Q NBS TECHNICAL NOTE 992

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standard

An InvestI Infiltration C

and Mechanisms for a

Townhouse

m

NO. 992

1979

C.2
NATIONAL BUREAU OF STANDARDS

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St Wttn^W^m

UT 2 2 1979

An Investigation of Air- Infiltration

Characteristics and Mechanisms

for a Townhouse

S. J. Treado, D. M. Burch, and

C. M. Hunt

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Sponsored by

Naval Facilities Engineering Command

U.S. Navy

Washington, D.C. 20390

Directorate of Civil Engineering

U.S. Air Force Washington, D.C. 20330 and

Office of Chief of Engineers

U.S. Army

Washington, D.C. 20314

.•.^"'-W^ °'' Cn ^f

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Teclnnology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued August 1979
National Bureau of Standards Technical Note 992

Nat. Bur. Stand. (U.S.), Tech. Note 992, 36 pages (Aug. 1979) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. 003-003- Price $

(Add 25 percent additional for other than U.S. mailing)
AN INVESTIGATION OF AIR-INFILTRATION CHARACTERISTICS AND

MECHANISMS FOR A TOWNHOUSE

S. J. Treado

D. M. Burch

C. M. Hunt

ABSTRACT

Air infiltration measurements using a tracer-gas technique and the pressurization technique were performed on a three-bedroom townhouse having

a gas-fired, forced-air furnace system, in order to quantify the amount

of air infiltration due to various mechanisms. These mechanisms include

combustion and draf t-diverter air requirements, air leakage from supplyair ducts, and air leakage through the solid parts of the building

envelope as well as air leakage through cracks around windows and doors.

A thermographic survey was also performed in conjunction with pressurization of the structure, in an attempt to identify specific leaks. An

apparatus for measuring the air permeability of building materials was used to analyze the significance of air permeation through solid building elements. Based on the findings of the study, general guidelines

are presented for reducing air infiltration in residences.

Key Words: Air infiltration, residential; air permeability of houses:

energy conservation; pressurization technique; tracer-gas

technique.

I

111

TABLE OF CONTENTS

PAGE

ABSTRACT ill

1. INTRODUCTION 1

2. DESCRIPTION OF THE TEST HOUSE 2

3. INSTRUMENTATION AND MEASUREMENT TECHNIQUE 5 4. RESULTS AND ANALYSIS 6

5. THERMOGRAPHIC SURVEY 13

6. CONCLUSIONS AND RECOMMENDATIONS 14

REFERENCES 21

ACKNOWLEDGMENTS 21

APPENDIX. AIR PERMEABILITY THROUGH BUILDING MATERIALS . . 22

NOMENCLATURE 30

SI CONVERSION CHART 31
II
1. INTRODUCTION

This project was carried out under the sponsorship of the Tri- Services

Committee to provide technical information that could be used in future

energy-conservation programs for existing buildings.

Air infiltration rates in residences characteristically range between

1/2 to 1-1/2 air changes per hour under typical winter conditions,

and they can account for as much as 1/3 of the heating energy requirement. It should be possible to reduce air-infiltration rates for

residences to 1/4 air change per hour without degrading the indoor

air quality or comfort conditions, providing there is no unusually

large emission or release of contaminants or moisture indoors. A

minimum fresh air supply of 5 cfm per person is recommended by ASHRAE,

a figure which corresponds to an infiltration rate of 1/8 air change

per hour for a family of four in an average size (10,000 ft ) house.

Recent studies [1,2] have shown that the use of conventional measures

(such as caulking and weatherstripping) to seal air leakage paths

around windows and doors does not always produce significant reductions

in the overall air-leakage rate. These results suggest that significant

amounts of air leakage occur by way of other mechanisms such as duct

leaks, combustion and draf t-diverter air requirements, and air leakage

through the walls, ceiling, and floor. Additional air leakage will

occur when the bathroom or kitchen exhaust fans are operated. These

other mechanisms are discussed in the following pages.

The seams where supply air duct sections of an air-distribution system

are joined often leak small quantities of air when the heating plant

operates. This may cause a small negative pressure to be imposed on portions of the living space of a residence, which in turn induces

additional air infiltration.

When fuel-fired furnaces are located within the conditioned space of

a residence, they often draw their combustion and draf t-diverter air

directly from the living space. Again, this imposes a negative pressure on the conditioned space and additional outside air is drawn into

the conditioned space when the furnace operates.

In a recent study by the Canadian National Research Council [3], the

pressurization technique was used to investigate the amount of air

leakage occurring through the various components of the exterior envelopes of six residences. Significant amounts of air leakage were

shown to occur through the walls and ceiling. In another study [4],

the use of low-permeability polystyrene sheathing instead of wood-fiber

sheathing on a residence produced a 29% reduction in the overall airinfiltration rate. As in the case of the previous study, these results

suggest that air leakage occurs through solid building components.

From these studies, it was not clear whether the air leakage through

the solid elements of the buildings was due to air permeation through

the building materials (similar to flow through porous media) or due

1 la
to fluid flow through hair-line cracks in building materials.

In treating and reducing air infiltration in residences, an understanding of the nature of the air penetration process is necessary.

The objective of the present study was to experimentally investigate

the amount of air infiltration occurring due to various mechanisms.

Using a two-story townhouse as a test house, air pressurization and

tracer-gas measurements were performed to quantify the amount of air

infiltration attributable to duct leakage, combustion and draftdiverter air requirements, leakage around doors, windows, and electrical outlets, and leakage through other pathways in the walls and

the ceiling. The study is described herein.

2. DESCRIPTION OF TEST HOUSE

The test house, built in 1970, was an end-unit two-story townhouse

having a slab-on-grade floor. The floor area of the living space was approximately 1,212 ft . A photograph and floor plan of the test

house are given in Figures 1 and 2, respectively.

The walls of the test house were of nominal 2x4 inch wood-frame con- struction with full thickness of glass-fiber blanket insulation placed

in the wall cavities. As shown in Figure 1, brick veneer covered a portion of the walls. The remaining upper portion of the side and rear walls was covered with aluminum siding. Five and one-half inches

of loose-fill glass-fiber insulation were installed over the secondfloor ceiling. Construction details of the walls and ceiling are given

in Table 1. The inside wall surfaces consisted of gypsum board, with

the exception of one exterior wall of the family room which was covered

with wood paneling.

Table 1 . Description of Exterior Building Components

( Ins ide-to-Outside

)

WALL

1/2-inch

2x4

1/2-inch

CEILING

1/2-inch

2x6

gypsum board

wood studs (with R-11 glass-fiber blanket insulation

with asphalt-impregnated kraft paper vapor barrier)

asphalt-impregnated wood-fiber sheathing

brick veneer or aluminum siding «i__. j

gypsum board

joists (with 5-1/2 inches of loose-fill glass-fiber

insulation)
Figure 1. Photograph of the test house,
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The window systems were single-pane, double-hung wood-sash windows with

screens, except for the bay window. The window area was 127 ft or approximately 10% of the floor area, which is representative of residential construction. The windows and doors were equipped with good

weatherstripping and appeared to be of reasonably tight construction.

The heating plant consisted of a gas-fired forced-air furnace having a rated input capacity of 90,000 Btu/h. The heating plant was located in

a utility closet on the first floor of the test house, as indicated in

Figure 2. Combustion and draf t-diverter air were drawn from the

living space by way of a louvered ventilation opening in the door of

the utility closet. Supply-air ducts passed through interior partition

walls. No supply-air ducts were located in the attic. Each floor was serviced by a single return register.

The front of the test house had a northwestern exposure and faced an

opposite row of townhouses, while the rear was partially sheltered by

a small hill and several evergreen trees.

3. INSTRUMENTATION AND MEASUREMENT TECHNIQUE

Air-infiltration rates under natural conditions were measured using a tracer-gas technique. For these measurements, a small quantity of

sulfur hexafluoride (SF^) gas (approximately 4 cc) was introduced into

the conditioned space of the test house by using a hypodermic syringe

at the common air return of the heating plant. When the tracer gas was injected, the blower of the heating plant was operated for approximately

10 minutes in order to produce a uniform distribution of tracer gas

throughout the house. The concentration of SF^ within the conditioned

space was monitored by taking periodic air samples at a supply air

register when the blower of the heating plant operated. The concentration of SF^ in air samples was measured using a semi-automated sampling

system and a gas chromatograph equipped with an electron capture

detector [5]

.

The decay rate of the tracer gas was treated as a first-order exponential process expressed by the equation:

^ = - i c ^^^

dt V

where v = rate at which air enters or leaves the enclosure, ft /h

V = volume of the enclosure, ft

c = concentration of tracer gas at time t.

Solving eq . (1) yields

-^.t

I

c = c_eo^ (2)
or

^O

Here I is the number of volume changes per hour, and c is the initial

concentration of tracer gas.

The natural logarithm of the relative concentration (c/c ) was plotted

as a function of time and the air change rate (I) was determined from

the negative slope of a best-fit straight line passing through the

data points.

Combustion and draf t-diverter air intake was measured by attaching a 1-foot long duct extension to the louvered opening of the utility

closet door and sealing the remainder of the opening. The air speed was measured at various subareas of the duct using a hot-wire anemometer. The total volume of air flow through the duct was determined by

summing the measured flows through the subareas. The air-delivery

rate of the furnace blower was measured in a similar fashion, at the

return registers located in the corner of the family room and in the

upstairs hall.

The air tightness of various parts of the exterior envelope of the tesi

house was determined using the pressurization technique. For this

technique, a centrifugal blower was mounted on a sheet of plywood that was installed so as to replace one half of a sliding glass door at the rear of the test house. The maximum air-delivery rate of the blower

was approximately 1860 ft /min, as installed in this test configuration. Outdoor air, upon passing through the blower, passed through an air-flow measuring device, after which it was delivered to the conditioned space of the test house. The air-flow measuring device consisted

of a honeycomb air straightener, an array of pitot tubes for measuring

the total pressure, and an another array of pressure ports for measuring

the static pressure. The pressure difference between these arrays,

which is the velocity head, was measured with an inclined manometer

having a resolution of 0.003 inches of water column. The air intake of

the blower was equipped with a damper for varying the air-delivery rate.

The operation of the blower pressurized the interior of the test house

and caused an inside-to-outside pressure gradient to be developed. This

pressure difference was measured using two equal length flexible tubes

(one positioned at the inside surface of the building envelope, the

other positioned at a corresponding location at the outside surface)

connected to an inclined manometer or an electronic pressure transducer.

4. RESULTS AND ANALYSIS

Sets of consecutive air-infiltration measurements were performed using

the tracer-gas technique to investigate the increase in the rate of air
Infiltration due to operation of the furnace blower. During the bloweron measurements, the furnace blower was operated continuously. For the

blower-off measurements, the furnace blower was operated for ten minutes

after injection of tracer-gas to provide uniform mixing. Then the

blower was turned off for 60 minutes, after which the fan was turned

on again for ten minutes to provide uniform mixing prior to sampling

the decay in tracer-gas concentration.

A comparison between infiltration rates for cases with and without

blower operation is given in Table 2. It can be seen that blower

operation caused an average increase in the rate of air infiltration,

AI, of approximately 0.11 air changes per hour or a 20% increase. The

principle driving forces for air infiltration, the inside-to-outside

temperature difference and wind speed, are seen to be approximately

equivalent for both the blower on and off measurements for each of the

sets of measurements. This increase in air infiltration was principally

attributed to air leakage at the seams of supply air ducts located in

interior wall cavities. The wall cavities were separated from the

attic by wood plates which ran horizontally along the top of the wall.

These data were obtained during both winter and summer measurements.

Table 2. The Effect of Blower Operation on the Rate of Air

Infiltration

T^ - Tq Wind Speed Furnace Blower I AI

°F mph Operation h~^^ h

-*-

-5 2.5 ON 0.38 0.13

-5

-5

2.5

2.5

OFF

ON

0.25

0.31

0.06

-6.4

-6.4

2.0

2.0

ON

OFF

0.29

0.13

0.16

0.11

0.09

-6.4 2.0 ON 0.24

-6.4 2.0 OFF 0.15

39.5

41.7

3.0

2.1

ON

OFF

0.62

0.50

0.12

where T. = indoor air temperature, °F. Av.Al = 0.11

T = outdoor air temperature, °F.

Under winter conditions assume average I = 0.56 h (obtained

by averaging the fan-on and fan-off values measured under the

40°F temperature differential condition)

^1 = -^ = 20% increase

I .56
Based on conservation of mass considerations, if we assume that air

which leaks from supply air ducts and furnace jacket is lost from

the conditioned space, then an equal amount of replacement air must

be drawn into the conditioned space through the exterior building

envelope, or

fi . V . f . ; (4)

where AI = induced air infiltration rate, h~

V = volume of the enclosure, ft

f = fraction of the air delivered by the furnace blower that

is lost by way of duct leaks

u = air-delivery rate of the blower, ft /min.

For this particular test house under the conditions tested,

V = 9369 ft-^, AI = 0.11 h"^ and u = 769 ft^/min. From eq. (4), the

fraction of the air delivered by the furnace blower that is lost by

way of duct leaks is found to be 2.2%. These results indicate that

duct leaks which appear to be of minor significance may substantially

increase the rate of air infiltration for a house.

The combustion and draf t-diverter air requirements were measured and

found to be 49 ft /min. This corresponds to an induced air infiltration

rate of 0.31 air changes per hour.

Most natural gases are comprised predominantly of methane. Neglecting

the presence of other hydrocarbons, the chemical equation for the complete combustion of methane is:

CH^ + 2O2 + 2 (3.76 N2) -> 2H2O + CO2 + 2 (3.76) N2.

The burning of 1 mole (1 ft ) of methane is seen to require 9.52 mole

(9.52 ft ) of air. Furnaces typically require 40% excess air in order

to achieve complete combustion. Therefore, a furnace typically requires

13.3 ft of combustion air for every cubic foot of gas burned. The

draf t-diverter air requirement will generally be 40% more than the com- bustion air requirement, or 1 ft of gas burned requires 18.7 ft of

draf t-diverter air. Therefore a total of 32 ft of air is required for

every cubic foot of gas burned.

When the furnace of this test house operated, it was rated to consume

gas at 90 ft /h. Based on the foregoing considerations, the combustion

and draf t-diverter air requirements are estimated to be 48 ft /min.,

compared to a measured value of 49 ft /min.

The relative air tightness of various parts of the exterior envelope of

the test house was determined using the pressurization technique. The

experimental procedure was to pressurize the test house to approximately

0.1 inches of water, using a centrifugal blower. A stagnation pressure

of approximately 0,1 inches of water will result when a 14-mph wind is

8

mi
decelerated by a wall. Various parts of the exterior envelope of the

test house were consecutively sealed using sheets of polyethylene film

attached with duct tape to the inside surface. During each phase of

these measurements, the air-delivery rate of the blower was adjusted

so that the inside-to-outside pressure difference remained constant.

Reductions in flow rates achieved by sealing various parts of the

exterior envelope are given in Table 3. In cases where only a fraction of the total area of a component was sealed, the corresponding

reduction that would have occurred if the entire interior surface of

that component had been sealed was estimated by multiplying the

observed reduction in flow rate by the ratio of total surface area

of the component to the surface area sealed. If 50% of the ceiling

was sealed, the measured reduction would be doubled to obtain an estimate of the total reduction expected had 100% of the ceiling

been sealed.

When windows were sealed, the sealing included the window frames.

When exterior walls containing windows were sealed, the polyethylene

sheet was stretched across the window and the window area was subtracted from the total area sealed, leaving the actual wall area

sealed. Electrical outlets were sealed for measurement and left

sealed, and no ceiling light fixtures were present. Since the windows,

doors and electrical outlets were left sealed during most of the additional testing, the total prorated reduction percentage values are adjusted to compensate for the reduced available leakage area.

Air flow through cracks in buildings can be represented by the following relation [6,7]:

Q = a(Ap)^ (5)

3 where Q = flow rate through crack, ft /min

a = flow coefficient

Ap = pressure differential across crack, in H2O

n = a parameter typically ranging between 0.6 and 0.8.

From this relation, it is seen that the rate of air flow does not vary

linearly with the pressure differential. As a result, the measured

reductions listed in Table 3 are not strictly comparable because the

measurements were obtained at different inside-to-outside pressure

differentials. However, analysis indicates that inaccuracies due

to this factor have a small effect on the results and conclusions

drawn from this table. An example is given below.

In order to adjust the air flow rates listed in Table 3 to a common

inside-to-outside pressure difference, it would be necessary to know

the flow rate relation for the individual leakage paths. Each leakage path may have different values of a and n from eq . (5). As a worst case example, examine the floor/wall interface component from

Table 3.

9
Table 3. Reduction in Flow Rates Achieved by Sealing

Various Parts of Exterior Envelope 1

total

measured prorated

reduction reduction reduction

area AP (initial for area over sealed in. H,0 value) sealed entire area Component % cfm % %

Exterior Wall^ 21 .12 105(1605) 6.5 27.9

Ceiling^ 35 .12 80(1655) 4.8 12.3

Party Wall 30 .13 25(1630) 1.5 4.5

Windows and Doors 100 .110 75(1680) 4.5 4.5

Electrical

° outlets m

° Exterior walls 100 .065 52(1603) 3.2 3.1 W

Party walls 100 .13 25(1805) 1.4 '

Floor/wall interface 25.5 .065 110(1860) 5.9

' J

Unsealed air leakage 23.3 1 paths such as wall/

ceiling interface,

or unaccounted-for

areas

^ Does not include wall/ceiling interface

Measurements of the leakage through this component were performed at

Ap = 0.065 in H^O. The initial flow rate was 1860 cfm and the reduction was found to be 110 cfm, for a reduction of 5.9%. The reduction in

flow rate is equal to the leakage through the floor/wall interface.

From eq. (5)

Q^ = 1860 = a^ (.065)

Qj^ = 110 = a^ (.065)^

where

Qrp = initial flow rate, cfm

Qj^ = reduction in flow rate, cfm

ar^ = total flow coefficient

10
a^ = reduction flow coefficient

nm = initial exponent

n^ = reduction exponent

In order to calculate flow rates which would occur at a different

AP, it would be necessary to know the values of nrp and nT^, which

could be determined through a series of measurements at different Ap

va lue s

.

If the values of n-r, and n^ are the same, the percentage reduction

would be the same at any AP, and would be independent of n. If the

values of n,p and n^ are different, the percentage reduction would

be different at a different AP. The magnitude of the difference

between the measured percentage reduction value and the percentage

reduction value which would occur at a different AP would depend

upon the difference between the value of Op and n^,, and the amount

of pressure difference adjustment. Larger pressure difference adjustments would cause larger differences between the percentage reduction

values at the initial AP and the percentage reduction values at the

adjusted Ap. Larger differences between nrp and nr> would also increase

the difference between percentage reduction values at different pressure differentials.

Since the values of n^, and n^ are not known in this case, it is

possible that the percentage reduction which was measured at AP = 0.065 in H2O would have been different at a different AP. To estimate the possible magnitude of this effect, flow rates were calculated

using various combinations of n values, ranging from 0.65 to 0.75.

Adjusting the flow rates to a pressure differential of 0.12 in H2O

yields percentage reductions which are in agreement with measured

values to within approximately 5.6% full scale (in this case 23.1% + 5.6%).

This example represents a worst case, a$ most of the measurements were made at pressure differentials of 0,12 +.01 in. H2O. The only other

measurement made at 0,065 in. H2O, exterior wall electrical outlets,

represents a total reduction of 3.1%, so error in its determination

would have a small effect on the overall percentage reduction values.

From Table 3, it is seen that 40.2% of the air leakage observed during

pressurization tests occurred through the exterior walls and the ceiling,

Only 8.9% of the air leakage occurred through cracks around the windows,

doors, and electrical outlets. It was hypothesized that air leakage

through solid components of the building envelope could be due to

permeation of air through building materials (similar to flow through

a porous media). Air permeation rates through building materials

similar to those comprising the envelope of the test house were measured

in the laboratory (see appendix). These measurements showed that air

permeation through solid building materials was insignificant compared

11
to the overall rate of air infiltration for the test house. It was concluded that air leakage through solid components of the building

envelope occurred through joints and cracks where building materials

come together.

Of particular interest is the amount of air infiltration that occurs

as a result of the various mechanisms under typical conditions. The

overall rate of air infiltration corresponding to a 50% duty cycle

for the furnace blower was obtained by averaging the blower on and off

data obtained during the coldest conditions listed in Table 2, yielding

0.56 h or 87.4 ft /min. The values given in Table 2 were obtained

when the furnace burner was off. The induced air—infiltration rate

needed to satisfy combustion and draf t-diverter air requirements was taken to be one half the figure of 49 ft /min. or 24.5 ft /min. Thus,

the overall air-«inf iltration rate for the test house is equal to 111.9

ft /min. or 0.717 h , under these conditions. Combustion and draftdiverter air accounts for 21.9% of the air leakage in this example.

This contribution will vary with different climatic conditions and

length of furnace cycle.

Similarly, operation of the furnace blower has been shown to produce

an increase in air leakage amounting to 0.11 air changes per hour, or

8.6 ft /min for a 50% furnace duty cycle. In this example, that would

account for 7.7% of the overall leakage. The remainder of the leakage

will "flow through" the exterior envelope of the house, entering the

house at some points and exiting at others.

For the case of air flow through various parts of the exterior envelope,

the rates of natural air infiltration through various components were

assumed to be proportional to corresponding reductions in flow rates

observed during the pressurization tests. This assumption may not be

representative of natural conditions as a large pressure differential

of constant magnitude across the entire envelope of a residential structure may rarely, if ever, occur, A more typical situation would be that

of high outdoor static pressure at a windward wall, low static pressure

at a leeward wall, and positive inside pressure at the ceiling due to

stack effect. However, the results presented here are believed to

indicate potential leakage paths under natural conditions and identify

areas which could be improved to reduce air infiltration. The results

of this analysis are given in Table 4.

12
Table 4. Breakdown of Potential Air Leakage Paths Under Winter

Conditions.

Assume Overall Infiltration Rate

I = 0.717 h"^ (or 111.9 ft^/min)

For a 50% Duty Cycle for Furnace

ft-^/min.

Operation of Furnace Blower 8.6 7.7

Combustion and Draf t-Diverter Air

Total

24.5

33.1

21.9

29.6

Leakage through Exterior Envelope

Components:

Total

ft^/min %

70.4

Exterior walls

Floor/wall interface

Ceiling of second story

Party wall

Cracks

° Windows and doors

° Electrical outlets

Unsealed air leakage paths

22.0 19.7

18.2 16.3

9.7 8.7

3.5 3.1

3.5 3.1

3.5 3.1

18.4 16.4

5. THERMOGRAPHIC SURVEY

A thermographic survey of the structure was performed, in combination

with operation of the pressurization fan, in an attempt to visually

document some of the air-leakage paths. An infrared television system,

which is sensitive to the total thermal radiation emitted and reflected

from a surface, was utilized for this purpose. This system has been

previously described [1] as well as the test procedure [8].

The testing method involved both interior and exterior scans of the

structure, first under normal conditions and subsequently with the

pressurization fan operating. Exterior scans were done with a positive inside pressure and any air leaks, if visible at all, were expected to be indicated by the appearance of warm spots. Interior

scans were performed in combination with a depressurization of the

house, with air leaks indicated by cold spots. By comparing the temperature patterns obtained before and after fan operation, any relative

13
changes could be attributed to air leakage. It should be noted that

the absolute values of the surface temperatures were not compared,

because without a reference, adjustment of the thermographic camera to

identical absolute radiance temperature level was difficult due to the

time interval separating the "before" and "after" comparisons. The use

of color thermograms rather than gray-tone thermograms was seen to be

of much value in enabling an accurate resetting of the absolute radiance

temperature level to allow comparisons of the relative temperature profiles of the "before" and "after" thermograms. The absolute radiance

temperature level was adjusted until the coldest spot was displayed with

the color corresponding to the coldest temperature band.

5.1 Results of Thermographic Survey

For the exterior scan, thermograms were initially made under normal

conditions (see Figure 3a). The rectangular object in the lower left

is a piece of foam insulating board, the surface temperature of which

would indicate the outside air temperature (29°F). The temperature

spectrum from the coldest region (coded black) to the warmest region

(coded yellow) is displayed at the bottom of each color thermogram.

The total temperature band covers 9°F, so that the difference in tem- _, perature between adjacent colors is 0.9°F. 11

After the pressurization fan was turned on, shots were made every ten

minutes for a period of one hour (see Figure 3b). Almost immediately

a warm spot appeared on the aluminum siding covering the upper part

of the second floor exterior end wall. Further examination found that _, located inside the warm spot was the upper portion of the furnace I

closet, which contained air ducts and vent pipes. This duct chase was nominally separated from the living space by drywall, but not sealed

from the attic, and also separated from the first floor by the ceiling

drywall.

From these factors, it was surmised that the air leakage path responsible for the warm spot is through duct leaks or leakage through the

interior walls to the duct chase.

The interior scan was performed in a similar manner. No significant

differences were observed that could be attributed to fan pressurization, with the exception of a slight effect near the floor-wall !

interface under a bay window (see Figure 4a, b) . Several traditional

leakage paths were documented, including the front door (no storm

door) (Figure 5), closet corner (Figure 6), and window/corner

(Figure 7). The hot air supply register appears very warm.

6. CONCLUSIONS AND RECOMMENDATIONS

This study showed that, when the furnace of the test house operated

with a 50% duty cycle, approximately 30% of the overall rate of air

infiltration for the test house was due to the operation of the blower

14
Figure 3a. Exterior thermographic scan under normal conditions,

Figure 3b. Exterior thermographic scan while pressurized.

15
Figure 4a. Wall/floor interface under

normal conditions.

Figure 4b. Wall/floor interface while

pressurized.

16

i\
d

I

Figure 5. Front door while pressurized,

17
Figure 6. Closet corner while pressurized,

18
Figure 7. Window corner while pressurized,

19
and the burner of the furnace. Air leakage through the walls and ceiling accounted for 31% of the overall air-infiltration rate. A surprising finding was that air leakage through cracks around windows, doors,

and electrical outlets accounted for only 6% of the overall airinfiltration rate. Separate air-permeability measurements performed on building materials similar to those of the test house showed that air

leakage through solid elements of the test house was not due to air

permeation. It was hypothesized that this leakage was due to air leakage through joints and cracks between building materials, such as baseboard cracks, etc.

Based on the results of this study, substantial reductions in the rate

of air infiltration could be achieved by:

° Modifying the combustion closet so that required combustion and

draf t-diverter air are taken from the outdoor environment instead

of the conditioned space of the house;

° Eliminating duct leaks. This may be accomplished by using a suitable duct sealant, either during construction or as a retrofit measure; and

° Eliminating air leakage through joints and cracks between build^l

ing materials and where the building envelope is penetrated by

service outlets of various types. At the time of construction,

this may be accomplished by installing a continuous membrane of

polyethylene between the wood framing and stud cavities and the

interior dry wall.

If these measures had been carried out on this particular test house,

the overall rate of air infiltration could have been reduced by more

than 50%.

20
REFERENCES

D. M. Burch and C. M. Hunt, Retrofitting an Existing Wood Frame

Residence for Energy Conservation — An Experimental Study, NBS

Building Science Series 105, (1978).

B. A. Peavy, D. M. Burch, F. J. Powell, and C. M. Hunt, Comparison

of Measured and Computer-Predicted Thermal Performance of a FourBedrooom Wood-Frame Townhouse, NBS Building Science Series 57,

(April, 1975).

G. T. Tamura, Measurement of Air Leakage Characteristics of House

Enclosures, ASHRAE Transactions, Vol. 81, Part 1, pp. 202-211,

(1975).

C. F. Sepsy, Energy Usage Comparisons of Identical, Non-Inhabited,

Residential Homes with Two Types of Insulation and a Program Simulation of the Required Heating Loads, Environmental Control Group,

Department of Mechanical Engineering, Ohio State Univ., Columbus,

Ohio, (1978).

C. M. Hunt and S. J. Treado, A Prototype Semi-Automated System for

Measuring Air Infiltration in Buildings Using Sulfur Hexafluoride

as a Tracer Gas, NBS Technical Note 899, (March, 1976).

Larson, G. L. , D. W. Nelson, and C. Braatz, Air Infiltration Through

Various Types of Brick Wall Construction, ASHVE Trans. 35_, (1929).

Larson, G. L. , D. W. Nelson, and C. Braatz, Air Infiltration Through

Various Types of Wood Frame Construction, ASHVE Trans. 36_, (1930).

Kronvall, J., Testing of Houses for Air Leakage Using A Pressure

Method, ASHRAE Trans. Vol 34, Part 1, (1978).

ACKNOWLEDGMENTS

The authors wish to acknowledge the contributions of Marc LeMay, who

constructed the permeameter and carried out the installation of instrumentation in the test house. The authors would also like to thank

Mrs. Mary Reppert for her many editorial comments which substantially

improved the quality of the manuscript.

21
APPENDIX

AIR PERMEABILITY THROUGH BUILDING MATERIALS 1

n

22
AIR PERMEABILITY THROUGH BUILDING MATERIALS

Theory

ssults of air-infiltration measurements performed on a three-bedroom

fbwnhouse indicate that substantial air infiltration occurs through the

solid components of a building such as its walls, ceiling, etc. A

possible explanation for this effect is that air may permeate directly

through the material. Fluid flow through a porous medium is governed

by Darcy's Law, or

= (1) "^ 3P

lere Q = superficial flow rate per unit surface area, ft/sec

k. = permeability of porous medium, ft

y = dynamic viscosity of air, lbf*sec/ft

X = distance, ft

2 P = pressure, Ibf/ft .

»r the case of constant y and k, the foregoing equation can be interated across a building component of thickness (L) to yield:

Q^ = ii . ^ = (4) . AP

li L vL

(2)

•Here AP is the pressure difference across the building element. In the

; foregoing equation, we see that the air flow rate through a building

: component is directly proportional to the pressure difference across

the component and the air permeance /_k\ of the component. ^

yL

Applying eq. (1) to a composite building component having n layers

and making use of continuity relations at the adjoining boundaries,

we obtain;

I

Qo

= AP

' k'-L (%)k'2 + + ^k 'n (3)

Here AP is the pressure difference across the composite building component and / yL \ is the resistance to air permeation of the i-th layer,

^k~'i

Taking the logarithm of eq. (2), we obtain:

N

InCQ^) = -In Z (i^) +ln(AP)

i=l k

^ (4)

From this relation, we see that ln(Q ) is a linear function of In (AP).

A plot of In(Q^) versus In(AP) yields a straight line of unity slope.

23
2. Description of Apparatus

A prototype device, called a permeameter, for measuring the air permeability (k) of building materials in the laboratory was designed.

A cross section of the apparatus is shown in Figure A-1. The device

consists of a motor-driven syringe with variable speed control which

delivers air to a plexiglas cylindrical specimen holder. When the

motor operates, the plunger of the syringe is pushed at a constant

rate, which causes a constant air flow to be delivered through a circular specimen. The apparatus contains a pressure port for measuring the corresponding pressure drop across the specimen. This pres-' sure drop was measured either with a mercury manometer or a slant

gage having a sensitivity of less than 0.003 inches of water.

The plunger of the syringe is depressed by a piston running on a threaded rod. The rod is rotated by an electric motor and communicates

with the motor through a cone-shaped drive assembly. Adjustment of the

point of contact between the drive wheel and the cone allows for a continuously variable "gear ratio" from 1:1 through 1:3. Further variation in flow rate is obtainable by alteration of the syringe size.

The apparatus was tested for leaks by mounting an impermeable specimen

in the holder and using a gas chromatographic leak detector. 'In this

procedure, SF/- tracer gas was introduced into the system via the syring«

simultaneously effecting an increase in air pressure. The assembly was subsequently scanned with the probe of the detector, allowing accurate

identification and elimination of any leaks.

3. Experimental Procedure

Each specimen was cut so as to fit tightly into the end of the cylindrical specimen holder and was sealed by applying a room-temperature

vulcanizing silicone rubber at the interface of the specimen and the

cylindrical holder. The full face area of the specimen was available

for flow.

After a specimen was mounted in the apparatus, it was exposed to

various flow rates corresponding to pressure differences between 0.1

and 0.5 inches of water. The logarithm of the flow rate (Qq) was

plotted as a function of the logarithm of the pressure difference

(AP). a least-squares procedure was applied to the raw data yielding

a best-fit straight line when a mean air permeance _k for the data set

is calculated from the relation: yL

N

4 = izl (5)

Z (APi)

i=l

24

2
U

(U

4-1 CO6

bO

•HiH

•H

•HiH

•HX>

Cfl

•H

XI

CU

CO

CO

+J

nJU

CO

<3

(U

60

•H

25
where N is the number of discrete measurements. Air permeabilities were determined by multiplying air permeance values obtained from eq. (5) by

the factor yL.

4. Results and Analysis

Volumetric flow rates per unit area (Qq) are plotted as a function of

pressure difference (AP) for various building materials in Figure A-2,

It is interesting to note that the data for each of the specimens are fitted with a straight line having unity slope. This finding indicates

that the fluid flow occurring through the specimens behaves consistently

with a Darcy's Law model of fluid flow through a porous medium.

Air permeability values for the various building materials were calculatei

using eq.(5) and are summarized in Table A-1,

Table A-1. Measured Air Permeabilities of Building Materials

Building Material Thickness Permeability

Inch 10"^^ -ft^

Building Paper 0.030 0.82

Asbestos Shingle 0,154 1.74

Brick 3.75 0.54

Gypsum Dry Wall 5/8 4.2

with vapor barrier paint

Gypsum Dry Wall 5/8 7.5

Wood-Composition Fiber Board 1/2 105.0

Wood-Composition Fiber Board 7/16 59.0

Asphalt-Impregnated Sheathing 1/2 296.0

Polystyrene Insulation 3/4 *

Pine Board 1/2 *

Plywood 1/2 *

Plywood 3/8 *

Cedar Shingle *

Redwood Shingle *

* too impermeable to measure.

26
.02 .025.03 .04 .05.06 070809.1 .15 .2 .25 .3 PRESSURE DIFFERENCE, INCH OF WATER

Figure A-2. Air permeation through various building materials as

a function of pressure difference,

27
These data were then used to examine whether or not air leakage through

the solid materials in the building elements could account for a significant part of the measured leakage through the ceiling and exterior and

party walls of the test house.

The induced flow rate through a brick wall of the test house is calculated below for the case of a 7-1/2-mph and a 15-mph wind blowing normal

to the wall. Details for these calculations are given in the following:

The overall resistance to air permeation through' the brick wall is calculated in Table A2

Table A2 . Calculation of the Resistance to Air Permeation Through

a Brick Wall \

Construction Details

Resistance to Air Permeation yL^

10^- Ibf- sec/ft^ k

1/2-in Gypsum Dry Wall

3-1/2-in Insulation

1/2-in Asphalt-Impregnated Sheathing

0.030-in Building Paper

3-1/2-in Brick Facing

2.67

negligible

0.054

1.18

208.0

Total Resistance to Air Permeation = 212 x 10 Ibf* sec/ft

When a 7-1/2-mph wind is decelerated at the wall, a stagnation pressure

of 0.139 Ibf/ft is developed. For the case of a 15-mph wind, a stagnation pressure of 0.556 Ibf/ft is developed. Therefore, the air flow

rate through the brick wall is given by

Qo = AP

(^) ^ k

' t^i

or

Q = 0J39 ^ g^3g ^ ^Q-8 £t (fQ^ 7-l/'2-mph wind)

,4 sec

Qo

212 xlO

0.556

212 X 10 4 26.2 X 10 ^ ^^ (for 15-mph wind).

sec

28
The surface area of the front brick wall of the test house is approximately 290 ft^. The flow rate through the brick wall is 1.1 x

10""^

ft /min for the case of a 7-1/2-mph wind and 4.6 x 10 ft /min for the

case of a 15-mph wind.

Suppose the resistance to air flow offered by the brick is neglected,

then we obtain:

Q„ = Q-^^^ = 3.6 X 10"^ 1^ (for 7-1/2-mph wind)

^4 sec

I

3.90 X 10

Qo = 2jl556 = 14.3 X 10"^ il- (for 15-inph wind).

3.90 X 10^ ^^^

When the brick is excluded, the flow rate is calculated to be 0.063

3 3 ft /min for the case of a 7-1/2-mph wind and 0.25 ft /min for the case

of a 15-mph wind. Utilization of stagnation pressures based on the

ASHRAE handbook probably overrates the actual pressure difference

across the wall due to wind, so the actual resulting leakage due to

permeation could be less. In any case, the leakage due to permeation

is seen to be of very low magnitude.

Under typical conditions, the rate of air infiltration for the test

house was measured to be 0.72 air changes per hour. This figure corresponds to 111.9 ft /min. The foregoing air leakage values due to

air permeation are insignificant in comparison to the overall rate of

air-infiltration.

It was therefore concluded that air leakage due to air permeation

through solid building materials did not contribute to the overall

rate of air infiltration for the test house. The foregoing analysis

neglects air flow through the joints where building materials come

together.

29
a

=

c =

f =

I =

AI =

k =

L =

n =

^R

=

AP =

Q

=

Qo

=

t =

To =

u =

•V =

V =

U

=

NOMENCLATURE

flow coefficient

total flow coefficient

reduction flow coefficient

concentration of tracer gas

initial concentration of tracer gas

fraction of air lost by duct leaks

rate of air infiltration

increase in rate of air infiltration

permeability of porous medium

thickness

flow rate relation exponent

initial exponent

reduction exponent

pressure difference

flow rate through crack

superficial flow rate per unit surface area

time

indoor air temperature

outdoor air temperature

air-delivery rate of blower

rate at which air enters or leaves the enclosure

volume of enclosure

dynamic viscosity of air

1

1

30
SI CONVERSION CHART

Physical Quantity

Length

Length

Area

To Convert

From

in

ft

ft^

To

m

m

m

Multiply by

2.54 (10-2)

3.05 (10"^

9.29 (10-2)

Volume ft3 m3 2.83 (10-2)

Volumetric Flow rate ft /rain m^/s 4.72 (10-^^)

Velocity ft/min m/s 5.08 (10-3)

Power Btu/h W 2.93 (10-^

Dynamic Viscosity lbf-sec/ft2 Pa* s 4.79 (lob

Pressure Difference in H2O Pa 2.49 (10-2)

Temperature °F °C T^ = (Tp-32)/l

31
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 992

J.Oov't Accession No. J. fiecipiisti'4A<ic:6$5=(onWo»

4. TITLE AND SUBTITLE

AN INVESTIGATION OF AIR-INFILTRATION CHARACTERISTICS AND

MECHANISMS FOR A TOWNHOUSE

5. Publication Date August 1979

S. PerftMming Orsanizatlon Code

7. AUTHOR(S) 8. Performing Organ. Report No. S.J. Treado, D.M. Burch, and CM. Hunt

^

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

18. Project/Task/Work Oflit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSrreef, City, state. ZIP) Naval Facilities Engineering Command, U.S. Navy, Washington,

DC 20390; Directorate of Civil Engineering, U.S. Air Force,

Washington, DC 20330; and Office of Chief of Engineers,

U.S. Army, Washington, DC 20314.

13. Type of Report & Period Coverecj

Final

14. %>onsoring Agaicy Code

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of most signi ficant infor. literature survey, mention it here.)

If document includes a signi ficant bibliography or

Air infiltration measurements using a tracer-gas technique and the pressurizatiorr

technique were performed on a three-bedroom townhouse having a gas-fired, forcedair furnace system, in order to quantify the amount of air infiltration due to

various mechanisms. These mechanisms include combustion and draft-diverter air

requirements, air leakage from supply-air ducts, and air leakage through the soli

parts of the building envelope as well as air leakage through cracks around windows and doors. A thermographic survey was also performed in conjunction with

pressurization of the structure, in an attempt to identify specific leaks. An

apparatus for measuring the air permeability of building materials was used to

analyze the significance of air permeation through solid building elements. Based

on the findings of the study, general guidelines are presented for reducing air

infiltration in residences.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons) Air infiltration, residential; air permeability of houses; energy conservation;

pressurization technique; tracer-gas technique.

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