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A correlation between air infiltration and air tightness for houses in a
developed residential area
Shaw, C. Y.
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Ser
TH1
N21d
no. 1100
National Research
Conseil national
c. 2
I
*
Council Canada
de recherches Canada
A CORRELATION BETWEEN AIR INFILTRATION AND AIR TIGHTNESS FOR HOUSES I N A DEVELOPED RESIDENTIAL AREA
by Dr. Chia-Yu Shaw
ANALYZED
Reprinted from
ASH RAE Transactions Vol. 87, Pt. 2. 1981 p. 333
-
341DBR Paper No. 1100
Division of Building Research
Price $1.00 OTTAWA
BLW.
RES.
L I B R A R Y
C N R C
-
. ) C t S : IL ' i n f i l t r a t i o n de l ' a i r e s t l e passage n a t u r e 1 de l ' a i r de l l e x t C r i e u r 3 l ' i n t e r i e u r de l a maison e n r a i s o n d e 1 1 6 c a r t de p r e s s i o n q u i s 1 6 t a b l i t de p a r t e t d ' a u t r e de s o n enveloppe s o u s l ' e f f e t de l a d i f f ' e r e n c e de t e m p e r a t u r e e n t r e l ' i n t ' e r i e u r e t l l e x t C r i e u r . On s a i t q u ' i l s ' a g i t de l ' u n e d e s p r i n c i p a l e s formes de p e r t e s d 1 6 n e r g i e pour une r e s i d e n c e , mais il e s t d i f f i c i l e de 1 1 6 v a l u e r .
Les d 6 b i t s d ' i n f i l t r a t i o n peuvent C t r e mesurgs au moyen d'un gaz t r q a n t , c e q u i n C c e s s i t e un materiel coQteux e t une longue p r 6 p a r a t i o n ; en o u t r e , l a d t h o d e e s t d'emploi d i f f i c i l e
2
grande B c h e l l e pour d e s e s s a i s c o u r a n t s . 11 e s t p l u s simple d ' u t i l i s e r l a m6thode de p r e s s u r i s a t i o n pour mesurer l e s f u i t e s d ' a i r e t t r a n s p o s e r l e s r B s u l t a t s e n d B b i t s d ' i n f i l t r a t i o n p a rl a c o r r C l a t i o n infiltration-Ctancheite. Cette comrmnication
p r e s e n t e une t e l l e c o r r i Z l a t i o n pour d e s maisons s i t u ' e e s dans une banlieue. Une Btude p r e l i m i n a i r e de c e t t e c o r r ' e l a t i o n a dCj2 f a i t l ' o b j e t d ' u n r a p p o r t . V o i c i maintenant les r B s u l t a t s d e f i n i t i f s .
No.
2655A CORRELATION BETWEEN AIR
INFILTRATION AND AIR TIGHTNESS
FOR HOUSES IN A DEVELOPED
RESIDENTIAL AREA
DR. CHIA-YU SHAW,
P.Eng.
INTRODUCTION
Air infiltration is the uncontrolled leakage of air into a house, that results from pressure differentials across its envelope induced by wind and inside-to-outside temperature difference. It is recognized as one of the major energy losses in residences but is difficult to estimate.
Infiltration rates can be measured using the tracer gas method which requires expensive equipment and long preparation time, but it is difficult to use on a large scale for routine tests. A more simple method is to use the fan pressurization~method to conduct air leakage tests and then translate the result into infiltration rates using an infiltration-air tightness corre- latian. This paper introduces such a correlation for houses in a suburban area. A preliminary study of the proposed correlation has been reported previously
[I].
The final result is now presented.TEST HOUSES
The two houses used in the study are the same size and have the same floor plan. They are lo- cated on adjacent lots (Fig. 1). The standard house (HI) was built to a construction standard similar to other new houses in the same area. The upgraded house (H4) was built with added insulation and a specially applied polyethylene vapor barrier to improve air tightness of the house shell. Both houses were equipped with an electric furnace and a forced air circulation system. The upgraded house (H4) also had a heat pump unit. The volume, floor and envelope area of each house are 386 m3, 118 m2 and 228 rn2 respectively. The ratio of the envelope area to the volume for the two houses is 0.59 m-1. A detailed description of the houses is given in Ref. 2. AIR TIGHTNESS VALUE
Air leakage tests were conducted on both houses using the fan pressurization method. Fig. 2 shows the air leakage rates per unit area of building envelope defined as the area of the exterior walls above grade plus that of the ceiling of the upper floor. The measured values of over-all air leakage rate and pressure difference were fitted to the following flow equation to obtain the flow coefficients and exponents for the two houses
where
Q = air leakage rate, L/s
C = flow coefficient
,
L/S -m2 (Pa)n A = area of building envelope, m2AP = pressure difference between inside and outside, Pa n = flow exponent
C.Y. Shaw, Research Officer, Energy and Services Section, Division of Building Research, National Research Council of Canada, Ottawa, Ontario, Canada, K1A 0R6.
The flow exponent, n, is 0.71 for both houses, and the corresponding flow coefficients are 0.11 for the standard house (HI) and 0.075 ~ 1 s . m ~ . (Pa)n for the upgraded house
(H4)
; t h e r a t i o of t h e flow coefficients i s 1.47.AIR INFILTRATION RATES
Air infiltration measurements were conducted simultaneously on both houses between January and April 1979 using the tracer gas decay method and C02 as the tracer gas. Additional measurements were made on the upgraded house in July and August of the same year to investigate the wind
effect on infiltration. Unfortunately the same tests could not be carried out on the standard house because it was occupied during that period. Details of the measurements are given in Ref.1.
Because air infiltration can be caused by temperature difference or wind acting alone or by the combined action of the two, the results are divided into three groups depending on the relative strength of the two weather parameters as follows:
Type I
-
Temperature Caused InfiltrationWhen the wind speed is less than 3.5 m/s, the measured infiltration rates for the two
houses were found to depend on the inside-outside temperature difference only
[I].
A plot of the infiltration rate vs. the temperature difference shown in Fig. 3 suggests that the infiltration rate increases with the temperature difference according to a power-law expression that was found to be[I]
where
I is the infiltration rate in air changes per hour (ach)
v is the volume in m3
At is the inside-outside temperature difference in OK
C is the flow coefficient as defined in Eq 1. Type I1
-
Wind Caused InfiltrationWhen the temperature difference is less than 20 K and the wind speed is greater than 3.5 m/s, the wind becomes the dominant driving potential causing air infiltration. For the wind induced infiltration, the effect of wind direction and wind shielding should also be considered. These effects were approximately accounted for by considering a house to be either exposed to or shielded from wind depending on the wind direction and the surroundings. For a house in a developed residential area, the front and back walls are normally less shielded from wind than the two side walls. Consequently, the infiltration through the front or back is greater than that through the other walls. Hence a house is considered to be exposed if the wind approaches from the front or the back of the house; similarly it is considered to be shielded if the wind comes from the side. At both houses (H1 and H4), however, the back wall was also shielded because of a high earth berm (Fig. 1).
Fig. 4 shows the infiltration rate as a function of wind speed for both the exposed and the shielded conditions. The results indicate that the infiltration rate of the upgraded house in- creases with wind speed according to a power-law expression. Because, as indicated by Eq 1, air leakage rate varies with the nth power of the pressure differential which, in turn, is a function of wind speed, the infiltration data were fitted to a power-law expression of the form
where
V = the on site wind speed in m/s a = a constant
The cup anemometer was located approximately 18 m aboveground and was about 10 m behind the houses (Fig. 1). The on site wind speed was about 15% less than that recorded at the Ottawa
International Airport approximately 32 km away from the site.
Preliminary calculation indicated that for the best fit curve as indicated by the dashed line, Fig.
4,
the values for b were 1.82 for the exposed conditions and 0.96 for the shielded; for simplicity they were rounded off to 2 and 1. The infiltration equation for the upgradedhouse was found to be
I = 0.42 (A/v) C
v~~
Exposed= 0.76 (A/v) C Vn Shielded Type I11
-
Wind and Temperature Caused InfiltrationWhen the wind speed is greater than 3.5 m/s and the temperature difference is greater than 20 K the influence of these two driving potentials on infiltration should both be examined. The infiltration data were, therefore, plotted separately against the two driving potentials in Figs. 5 and 6. The results indicate that the variation in infiltration rates for various combi- nations of wind speeds ranging approximately from 3.5 to 7 m/s and temperature differences varying from 20 to 40 K is about 220% of the mean value. As the results do not indicate any definite relation between infiltration and wind speed or temperature difference the simplest model support- ed by the limited data is a constant infiltration rate. Thus, the infiltration rate for each house was assumed to be approximately equal to the mean infiltration rate for the data shown.
I = 4.31 (A/v) C Standard House
= 4.74 (A/v) C Upgraded House
For comparison, Eqs 2 and 4 are also shown in Figs. 5 and 6. The results indicate that when the wind speed is greater than 3.5 m/s the infiltration rate exceeds the values predicted by Eq 2 for the same At with low wind speed. On the other hand, when the temperature difference is great- er than 20 K, the infiltration rate predicted by Eq 4 under high wind conditions can be higher than that given by Eq 5. This suggests that for ranges of wind speed and temperature difference greater than that specified for Eq 5, the mean value for the constant infiltration rate model should be increased. Additional data are required to estimate the mean infiltration rates for these ranges.
Correlation Between Infiltration and Air Tightness
Because the envelope of the two houses differs in air tightness value a correlation between infiltration and air tightness can be obtained by comparing the simultaneously measured infiltra- tion data. Fig. 7 compares the infiltration rates for both houses at two ranges of wind speed. The results and Eq 2 indicate that for houses with the same flow exponent, the ratio of infiltra- tion rates equals the ratio of the flow coefficients for a wind speed lower than 3.5 m/s. A similar trend is apparent for high wind conditions even though the ratio of the mean infiltration rates given by Eq 5 is about 10% less than the ratio of the flow coefficients. If we assume that this relation is also valid for other houses, the infiltration-flow coefficient correlation can be modified to account for different flow exponent, area of building envelope, and volume as follows
where
b = a constant with a numerical value of 1 or 2 A = the envelope area of the house
v = the volume
x = represents At, V or 1 corresponding to Types I, 11, or I11 infiltration and o = (subscript) represents a reference house
Selecting the mean value of the two houses as the reference and substituting Eqs 2, 4 and 5 into Eq 6, the correlation between infiltration and air tightness as measured by the fan pressurization method for different weather conditions is:
Case I
-
Type I infiltration; V less than 3.5 m/sCase I1
-
Type I1 infiltration; At less than 20 K and V greater than 3.5 m/s 1/v2n = 0.42 (A/v) C ExposedI/Vn = 0.76 (A/v) C Shielded
Case 111
-
Type 111 infiltration; At between 20 and 40 K; V between 3.5 and 7 m/sEqs 2a, 4a, 4b and 5a are plotted in Figs. 8 and 9.
DISCUSSION
The proposed infiltration-air tightness correlation was tested using some of the measured infil- tration data of 25 houses from published and unpublished sources [3, 4, 51. These houses included two other Ottawa houses [3], 18 Swedish houses [4] and five Saskatoon houses [5] covering a range
of flow coefficients from 0.011 to 0.3 ~/s.m2.~an and a range of flow exponents from 0.52 to 0.92.
The Ottawa houses were bungalows with oil furnaces; the Saskatoon houses consisted of four bunga- lows and one split level all with gas furnaces; the Swedish group consisted of one two-story, ten one and a half-story, and seven single-story houses. The highest wind speed and the largest tem-
perature difference under which the measurements were made were 10 m/s and 54 K respectively.
Except for the Saskatoon houses where the tracer gas constant concentration method [6] was applied, infiltration rates were measured using the tracer gas decay method. In the Swedish houses the chimney and the fireplace openings were sealed during the tracer gas and the fan pressurization tests. The operation of the furnace was not controlled during the tests conducted on the Ottawa and the Saskatoon houses.
Fig. 8 shows a comparison between the predicted and the measured infiltration rates for
Cases I and 111. As shown, the agreement in general is within 25% of the predicted value. Fig. 8 also shows that Eq 5a overpredicted the infiltration rates for the two Ottawa houses (Nos. 1
and 2). This was probably caused by the uncertainty in estimating the flow coefficients which
were calculated from Eq 1 based on the air tightness value measured at a pressure differential of 75 Pascals and an assumed value of 0.65 for the flow exponent.
Fig. 9 shows that, in general, the agreement between the prediction and the measurement is also within 25% of the predicted value for the wind dominant infiltration regime. Since neither wind direction nor house orientation was given for the Swedish houses, it was assumed that the wind blew on the exposed walls of the houses. This can be partly responsible for the poor agree- ment for some of the Swedish houses.
No correction was made to account for the differences in the location of the wind speed sensor for these houses. This may also have contributed to some of the differences between the predicted and the measured infiltration rates. For simplicity, furnace operation, height of neutral pressure plane, house shape and distribution of leakage openings were neglected in the analysis. Even so good agreement between the predicted and the measured infiltration was obtained
(Figs. 8 and 9). Taking into account these variables may improve the accuracy of the predicted value but at the expense of the simplicity of the correlation.
SUMMARY
The infiltration rates for a house situated in a developed residential area can be divided into three types: temperature-caused infiltration, wind-caused infiltration, and a combined wind- and temperature-caused infiltration. The first and third types normally occur during the heating
season. If the wind speed is less than 3.5 m/s, infiltration (Type I) increases with At. As the
wind speed approaches 3.5 m/s and the outside air temperature drops to OOC, Type I infiltration transfers to Type I11 and the increase with wind speed or temperature difference becomes obscure. Type I1 infiltration occurs mainly in mild climates when the outside air temperature is above OOC. This type of infiltration is induced by wind and is, therefore, also affected by the surrounding houses and landscaping. If the wind blows on the exposed walls (usually the front and back), infiltration rate varies with ~ 2 n . If it strikes the shielded walls (usually the sides) the rate increases with Vn.
A comparison of the infiltration data measured simultaneously on the standard and the up--
graded houses (H1 and H4) suggests that for houses with the same flow exponent, the ratio of the infiltration rates is similar to the ratio of the flow coefficients. On this basis a model was developed to correlate infiltration with air tightness as measured by the fan pressurization method. It was found that the normalized infiltration rate (with respect to wind speed or tempe- rature difference) varies linearly with the flow coefficient if the dominant driving potential behind infiltration is either wind or temperature. It increases linearly with the flow coeffi- cient for a given range of wind speed and temperature difference if it is caused by the
combined action of wind and temperature.
The infiltration data obtained from 25 houses in various geographical locations were used to check the proposed correlation. Most of the measured infiltration rates were within 25% of
the predicted values. APPENDIX A CONVERSION FACTORS Multiply by To Obtain cfm ft2 cfmlf t2 cfmlf t? (Inches of Water)" Inches of Water OR cfml (Inches of Water)n- ft3 A C ~ / O R ~ Ach/ (mph) 2n Ach/ ( m ~ h ) ~ REFERENCES
1. Shaw, C.Y. and Tamura, G.T., "Mark XI Energy Research Project Air Tightness and Air Infil- tration Measurements", National Research Council of Canada, Division of Building Research, BR Note 162, July 1980.
2. Quirouette, R.L., "The Mark XI Energy Research Project
--
Design and Construction", National Research Council of Canada, Division of Building Research, BR Note 131, October 1978.3 . Tamura, G.T. and Wilson, A.G.
,
"Air Leakage and Pressure Measurements on Two Occupied Houses", ASHAE Transactions, Vol. 70, pp. 110-119, 1964.4 . Kronvall, J., "~esting of Houses for Air Leakage Using a Pressure Method", ASHRAE Transactions, Vol. 84, I, 1978.
5. Dumont, R.S., Unpublished Data.
6. Kumar, R:, Ireson, A.D. and Orr, H.W., "An Automated Air Infiltration Measuring System Using SF6 Tracer Gas in Constant Concentration and Decay Methods", ASHRAE Transactions, Vol. 85,
11, 1979. ACKNOWLEDGMENT
The author wishes to thank R.S. Dumont for permission to use his unpublished infiltration data, and to acknowledge the contribution of D.G. Stephenson, G.T. Tamura, and M. Bassett in the preparation of this paper. The two houses used in this study were part of the HUDAC/NRC Mark XI Energy Research Project. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.
I. INFILTRATION, ac/h
q, OVER-ALL AIR LEAKAGE RATE PER UNIT AREA OF BUILDING ENVELOPE, 11s. m2 h a
2
VI B k. wb-4 a h2
a*
0 r t k . q w 5."g5;
m P i-tz
" t og
h m nJ E ZN G
u C h P- O t n N 1 ~ 1-
~ 1 n-
0 * - * I d-
\ p., '2 \I : I I,-
;
-
1- -
H I M I Y-
1 O l - m \ 1 0 : $ 0 1-
-
-
'r
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A $ $-
\
a 0 0 1 I l l l l A oI I V L 3 . 5 ./1
-
/
-
-
C = F L O W C O E F F I C I E N T I I-
0 . 1 0 . 2 0 . 3 0.4 0.5 u 0 . 1 2 I 1 I I I CASE I . V ~ 3 . 5 m , ~ , An bO°K0 , 10
-
H I AND H4. REF I, OTTAWA0 REF 3. OTTAWA
.
REF 4. SWEMN2
0. 08-
REF 5. SASKATOON-
I NUMBER OF DATA s-
U 0 . 0 6 ' C-
a 0.04-
-
V) 4 "7 3 0 . 4 - o I w 0 . 3 0 . 2 T I I 1 o 3 . 5 5 V r 5 . 0 r n / l V 2 5 . 0 m / r } A t - o e K-
* / '1
-
.
40
.+I 0 * I 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 o 0 . 0 2-
-
Y I H O U S E H I , ac/h U 4 P: F i g . 7 C o m p a r i s o n o f-
oI
0 0 . 0 5 0. 10 0. 15 0 . 2 0 0.25 0. 30 s i m u l t a n e o u s l y m e a s u r e d i n f i l t r a t i o n r a t e s o f z o 1 . 2 H 1 a n d H 4-
C I I I I I 4 CASE m , ~ . ~ L V L I O ~ / , . Z O L A I L ~ O ' K E b.-2 1, 0-
H I AND H4. REF I d-
L 0 REF 3 Z-
.
REF 4 0 . 8-
REF5-
c-
-
0 . 6 --
8-4-
d I 0 0.05 0. 10 0. 15 0 . 2 0 0.25 0 . 3 0 C . A / " . L / S . p a n . rn3 0 0.05 0. 10 0. 15 0. 20 0 . 25 0 . 3 0 Z 0 A I R T I G H T N E S S - F A N P R E S S U R I Z A T I O N-
I-
C 0 . 4 4 w c 2-
$ C- 0. 3 ",-
-
E-
\.
REF 5 1-
0 . 2-
m C >-
-
0 . 1-
O D 0.05 0. 10 0. 15 0 . 2 0 0 . 2 5 0 . 30 C . A / ".
1 1 s . p a y n 3 A I R T I G H T N E S S - F A N P R E S S U R I Z A T I O N F i g . 9 C o m p a r i s o n o f p r e d i c t e d a n d m e a s u r e d i n f i l t r a t i o n r a t e s f o r C a s e 11 340 i * I I I I I SHIELDED, A t ~ 2 0 ~ K . V ~ 3 rn/s-
H I A N D H4. REF I-
0 REF 3 Fig. 8 C o m p a r i s o n of p r e d i c t e d a n d m e a s u r e d i n f i l t r a t i o n r a t e s f o r c a s e s I a n d 111DISCUSSION
G.O. HANDEGORD, Coordinator, Building Technology, DBR Nat. Res. Council, Ottawa, Canada: This correlation should be valid only for houses without chimneys, as it was based on the infiltration data of two electrically heated houses and did not consider the fundamental difference between air leakage openings and pressure difference patterns between fuel heated houses and those heated electrically.
C.T. SHAW: Mr. Handegord is correct in saying that the correlation is based only on the in- filtration data from two electrically heated houses. However, the equations also consider, to a certain degree, the difference between houses with fuel-fired heating systems and those heated electrically. The main difference between the two is, as Mr. Handegord notes, that the fuel-heated houses have chimneys that affect both the air leakage characteristic and the pres- sure difference pattern of the house. The former effect is included in the equations through the terms "flow coefficient" and "exponent" (C and n). Even though the pressure difference pattern is neglected in the equations, the agreement between the predicted and the measured infiltration rates for fuel-heated houses, Figs. 8 and 9, appears to indicate that the corre- lation is approximately valid for them.
Further studies are presently being conducted. From these results, we hope to incorporate pressure difference patterns to refine the pressurization-air infiltration correlation.
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