured to be 92 cfm25, the 

leakage to the attic was only 45 cfm25. The air handler flow ranged from 450–600 depending on 

operational mode of the unit.

Table 8. Duct Leakage Test Results at House 2

Total Duct Leakage cfm

Total 92

Supply 89

Return 3

Leakage to Attic cfm

Total 45

Supply 44

Return 1

3.1.3 House 3

House 3 had air sealing improvements to the attic only. The exterior walls had not been air 

sealed, so the overall building tightness of this home was the lowest, even though it was the 

smallest test home. Building leakage at –50 Pa was 1,365 cfm50, which translates to an ACH50 of 

~8.3 for the home. Of that total building leakage, 14 cfm50 was through the attic to outdoors.
17

Table 9. Whole-Building Depressurization Test Results at House 3

House Depressurization

(–1.6 Pa Baseline) CFM10 CFM20 CFM30 CFM40 CFM50 CFM60

House Depressurization Airflow (CFM)

Open Attic Hatch 427 725 993 1,165 1,351 1,535

Closed/Sealed Attic Hatch 432 730 989 1,173 1,356 1,523

Pressure Differential of Attic With Respect to House (Pa)

Open Attic Hatch 0 0 0 0 0 0

Closed/Sealed Attic Hatch 0.6 2.2 4.2 6 8 10

Attic Depressurization 

Airflow for Neutral Pressure 

to House (CFM)

– – 12 14 18 20

House Depressurization 

Airflow (CFM) 465 755 986 1,200 1,365 1,530

3.2 Field Monitoring

The results of the temperature and RH monitoring for each test home follow.

3.2.1 House 1

Figure 17 shows conditions at House 1 from mid-December 2013 through the end of June 2014

(a data collection issue resulted in the loss of July data for outdoor conditions, but peak design 

conditions occurred in the June data, so the data should be representative of the summer months). 

The living space temperature fluctuates a little during December as heating was being supplied 

by an inverter-driven air-source heat pump. Starting in January, the hydronic baseboard system 

was used for the space heating. The heating set point was 70°F. In the summer months, the airsource heat pump was once again used to now provide space cooling at a set point of 75°F.

Looking at this temperature differential as a function of outdoor air temperature (Figure 18), the 

general trend is for the unvented attic to be slightly cooler than the living space in winter (down 

to ~7.5°F), roughly the same in the spring, and slightly warmer in the summer (up to ~7.5°F).

Figure 19 shows the humidity ratio conditions at House 1. Humidity ratio, rather than RH, is 

presented because it is the total amount of water vapor present in a given volume and does not 

take temperature into consideration. Except for the beginning of summer, the humidity ratio of 

the unvented attic was fairly similar to the living space conditions (Figure 20).
18

Figure 17. Temperature conditions at House 1

Figure 18. House 1 temperature differential with respect to outdoor temperature
19

Figure 19. Humidity conditions at House 1

Figure 20. House 1 humidity ratio differential with respect to outdoor humidity ratio
20

3.2.2 House 2

Even more so than House 1, the attic conditions in House 2 track fairly closely to living space 

conditions (Figure 21). The small amount of space conditioning that is leaking from the 

distribution system to the attic likely caused this small range in temperature differential (Figure 

22). The home operated at a set point of 70°F for most of the winter and a set point of 72°F for 

much of the summer.

Figure 23 shows the humidity ratio conditions at House 2. Again, except for the beginning of 

summer, the humidity ratio of the unvented attic was fairly similar to the living space conditions 

(Figure 24).

Figure 21. Temperature conditions at House 2
21

Figure 22. House 2 temperature differential with respect to outdoor temperature

Figure 23. Humidity conditions at House 2
22

Figure 24. House 2 humidity ratio differential with respect to outdoor humidity ratio

3.2.3 House 3

House 3 is a finished attic of a Cape Cod-style house with the monitoring being done in the 

upper attic, as illustrated in Figure 25. While the upper attic and knee wall attic were the 

combination of ccSPF against the roof deck and the remainder insulated with ocSPF, the 

insulated rafter sections were only 5-1⁄2-in. ocSPF (what was possible during this retrofit without 

taking down the interior drywall). 

Figure 25. Sketch of House 3 attic

House 3 was monitored from January 2014 through April 2014 only. Figure 26 through Figure 

29 show the temperature and humidity ratio conditions at House 3. While the temperature trends 

at House 3 were fairly similar to House 1, the swings in living space temperature (between 

~62°F and 68°F during the winter, as seen in Figure 26), lower insulation in the insulated rafter 

upper attic

insulated 

rafters

finished attic

knee wall 

attic
23

space, and higher whole-building leakage likely led to the slightly steeper temperature 

differential curve with respect to outdoor temperature.

With respect to humidity ratio (Figure 27), while the living space and unvented attic tracked 

fairly closely throughout the winter, from the spring onward, the attic space saw large spikes in 

the humidity ratio. In the other two test homes, this fluctuation range in the attic humidity ratio 

didn’t occur until the summer months. The higher perm rating of this insulated rafter roof section 

may have allowed moisture to invade the upper attic because of solar drive. Alternatively, this 

could have been a result of the attic being sealed but the exterior walls not being sealed. Humid 

air could have risen to the unvented attic, and because the sensible cooling load was insufficient 

to require air conditioning, no dehumidification occurred. 

Figure 26. Temperature conditions at House 3
24

Figure 27. House 3 temperature differential with respect to outdoor temperature

Figure 28. Humidity conditions at House 3
25

Figure 29. House 3 humidity ratio differential with respect to outdoor humidity ratio
26

4 Discussion

While this is only a small sample, comparing the results from the three test homes may provide 

researchers and modelers valuable insight into predicting unvented attic conditions in homes in 

the Northeast. In Figure 30 and Figure 31, temperature and humidity ratio trends are provided for 

the three test homes and the BEopt model discussed in Section 1.3. Especially in the case of 

humidity ratio, the current BEopt modeling assumptions in an unvented attic vary from the three 

test homes for this climate region. 

Figure 30. Temperature trend lines for the three test homes and a BEopt model

Figure 31. Humidity trend lines for the three test homes and a BEopt model
27 

Based on this initial field monitoring and assuming an attic-to-living space coupling factor near 

to 1, the equations in Table 11 are suggested for estimating attic conditions in the Northeast. 

Table 10. Temperature and Humidity Ratio Trends for Various House Types 

House Type 

Unvented Attic 

Temperature 

(°F) 

Unvented Attic Humidity 

Ratio 

(lbw/lbda) 

Energy-Efficient Home 

With Ducts in Unvented 

Attic (Supply Side 

Leakage) 

TL + 0.0009(TOA)

2

 – 

0.0508(TOA) – 0.1281 HRL + 14.606(HROA)

2

 – 

0.1424(HROA) + 0.00005 Energy-Efficient Home 

Without Ducts in 

Unvented Attic 

TL + 0.0007(TOA)

2

 + 

0.0751(TOA) – 7.5243 

Existing Cape Style 

Home With Only an 

Unvented Attic Upgrade 

and No Ducts in Attic 

TL + 0.0026(TOA)

2

 – 

0.005(TOA) – 7.2737 

HRL + 12.062(HROA)

2

 + 

0.1192(HROA) – 0.0002 

* variables: T = temperature, HR = humidity ratio 

* subscripts: L = living space, OA = outdoor air
28

5 Conclusion

The key research questions for this data collection effort and their answers resulting from the 

performed experiment were:

Is it appropriate to assume that the unvented attic conditions are the same as the fully 

conditioned space below?

The unvented attic is unlikely to be at the same conditions as the living space, unless it is directly 

or indirectly conditioned. In the case of ducts located in the attic, wintertime conditions of the 

unvented attic tracked fairly well with the living space conditions. In the summertime conditions, 

the cooling duct leakage didn’t have a noticeable impact on humidity ratio. This is likely because

it was primarily supply-side leakage in this study. If the leakage had been on the return side, 

there might have been the potential for a more noticeable impact on the humidity ratio in the 

unvented attic. 

From a field protocol perspective, a single point measurement in the center of an unvented attic 

is suitable for characterizing the overall unvented attic conditions.

What interior boundary conditions should be used when modeling unvented roof assemblies, i.e. 

what are the appropriate attic space temperature and humidity ratio conditions?

While this is only a small sample, comparing the results from the three test homes provides

researchers and modelers valuable insight into predicting unvented attic conditions in homes in 

cold climates. From this limited study, the original modeling assumption of the same conditions 

for the living space and unvented attic do not appear to be appropriate. Based on this initial field 

monitoring, and assuming an attic-to-living space coupling factor near to 1, the equations in 

Table 11 are suggested for estimating attic conditions in the cold climate region.

Table 11. Temperature and Humidity Ratio Trends for Various House Types

House Type

Unvented Attic 

Temperature

(°F)

Unvented Attic 

Humidity Ratio 

(lbw/lbda)

Energy-Efficient Home With 

Ducts in Unvented Attic (Supply 

Side Leakage)

TL + 0.0009(TOA)

2 –

0.0508(TOA) – 0.1281 HRL + 14.606(HROA)

2 –

0.1424(HROA) + 0.00005 Energy-Efficient Home Without 

Ducts In Unvented Attic

TL + 0.0007(TOA)

2 + 

0.0751(TOA) – 7.5243

Existing Cape Style Home With 

Only an Unvented Attic Upgrade 

and No Ducts in Attic

TL + 0.0026(TOA)

2 –

0.005(TOA) – 7.2737

HRL + 12.062(HROA)

2 + 

0.1192(HROA) – 0.0002

* variables: T = temperature, HR = humidity ratio

* subscripts: L = living space, OA = outdoor air

What variables have the greatest impact on the difference between the unvented attic and 

the conditioned space conditions?
29

Because this was a small sample set of three homes, all variables were unlikely to be adequately 

addressed. With that said, the three main variables identified in this data collection study were 

indirect space conditioning through duct leakage, the building leakage of the living space, and 

the attic-to-living space coupling factor.

If the unvented attic receives indirect space conditioning from duct leakage, the attic conditions 

track closer to the living space in terms of temperature. Trends in the humidity ratio didn’t seem 

to be significantly impacted by supply-side duct leakage. If the duct leakage had been on the 

return side, it would have likely had a greater impact on humidity ratio, but might have 

minimized the impact on the temperature side.

Because Houses 1 and 2 had similar ACH50 building leakage values and tracked reasonably 

closely with regard to humidity ratio (while House 3 saw much higher humidity ratios even in 

the spring months), the building leakage of the living space (all unvented attics were reasonably

tight) likely resulted in this difference between the test sites.

While all three test homes were similar in terms of the attic-to-living space coupling factor 

(0.84–0.89), it makes logical sense that the more connected the two spaces are to each other, the 

closer they will track in terms of space conditions. This would be confirmed only with additional 

test sites that have unvented attics, but the ceiling plane has also been air sealed (not typically 

found in homes).
30 

References 

Metzger, C.E.; Wilson, E.; Horowitz, S. (2012). Addendum to the Building America House 

Simulation Protocols. Golden, CO: National Renewable Energy Laboratory. DOE/GO-102012-

3855. Accessed December 16, 2014: http://www.nrel.gov/docs/fy13osti/57450.pdf. 

Schumacher, C. (2007) “Unvented Roof Assemblies for All Climates.” Building Science 

Digests. BSD-149. April 23, 2007. Accessed May 30, 

2014: http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assembliesfor-all-climates. 
DOE/GO-102015-4611 ▪ February 2015

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