NISTIR 7072

Air and Pollutant Transport from

Attached Garages to Residential Living

Spaces

Steven J. Emmerich

Joshua E. Gorfain

Mike Huang

Cynthia Howard-Reed
NISTIR 7072

Air and Pollutant Transport from

Attached Garages to Residential

Living Spaces

Steven J. Emmerich

Joshua E. Gorfain

Mike Huang

Cynthia Howard-Reed

Building Environment Division

Building and Fire Research Laboratory

National Institute of Standards and Technology

Prepared for:

U.S. Department of Housing and Urban Development

Office of Lead Hazard Control and Healthy Homes

December 2003

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Undersecretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
ABSTRACT

NIST is conducting a study on the indoor air quality (IAQ) impacts and engineering solutions 

related to the transport of pollutants from attached garages to residential living spaces. Natural 

or equipment-induced pressure differences across air leakage paths in house-garage (HG) 

interfaces can result in the transport of the contaminants generated in garages into adjacent living 

spaces. This paper summarizes a literature review on the transport of pollutants from garages to 

residential living spaces and describes a field study to estimate the range of airtightness of 

attached garages and of HG interfaces in a sample of U.S. homes. 

Although the body of literature on pollutant transport from attached garages to residential 

buildings is limited, the studies reviewed provide substantial evidence that transport of 

contaminants from garages has the potential to negatively impact residential IAQ in either an 

acute (e.g., carbon monoxide from automobiles) or chronic manner (e.g., storage of chemical 

products). However, the literature contains more questions than answers on issues such as the 

airtightness and geometry of the HG interface, the impact of heating and cooling equipment in 

the garage, and the effectiveness of potential engineering solutions. 

In order to address one gap in understanding these issues, the airtightness of garages and HG 

interfaces was measured in five residences using fan pressurization. While the small sample of 

houses limits generalization of the results, a range of house ages, styles, and sizes was included. 

For all homes tested, the garage was found to be at least twice as leaky as the house, based on air 

changes per hour at 50 Pa. The leakiness of the garage envelope, based on surface area 

normalized effective leakage area at 4 Pa (ELA4/SA), ranges from a high of nearly eleven times 

to a low of two and a half times that of the house exterior envelope leakage. On average, the HG 

interface was almost two and a half times leakier than the rest of the house envelope, when based 

on ELA4/SA. However, this average is somewhat skewed due to one HG interface measured in 

this study that is almost eleven times leakier than the rest of the house envelope. Conversely, a 

larger Canadian study found HG interfaces to be comparable to house envelopes but found the 

average garage to be about ten times leakier than the houses – possibly because Canadian houses 

are consistently tighter than U.S. houses (Fugler et al. 2002). 

The knowledge gained from this review and the field study will be used in a simulation study of 

the potential occupant exposure to pollutants from attached garages and to explore potential 

engineering solutions to associated IAQ problems. 

Key Words: airtightness, attached garage, blower door, house-garage interface, indoor air 

quality, living space, pollutant transport. 

 iii
 iv
TABLE OF CONTENTS

Abstract iii

List of Tables vii

List of Figures vii

Background 1

Literature Review 2

Summary 5

Field Study 8

Introduction 8

Houses 8

Method 9

Analysis 11

Results 14

Discussion 18

Acknowledgements 19

References 19

 v
 vi
LIST OF TABLES

1 Building Description Summary 9

2 Building Orifice Parameters 15

3 House and Garage Envelope Leakage Summary 17

LIST OF FIGURES

1 Blower Door Configuration for Buildings B, C, D, E 10

2 Blower Door Configuration for Building A 11

3 Sample Plot of Blower Door Data for Two Test Configurations 12

 vii
Background 

Many pollutant sources are commonly stored or used in residential attached garages such 

as gasoline-fired engines (automobiles, lawnmowers, etc.), paints, and solvents. Pressure 

differences across air leakage paths between the garage and adjoining living space can 

result in the transport of these contaminants to the living space. Factors influencing this 

transport include temperature differences, wind, the placement of the air handler or ducts 

in the garage, duct leakage, and equipment operation, such as exhaust fans and vented 

combustion appliances. Although this issue has long been identified in residential indoor 

air quality (IAQ) studies (Wallace 1987, Traynor and Nitschke 1984, Hawthorne et al. 

1986, Lindstrom et al. 1995, Colome et al. 1994, Levsen et al. 1999, Lebowitz et al. 1999 

and Brown 2002), there has not been extensive study to identify key parameters 

impacting occupant exposure to emissions from sources in attached garages or methods 

to reduce this exposure. 

The objective of this project is to use a multizone model to study the potential for 

occupant exposure to contaminants transported from residential attached garages. 

Multizone model simulations will identify important parameters affecting the 

concentrations in the home and potential methods to reduce occupant exposure.

This report summarizes a review of the published literature on the transport of pollutants 

from garages to residential living spaces and presents the results of a small field study of 

garage airtightness. These tasks were performed in support of the planned simulation 

effort. 

1 
Literature Review

A literature search was conducted on the impact of attached garages on residential IAQ 

with the objectives of providing a sound base for this research effort and identifying 

potential case study buildings upon which to base the modeling efforts. The body of 

published literature on this topic is small and only reports that treated the topic in detail 

were reviewed. Note that there has also been work published on large, commercial 

garages including a recent review (Limb 1994). Although there are some issues in 

common between residential and commercial garages, there are many differences such 

that this commercial building work is not directly relevant to the current effort and thus is 

not reviewed here. 

One of the most significant studies of the transport of contaminants from attached 

garages to houses was conducted in Canada (Graham 1999, Graham et al. 1999, 

Noseworthy and Graham 1999). Graham (1999) described measurements performed to 

characterize the tailpipe emissions of a single car, which was used in later experiments. 

Graham et al. (1999) reported on the transport of vehicle emissions from attached garages 

in 16 Canadian houses over two winter seasons. Pollutants measured included carbon 

monoxide (CO), carbon dioxide (CO2), volatile organic compounds (VOCs) and carbonyl 

compounds. Two test procedures were followed including a hot soak test (the warmed-up 

vehicle was parked in the garage in the afternoon) and a cold start test (the vehicle was 

started in the garage in the morning then backed out of the garage and the garage door 

was closed). Contaminant monitoring continued for 4 h for each test procedure. 

Additionally, sulfur hexafluoride (SF6) was used as a tracer gas during some tests to 

estimate garage air change rates. 

They found strong evidence that IAQ in the homes was influenced by the vehicle 

emissions with the largest influence observed during morning cold start of vehicles. 

During cold start tests, the average CO concentration in the garages ranged from 16 

mg/m3

 to 298 mg/m3 over 4 h, while the average CO concentration in the houses ranged 

from 0.5 mg/m3

 to 38 mg/m3 (note: 1.0 mg/m3 = 0.86 ppm(v)). The average CO 

concentrations in both garages and houses was 2.2 mg/m3

 or less during all hot soak tests. 

The ratio of house SF6 concentration to garage SF6 concentration during both types of test 

ranged from about 0.2 % to a high of 16 %. However, questions were raised about the 

accuracy of some SF6 measurements in the garage due to potentially incomplete mixing. 

Significant amounts of VOCs entered the house from the garage, with the increases 

varying widely depending on the compound, house, and test condition. It was difficult to 

detect a contribution of the vehicle to carbonyl compounds due to already significant 

levels in the houses. 

Two sets of tests were performed at one house that showed qualitatively greater transport 

from the garage to the house during colder weather due to a larger stack effect. Although 

one house consistently showed greater transport of contaminants from the garage to the 

house, no explanation was offered. Almost no details of the houses (floorplans, 

construction, heating and cooling system, etc.) were reported. Estimates of garage air 

change rates from SF6 measurements for 4 of the garages ranged from 1.8 h-1 to 2.7 h-1. 

Neither ambient conditions nor garage temperatures were reported for these estimates. 

2 
Noseworthy and Graham (1999) used a chemical mass balance model to apportion 

measured VOC concentrations in the houses to their sources (i.e., outdoor air, garage air, 

and pre-test air). They found that from 10 % to 75 % of the house concentrations were 

attributable to the garage air depending on the house and test type. 

Fugler et al. (2002) summarizes the research described above and also briefly describes 

measurements of air leakage from attached garages in 25 houses (a mix of ages, sizes, 

number of stories, and configuration of shared house-garage (HG) interface) and a related 

modeling effort. Researchers tested the airtightness of the HG interface during summer 

and winter seasons using depressurization techniques. They found that, normalized by 

wall surface area, the garages were about 10 times leakier than the houses on average and 

the HG interface was about equal in leakiness to the rest of the house envelope. However, 

the ratio of air leakage through the HG interface to the total air leakage into the house had 

a wide range with a low near zero and a high near 45 %. The pressure difference across 

the HG interface was also measured during summer and winter conditions. The pressure 

difference averaged 1.6 Pa during the winter and 0.5 Pa during the winter (both with a 

higher pressure in the garage than the house). Little detail is provided on a modeling 

effort that was intended to extend the range of exposure scenarios considered beyond the 

limited cases that were tested. After a model calibration exercise, the model was used to 

examine the potential effectiveness of operating a 100 L/s exhaust fan in the garage 

during the first half hour of an automobile cold start test. The model showed that the 

exhaust fan was not useful for the typically leaky garages but could limit transport of CO 

into the house from the tightest garage. 

Moore and Kaluza (2002) monitored carbon monoxide, among other parameters, inside 

65 homes in four Alaskan cities that were built since the adoption of the Alaska Building 

Energy Efficiency Standard in 1992 (AHFC 1992). This study observed that, for the 

majority of homes for which they had data, the house CO reading followed the same

temporal pattern as the garage CO. Since the house CO levels tracked garage CO levels, 

they assumed that most of the CO found in the living space could be attributed to CO 

produced by cars in the garage migrating into the living space. No other architectural, 

behavioral, or environmental factor was as strongly associated with elevated CO as 

garage concentrations. This study also suggests installing a garage mounted exhaust fan 

as a potential solution to this problem. 

Lansari et al. (1996) describes a study predicting pollution concentrations in a residence 

with an attached garage using an early version of NIST’s CONTAM IAQ model. 

Experiments were also performed in a test house to validate the model. The experiments 

involved evaporating 5 mL of methanol over a 13 min period in the two-car garage. The 

garage was located in the back of the house with storage areas in the back of the garage. 

The attached garage is adjacent to the kitchen and dining room. The floor plan consists of 

a single level with one bedroom, one bathroom, a living room, family room, and dining 

room. There was no mention of any HAC equipment or ductwork in the garage. The 

garage concentrations of methanol were measured every 5 min for a period of 90 min. 

One limitation of the study was the lack of measurements of methanol in the remainder of 

the house. Dispersion of methanol throughout the house was then simulated. Air leakage 

and ventilation flows in the model were not based on test house measurements. Instead of 

additional methanol measurements, SF6 was used to test the model’s performance by 

3 
measuring the percentage of contaminant that enters the house from the garage, 

determining how rapidly the concentration in the garage drops after the door is opened, 

and testing the assumption that the garage contaminants were well mixed. The kitchen 

area adjacent to the garage was predicted to have the highest methanol concentration in 

the house. The study concludes that the model over-predicted garage methanol 

concentration during the release period and under-predicted methanol concentrations by 

about 15 % after evaporation was complete. 

Tsai and Weisel (2000) reported on a study on the potential transport of methanol, 

benzene, and toluene into a residence from a methanol-fueled vehicle parked in an 

attached garage. The garage was attached at the first floor to a two-story house. Although 

the HAC system did not include intentional connections in the garage, it was not reported 

whether any ductwork was located in the garage. Two ventilation conditions were 

studied, HAC fan on and off. Other conditions considered included the integrity of the 

evaporative emissions control, the ambient air temperature, and the fuel tank temperature. 

Experiments were performed by driving the methanol-fueled vehicle until warm, then 

parking it in the garage and shutting both the garage door and the door to the residence. 

Pollutant concentrations were measured at three sample locations: in the garage, the room

adjacent to the garage, and the living room on the other side of the house. The 

experiments each lasted 3 h and were done over 16 sampling days during the summer in 

New Jersey with each condition pair (ventilation on or off and canister connections on or 

off) conducted four times. The canister connection was altered to simulate a poorly 

maintained or spent emission control device. On average, pollutant concentrations in the 

room adjacent to the garage ranged from one-tenth to one-fifth of the garage 

concentrations over the 3 h tests. Concentrations in the living room were about one-third 

the adjacent room concentrations on average. The authors cited sink effects as a potential 

reason for differences between concentration ratios for the three pollutants. Due to 

increased mixing, pollutant concentrations in the adjacent room were lower when the 

HAC fan was on than when it was off, while the average concentrations in the living 

room were higher. The ambient air temperature had no effect on concentrations, but only 

varied over a limited range from 24 °C to 32 °C during the tests. An interesting 

observation was a sharp rise in the adjacent room concentrations near the end of the test 

coinciding with a sharp drop in garage concentrations due to the garage door opening. In 

an earlier pilot study, Weisel and Lawryk (1993) also measured transport of methanol and 

benzene from a garage to an adjacent room, but little detail was provided. 

Marr et al. (1998) and Nazaroff et al. (1996) both reported a modeling study of the risk of 

accidental death due to CO poisoning from automobiles in residential garages. The study 

employed a Monte Carlo simulation technique and looked at 1 h and 3 h scenarios of CO 

emissions in either a 90 m3

 (two-car garage) or a 400 m3 (single-family house with 

garage) well-mixed enclosure. CO emission rates were based on a distribution using 

measured California data, and whole house air change rates were based on a distribution 

from a study of U.S. housing. Due to the lack of reported garage air change rates, the 

garage air change rate distribution was assumed to range from the same as the house air 

change rate distribution up to 3 times the house air change rate distribution. A model for 

blood carboxyl-hemoglobin (COHB) was used to determine the risk of fatality from the 

predicted CO concentration data. The authors found no risk of death from the 1 h 

4 
exposure in the house scenario but a risk ranging from 3.5 % to 21 % for the other 

scenarios. A CO emission rate of 1 g/min was reported to be an approximate threshold for 

causing accidental deaths from CO poisoning. 

Kaluza (1999) provides an overview of the issues involved in the transport of pollutants 

from attached garages. Issues discussed include contaminant sources in garages, stack 

effect, “tuck-under” garages (i.e., a garage located both below and adjacent to living 

space), furnaces and ductwork in garages, airtightness of walls between houses and 

garages, and house depressurization due to exhaust fans. Kaluza tested one residence in 

Alaska using CO generated by a car in the garage. The garage was a tuck-under design 

with no heating appliance located in it. A CO peak about one-tenth of the peak in the 

garage was measured in the house and occurred about 5 h after the garage peak. 

Operation of a whole-house ventilation system caused a 50 % reduction in the peak CO 

level in the home. Operation of a garage exhaust fan of unspecified capacity to maintain a 

very slight negative pressure in the garage relative to the house (1 Pa) prevented CO from

entering the house. 

Furtaw et al. (1993) reported a measurement and modeling study of the potential 

exposure in residences to evaporative emissions of benzene from gasoline-fueled vehicles 

parked in attached garages. Measurements of benzene concentrations and air change rates 

based on tracer gas tests were made in several homes although very little detail was 

reported on the houses, garages, internal conditions and ambient conditions during the 

tests. They reported garage air change rates ranging from 0.3 h-1 to 1.5 h-1 with closed 

garage doors, garage air change rates ranging from 17 h-1 to 103 h-1 with open garage 

doors, and house air change rates ranging from 0.5 h-1 to 0.8 h-1. They also found that 

about 1 % of the air entering the house came from the garage during warm weather 

compared to 8 % to 9 % during cool weather. A simple two-zone (i.e., the living space 

and the garage) mass balance model was also developed with an evaporative benzene 

source dependent on the ambient temperature. Two cases were modeled – a winter case 

with house and garage air change rates constant at 0.5 h and 10 % of the house air 

entering from the garage and a summer case also with garage and house air change rates 

of 0.5 h but with only 1 % of the house air entering from the garage. In this example case, 

calculated average benzene concentrations were much higher in the garage during the 

summer (240 μg/m3

 in summer vs. 70 μg/m3 in winter) but were higher in the house 

during the winter (6 μg/m3

 in winter vs. 2 μg/m3 in summer). Based on the study, the 

authors recommend ventilating the garage to the ambient by some means although 

specific amounts of ventilation are not recommended. One alternative suggested is for 

occupants to leave the garage door open after parking a vehicle. 

Greiner and Schwab (1998) describe the investigation of CO contamination of a home 

from a source in the garage (combustion engine vehicle startup). They measured various 

home appliances, heating equipment, and a fireplace for CO emission to determine the 

source(s) of the elevated CO concentrations found in the house, none of which were 

found to be the sources. Blower door and other tests were used to characterize the house, 

the garage, and the house-garage interface. Blower door tests revealed a large proportion 

(over 40 %) of the air entering the house came from the garage. From pressure 

measurements, smoke tests, and a tracer gas test, startup of the homeowner’s car was 

found to be the only source of CO. When first started, the vehicle produced high 

5 
concentrations of CO (102 000 mg/m3

) at the tailpipe. The CO was pushed into the living 

space due to the high pressure of the garage relative to the house. This study also found 

that a garage exhaust fan with a measured flow of 131 L/s maintained a negative pressure 

of 4.0 Pa in the garage relative to the house and was effective at preventing CO transport 

from the garage into the house. Since operation of the garage exhaust fan also caused 

depressurization of the house and raised concerns about water heater vent backdrafting, a 

150 mm combustion air/make-up air opening was added to the house and a powered 

induced-draft fan blower, with safety shut-off, was added to the water heater. 

A study by Wilber and Klossner (1997) for a natural gas utility consisted of monitoring 

houses in the greater Minneapolis area where two or more carbon monoxide complaints 

had been investigated by technicians who were unable to identify the CO source. This 

study found CO transport from attached garages was a potential CO source in 74 % of the 

study homes based on an evaluation of the houses and CO events including consideration 

of homeowner interviews, the timing of the events, depressurization tests, CO production 

from appliances, and measurements of pressures between the house and garage. 

However, 30 % of the homes were found to have multiple potential CO sources. It was 

found that the average air leakage from the garage to the homes was over 25 % of the 

total house leakage. Along with the stack effect, this leakage played a key role in CO 

migration into the houses. 

Thomas et al. (1993) conducted indoor, personal, and outdoor monitoring of benzene 

exposure at eleven homes in New Jersey over multiple 12 h periods. A major finding of 

this study is that the concentrations of a majority of the VOCs found in indoor air exceeds 

those measured in outdoor air. This study also found mean benzene concentrations in the 

garages two to fifteen times higher than outdoor air levels and two to three times higher 

than main living area concentrations in three of the four homes having attached garages. 

Indoor benzene levels at homes that did not have attached garages were not elevated far 

above outdoor levels during most monitoring periods. Data from this study suggest that 

under some conditions, an attached garage can introduce as much or more benzene into a 

home’s living areas as the other indoor sources, including tobacco smoke. However, 

since only a few homes could be monitored extensively, the authors cautioned that the 

results here couldn’t be used to make inferences to the general population. 

Mann et al. (2001) recently reported a study on the transport of benzene from

automobiles in attached garages in five homes in the United Kingdom. Concentration 

measurements were made in the house (living room, main bedroom, and room above the 

garage), in the garage, in the car, and outdoors. In the four homes that had cars parked at 

least some of the time in the attached garage, garage concentrations of benzene averaged 

at least 25 times the outdoor level. Concentrations in the houses were much lower than in 

the garages but still averaged at least several times the outdoor concentration. 

Summary 

6 

The primary goal of the literature review was to support a planned simulation study on 

the transport of pollutants from attached garages to the living spaces of homes. Although 

the body of literature on pollutant transport from attached garages to residential buildings 

is small, the studies reviewed provide a good overview of the issues on which to base the 
simulation effort. There is substantial evidence that transport of contaminants from

garages has the potential to negatively impact residential IAQ in either an acute (e.g., CO 

from cold-starting a vehicle) or chronic manner (e.g., VOCs from storage of household 

chemical products). Many questions are raised including the leakage of the HG interface, 

presence of heating and cooling equipment and ductwork in the garage, potential 

contaminant sources in the garage, and potential IAQ control options. 

One conclusion drawn from this literature review is that additional measurements in 

houses with attached garages are needed prior to undertaking the simulation effort. These 

measurements will supplement the data characterizing garage airtightness and the housegarage interface. Another conclusion is that the simulation effort should be focused on 

the airflow modeling of the house and garage rather than the contaminant sources. The 

literature reveals that the potential source types and strengths in garages vary too widely 

to adequately study them in detail. Also, one can assume that any significant airflow into 

the living space from an attached garage is undesirable, and therefore, that airflow is 

more important than any particular source or source strength. However, contaminant 

modeling will be included to illustrate the potential impact of the airflows on contaminant 

concentrations. 

7 
Field Study 

This section describes a small field study of five homes located in the Washington D.C. 

metropolitan area. The purpose of this study is to supplement the data in the literature on 

the airtightness of garages and house-garage interfaces. The data will serve to establish a 

range of conditions for a planned simulation study. 

Houses 

In this study, four single-family homes; buildings A, C, D, and E, and one townhouse; 

building B, were tested. Buildings B and D have tuck-under garages (i.e. adjacent and 

under living spaces), while the remaining buildings have garages adjacent to the living 

spaces. A summary of the building descriptions for the five buildings including floor 

areas and envelope surface areas can be found in Table 1. 

Building A is a split-level single family home with the attached garage adjacent to one 

wall of the living space. This building was constructed in 1964, and has the heating and 

air-conditioning (HAC) equipment and ductwork located in the basement and crawlspace 

respectively. The garage has three openings (two doors and a window) to the outside, in 

addition to the garage door, and is a converted carport. 

Building B is a middle unit, three-level townhouse built in 1988. This building has a 

finished tuck-under garage that takes up part of the lowest level, and shares three surfaces 

with the living space. Due to the tuck-under design, the garage has only one side exposed 

to the outdoors. One of the sidewalls is shared with the unit next door while the 

remaining two sidewalls are shared with the living space. The garage ceiling is also 

shared with the living space of the level above. The lowest level is thermally conditioned 

living space. Both the HAC equipment and the ductwork are located in the living space. 

Building C is a single-family home built in 1908 that has a basement and an attic in 

addition to two main levels. The HAC equipment is located in the basement while the 

ductwork is located in the living space and the attic. The garage is unfinished and is 

adjacent to the living space, sharing one surface with the living space. 

Building D is a single-family home with a tuck-under garage built in 1995. This building 

consists of two floors and a basement, which is mostly below grade. The HAC equipment 

is located in the basement and the ductwork is in the living space. The garage shares three 

surfaces with the living space. The garage is finished and was previously used by the 

builder as an office. 

Lastly, building E is a single level single-family home built in 1968. The garage is 

adjacent to the living space, sharing two surfaces with the living space and one surface 

with a laundry/furnace room that extends out from the living space. The garage was 

formerly used as a living space. 

8 
Table 1. Building Description Summary 

Building A B C D E

Year Built 1964 1988 1908 1995 1968

Type Single-Fam.

split level 

Townhousemiddle unit SingleFamily SingleFamily SingleFamily 

Garage Type Adjacent Tuck-under Adjacent Tuck-under Adjacent 

House-Garage Shared 

Surfaces, # 1 3 1 3 2

House Floor Area, m2 150 140 420 190 160 

Garage Floor Area, m2 49 17 54 20 21 

House Envelope 

Surface Area, m2

 280 140 370 340 330 

Garage Envelope 

Surface Area, m2

 98 7.4 120 29 83

HG Interface Surface 

Area, m2

 22 38 22 45 27 

Method 

Pressurization tests were conducted on the houses to measure the leakage of the house, 

garage, and the HG interface using various configurations to target specific zones and 

boundaries. The pressurization tests were generally conducted according to ASTM 

Standard E 779-99 (ASTM 1999) using blower doors. Three configurations were used 

with buildings B, C, D, and E while two different configurations were used with building 

A. The three configurations used on each of the three houses and one townhouse, shown 

in Figure 1, are as follows: 

1. Blower door in the living space with the garage door open. 

2. Blower door at the house-garage interface with the living space doors open. 

3. Blower door in the living space with the HG interface door open. 

In the figure and the analysis, each building is represented as two zones; a house zone 

and a garage zone separated by the house-garage interface. Arrows indicate the location 

of the blower door for each test and the direction of the airflow from the blower. The 

pressure difference, ∆PHG, is the pressure difference across the house-garage interface. 

The pressure differences across the living space exterior envelope for each test 

configuration are designated as ∆PH, while ∆PG designates the pressure differences across 

the garage exterior envelope. 

9 
Figure 1 Test Configurations for Buildings B, C, D, and E 

Symbol Legend for Figures and Equations

H – exterior envelope of house (living space) 

H’ – house exterior envelope and HG interface combined (H+HG) 

G – exterior envelope of garage 

G’ – garage exterior envelope and HG interface combined (G+HG)

HG – HG interface 

Q# or L – Airflow rate from blower door in configuration # or for surface 

designation (L) 

∆P# or L – Pressure difference across a surface 

C# or L – flow coefficient for Q# or L

n# or L – flow exponent for Q# or L

For building A, the blower door configurations are different because of an exterior door 

located in the garage. This extra door allows for the use of a blower door in the garage in 

addition to the blower door located in the house. The three configurations used on 

building A, shown in Figure 2, are as follows: 

1A.Blower door in the living space with the garage door open (same as configuration 

1 in Figure 1). 

2A.Blower door in the garage with the living space doors open. 

3A.One blower door in the living space and a second blower door in the garage . 

10 
Figure 2 Blower Door Configurations for Building A 

Analysis

For each blower door test, measured airflows were recorded at four to seven pressures 

ranging from 10 Pa to 70 Pa (not all spaces could be pressurized to 70 Pa). The airflows 

and pressures are then fit to a power law equation: Q = C (∆P)n

. The airflow and pressure 

data from each blower door test are logarithmically transformed and fitted with a linear 

regression, yielding the flow coefficients, C, and flow exponents, n, for each 

configuration. A sample plot of typical data is illustrated in Figure 3 for configurations 1 

and 3 in Building E. Configuration 1 includes flow through the house exterior envelope 

and HG interface combined (called H’) and configuration 3 includes flow through the 

combination of the house exterior envelope, H, and the garage exterior envelope, G. 

The flow coefficients and exponents are then used to calculate various leakage 

parameters for each of the buildings. Also, from the blower door data and conservation of 

mass equations, effective leakage areas (ELAs) are calculated for all of the distinct 

interfaces (house and garage envelopes and the house-garage interface) for each of the 

houses. Error analysis and confidence intervals are also calculated for all the parameters 

and the blower door data, as prescribed in the ASTM Standard E779-99 (ASTM 1999).

11 
R2

 = 0.9978

R2 = 0.9926

7.2

7.4

7.6

7.8

8.0

8.2

8.4

8.6

8.8

9.0

2.1 2.6 3.1 3.6 4.1 4.6

ln(∆P) Pa

ln(Q) m3/h

Configuration 3 Configuration 1

Figure 3. Sample plot of blower door test of Building E for two test configurations. 

Calculations for the envelope and house-garage interface parameters for building A are 

different than those for buildings B, C, D, and E due to the different blower door test 

configurations performed on building A (see Figure 2). The calculations for building A 

are described first. 

For test 1A, depressurizing the living space (H), while the garage door is open, 

effectively changes the garage space to an ambient zone. The house zone is bounded by 

the exterior envelope surfaces of the living space, H, and the HG interface surface. The 

reverse is performed for the garage in test 2A, in which the garage zone is bounded by the 

exterior envelope surfaces of the garage, G, and the HG interface surface. Test 3A 

consists of two blower doors simultaneously operating; one in the living space, and 

another in the garage. The pressure difference across the house-garage interface is 

monitored with both fans operating. The pressures in both zones relative to the ambient 

are raised at multiple points while the monitored pressure difference across the HG 

interface is held at approximately zero as measured by a digital micro-manometer. This 

effectively eliminates airflow through the HG while the two blower doors separately 

yield the flows and pressures for H’ and G’. 

Using continuity equations, the power law orifice equation, and Bernoulli’s equation, the 

ELAs for the exterior envelopes of the living space and the garage, as well as the ELA for 

the HG interface is calculated. The flow coefficient and flow exponent, CH and nH are 

determined from the linear regression to the data from the blower door positioned in the 

living space during the simultaneous blower test (test 3A). The coefficient and exponent, 

CG and nG, are determined from the linear regression to the data from the blower door 

positioned in the garage during the simultaneous blower test (test 3A) also. The flow 

coefficients and exponents, CG’, nG’, CH’, and nH’, are obtained from the linear regressions 

12 
to the data from tests 2A and 1A respectively. Corresponding uncertainties at 95 % 

confidence are also obtained from the regressions as described in ASTM Standard 779-

99. The effective leakage areas for the exterior envelopes of the living space and the 

garage, subscripted H and G respectively, are calculated from equation (1): 

( ) 2

n C P ELA

P

ρ

⋅∆ =

⋅∆

 (1)

where C = the flow coefficient for surface H or G, n = the flow exponent for surface H or 

G, ∆P = the reference pressure difference, and ρ = the density of air at standard 

conditions. 

To solve for the ELA of the HG interface, continuity equations at a reference pressure of 

25 Pa combined with the orifice equation are solved for both the house zone and the 

garage zone. The reference pressure of 25 Pa is chosen as opposed to 4 Pa, which is often 

used in ELA calculation, with the intent to eliminate errors associated with extrapolating 

values from outside the range of the experimental data. The pressure of 25 Pa is 

approximately midrange of the blower door data for this building, thus it is used as the 

reference pressure for this part of the calculation. The equation can be written as two 

different yet equal expressions: 

 (2) '

,25 ',25 ,25 ' (25) (25) G GHG G G G G

n n Q Q= −Q = C ⋅ −C ⋅

 '

,25 ',25 ,25 ' (25) (25) H HHG HHH H

n n Q Q= −Q = C ⋅ −C ⋅ (3) 

where QHG, 25 = the airflow rate of the HG interface at 25 Pa. 

Assuming a flow exponent of 0.65 for the HG interface, the flow coefficient at this 

interface is found from: 

0.65

,25

(25)

HG

HG

Q C = (4)

This coefficient is solved two ways, one using the G’ – G equation and one using the 

H’ – H equation. Analytically, the two values for the coefficient should be identical. 

From the calculated value of CHG, the ELA of the HG interface at any reference pressure, 

ELAHG, can be calculated from Equation 1. 

This ELA value is also calculated two ways, one using CHG derived from equation (2) 

and one from equation (3). The reported ELAHG is the average of the two values. 

Uncertainties in the ELA values, at 95 % confidence level, are also propagated through 

these calculations from the errors in the flow coefficients and exponents obtained from

the linear regressions. 

Calculations for the remaining buildings, B, C, D, and E, are also based on continuity, 

ELA, and power law orifice equations. The exterior envelope surfaces of the living space 

and the garage are again designated H and G respectively, and the house-garage interface 

13 
is again designated HG. Applying the law of conservation of mass and the power law 

orifice equation yields a three by three matrix of airflow rates: 

 (5) QH QHG

n Q = C ⋅∆P = + 1

1 1

 (6) QG QHG

n Q = C ⋅ ∆P = + 2

2 2

 (7) QG QH

n Q = C ⋅∆P = + 3

3 3

Here, C1 and n1 are the flow parameters obtained from the data for blower door test 1, C2

and n2 are obtained from the data for test 2, and C3 and n3 are obtained from the data for 

test 3. With three equations and three unknowns, this matrix can be solved for QH, QG, 

and QHG, all at a specific reference pressure across surfaces H, G, and HG. Using these 

three airflow rates, the zone envelope and HG interface ELA’s are calculated at the same

reference pressure using the same ELA equation used with building A: 





 



 ⋅ ∆

=

ρ

P

Q ELA H

H

2

 (8)

Again, uncertainties in the ELA values, at 95 % confidence level are also propagated 

through these calculations from the errors in the flow coefficients and exponents obtained 

from the linear regressions that were applied to the blower door data. 

Results

All flow coefficients and flow exponents used in the calculations for each building are 

shown in Table 2. 

14 
Table 2. Building Orifice Parameters 

 (Units for flow coefficients, C, and flow coefficients uncertainty, ∆C, are m3

/h·Pan) 

1

Assumed value (see Analysis section). 

CH’ 535 ±∆CH’ 45 nH’ 0.67 ±∆nH’ 0.016 

CH 300 ±∆CH 32 nH 0.75 ±∆nH 0.027 

CG’ 1330 ±∆CG’ 72 nG’ 0.59 ±∆nG’ 0.011 

CG 1670 ±∆CG 220 nG 0.46 ±∆nG 0.020 

(CHG)H’- H 160 (±∆CHG)H’- H 67 (nHG)H’- H 0.651 ---- ----

Building 

A 

(CHG)G’- G 176 (±∆CHG)G’- G 136 (nHG)G’- G 0.651 ---- ----

C1 194 ±∆C1 12 n1 0.77 ±∆n1 0.014 

C2 58 ±∆C2 1 n2 0.76 ±∆n2 0.005 

Building 

B 

C3 168 ±∆C3 8 n3 0.81 ±∆n3 0.011 

C1 299 ±∆C1 34 n1 0.80 ±∆n1 0.026 

C2 750 ±∆C2 68 n2 0.65 ±∆n2 0.018 

Building 

C 

C3 1010 ±∆C3 180 n3 0.67 ±∆n3 0.043 

C1 291 ±∆C1 28 n1 0.73 ±∆n1 0.020 

C2 104 ±∆C2 4 n2 0.63 ±∆n2 0.007 

Building 

D 

C3 309 ±∆C3 21 n3 0.74 ±∆n3 0.014 

C1 283 ±∆C1 29 n1 0.73 ±∆n1 0.021 

C2 221 ±∆C2 14 n2 0.63 ±∆n2 0.012 

Building 

E 

C3 474 ±∆C3 27 n3 0.70 ±∆n3 0.011 

Table 3 summarizes the house-garage interface leakage, and its relative magnitude to the 

house and garage envelope leakages and other results. The average air change rate at 50 

Pa (ACH50) for the living space of the five homes in this study is 9.6 h-1, with a range 

from 5.6 h-1 to 12.5 h-1. For the garages, the average ACH50 is 48.4 h-1 with a range from

20.8 h-1 to 106 h-1. The average effective leakage area at a reference pressure of 4 Pa 

(ELA4) for the house exterior envelope is 736 cm2

 with a standard deviation of 169 cm2

and a range from 491 cm2

 to 917 cm2. For the garage exterior envelope, the average 

ELA4 and standard deviation are 1220 cm2 and 1440 cm2

. The corresponding range of 

ELA4 is from 64 cm2

 to 3430 cm2. For the HG interface, the average and range of ELA4

is 157 cm2 and from 33 cm2 to 446 cm2 respectively.

15 
Table 3. House and Garage Envelope Leakage Summary 

Building A B C D E Average σ 

ACH50,H (h-1) 11.6 11.1 7.6 5.6 12.5 9.6 2.9

±∆ (h-1) ----- 1.0 1.2 0.8 1.8

ACH50,G (h-1) 106 26.8 36.7 20.8 51.5 48.4 38

±∆ (h-1) ----- 0.9 5.2 1.1 4.6

ELA4,H (cm2) 917 491 885 756 811 736 169

±∆ (cm2) 100 9 98 19 22

ELA4,G (cm2) 3430 64 1893 166 540 1220 1440

±∆ (cm2) 454 9 98 19 22

ELA4,HG (cm2) 446 114 90 103 33 157 165

±∆ (cm2) 154 9 98 19 22

(ELA4/SA)H (cm2/m2) 3.00 1.71 3.09 2.64 2.24 2.54 0.57

±∆ (cm2/m2) 0.44 0.17 0.46 0.27 0.23

(ELA4/SA)G (cm2/m2) 28.6 1.08 31.79 2.79 4.81 13.8 15.1

±∆ (cm2/m2) 4.75 0.18 3.58 0.42 0.52

(ELA4/SA)HG (cm2/m2) 20.4 2.97 2.35 2.67 1.20 5.91 8.10

±∆ (cm2/m2) 7.32 0.37 2.56 0.56 0.81

NLAG-H 9.5 0.63 10.3 1.1 2.2 4.7 3.8

NLAHG-H 6.8 1.7 0.76 1.0 0.5 2.6 0.8

Leakage values of different buildings can be compared by normalizing the ELA4 by the 

surface area associated with the leakage, designated as ELA4/SA. For the house exterior 

envelope, the average and standard deviation for the ELA4/SA is 2.5 cm2

/m2 and 0.57 

cm

2

/m2 respectively. The average ELA4/SA for the garage exterior envelope is 14 cm2/m2

with a standard deviation of 15 cm2

/m2. The average and standard deviation of the 

ELA4/SA for the HG interface is 5.9 cm2

/m2 and 8.1 cm2/m2 respectively. The relatively 

large standard deviations of the ELA4 and ELA4/SA for the three surfaces can be 

attributed to building A, which when compared to the rest of the homes, appears to be 

very leaky in all respects. 

To quantify the amount of leakage of the garage envelope relative to the living space 

envelope and the HG interface relative to the living space envelope, two parameters are 

used: NLAG-H ratio and NLAHG-H ratio. The NLAG-H ratio is the garage ELA4 normalized 

by the garage envelope surface area divided by the living space ELA4 normalized by the 

16 
house envelope surface area. The NLAHG-H ratio is the HG interface ELA4 normalized by 

the surface area divided by the living space ELA4 normalized by the house envelope 

surface area. The average and standard deviation of the NLAG-H is 4.7 and 3.8, 

respectively. For NLAHG-H, the average and standard deviation is 2.6 and 0.8, 

respectively.

17 
Discussion 

Broad conclusions cannot be drawn from this study due to its small sample size; but this 

study does provide useful data related to the transport of air and pollutants from attached 

garages. Despite the small sample size, the broad range of building ages and designs and 

airtightness values serve to establish a range of conditions useful to the simulation effort. 

As a reference point, these results are compared to a similar set of data from 25 homes in 

Canada (Fugler et al. 2002). Garage airtightness for the Canadian homes average about 

47 h-1 at 50 Pa with a range from 11 h-1 to 97 h-1, while the five homes studied here 

average 48 h-1 with a range from 20 h-1 to 100 h-1. These reported values for both studies 

are consistent. Fugler also reported a range of house-garage interface ELAs from 4 cm2

 to 

400 cm2

 with an average of 140 cm2. This study shows a range of HG interface ELAs 

from 33 cm2

 to 450 cm2 with an average of 160 cm2. This study indicates the leakages of 

the house-garage interface and of the house exterior envelope, when normalized by 

surface area, are similar for 4 of the 5 houses. Fugler also found house-garage interface 

leakage to be similar to house exterior envelope leakage, which serves to confirm the 

reasonableness of this study’s results. A difference between this study and that by Fugler 

is that homes in Canada are typically tighter than homes in the U.S. 

Sherman and Dickerhoff (1998) found for 12 902 homes throughout the U.S., the average 

ACH50 is 29.7 h-1 with a range from 0.47 h-1 to 83.6 h-1. While the average (9.6 h-1) 

ACH50 for the five houses in this study are significantly lower than that reported by 

Sherman and Dickerhoff, the values measured in this study are well within their range. 

However, a few conclusions may be drawn for the buildings in this study. Of the five 

buildings examined here, the garages are never as tight as the living space. In fact, the 

garages were all at least twice as leaky as the living spaces, as indicated by the measured 

ACH50.This is also the case when looking at the surface area normalized ELAs of the 

house and garage exterior envelopes. If the very leaky building A is ignored, the older 

houses generally tend to be leakier than newer homes, possibly indicating age as a factor 

residential building airtightness. 

The five houses tested range in garage type, size, and use. This large variation carries 

through to the airtightness results. One of the garages (that of building E) was previously 

a living space, while another garage (that of building D) was originally used as an office 

by the builder. The garage envelopes in these buildings may be purposefully tighter than 

is generally the case as they have lower ELAs as normalized by surface area (ELA/SA). 

On the other hand, one garage (that of building A) is a converted carport, which could 

explain why it is extremely leaky. Some garages examined in this study are finished and 

have weather stripping while others have exposed studs with visible leaks. This wide 

variation in garage construction and use, considered along with the variation in leakage 

parameter values, suggests more extensive field measurements are needed. 

18 
Acknowledgements 

This work was sponsored by the U.S. Department of Housing and Urban Development, 

Office of Healthy Homes and Lead Hazards Control under Interagency Agreement No. IOPC-21525. 

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21