Terms
Air tightness
The air tightness of a home is how much air leaks in and out of a home through
unplanned openings.
Air leakage
Air leakage is a measure of air tightness. It is the size of the holes in the outer
envelope of a building. Its units are generally cm2.
Normalized Leakage
The following equation is a commonly seen method of normalizing air leakage.
NL = Normalized Leakage
ELA = Effective Leakage Area (Air Leakage), cm2
Af = Floor Area, m2
H = Height, m
Infiltration
Infiltration is another measure of air tightness. It is the volume of air flowing
through the holes in the outer envelope when the home is at a specified pressure
difference. Generally, the infiltration is reported at a pressure difference of either 50
Pascals, when an accurate reading can be made, or extrapolated to 4 Pa, which is a
typical condition. It is generally reported in m3/s.
Air Changes Per Hour
The following equation is a method of normalizing infiltration.
Infiltration x 3600 sec
Volume
hour
Blower door tests
Blower door tests are used to determine air leakage and infiltration. The planned
openings (ie windows) in a home are sealed and then the home is pressurized for a
range of values using a fan installed in a doorway. The flow through the fan is then
measured. Using a method from the American Society for Testing and Materials,
E779, the air leakage can be calculated from these measurements.i Please see
Appendix X for information on this calculation. The infiltration is the measurement
at whichever pressure is of interest.
Measuring Air Leakage of U.S. Homes
There is a large database of blower door tests collected by the Lawrence Berkeley
National Laboratory. Persily et al. completed the most recent analysis of this
database in 2006, in order to classify the typical normalized leakage of single-family
homes in the United States.ii The final result of this paper is the following table:
Table 1. Normalized Leakage based on floor area and year
The approach he took to calculate these numbers is based on the following chart
from a paper by Chan et al. in 2003.
Table 2. Statistics of the normalized leakage of conventional houses that are not participants of low-income or an
energy efficiency program
The values in the box are the geometric means of normalized leakage for the type of
home specified by area and year. Persily then averaged the numbers by year for the
floor areas below and above 139 m2. This produces the chart seen in Figure X.
Figure 1. Normalized Leakage Versus Year Built
Persily did a linear regression of these lines and found the values of normalized
leakage in 1940, 1955, 1979, and 1998, approximately. These values are reported in
the table, above.
Values of Normalized Leakage
As discussed above, there have been measurements made on the United States
housing stock to determine typical air leakage. In addition to the values found in the
Persily table above, Sherman measured the average normalized leakage for new,
conventional homes to be 0.55 in a paper in 2002. It is important to note that a
conventional home in this context is one that was “not built as part of any energyefficiency program. Most of them were measured as part of some voluntary
program. Many of the builders knew that their houses would be tested, but no
special energy efficiency features were installed.” [Sherman, 2002]
There is one standard, ASHRAE 119, which bases its standard on normalized
leakage. This is by region and, in Chicago, any value of normalized leakage below
0.57 is considered sufficient.
In addition, there are two methods of determining whether mechanical ventilation
is required based on normalized leakage. The first is found in ASHRAE 119 and has
three categories: required ventilation, unknown, and ventilation not required. The
second is in a paper by Sherman, which has four divisions including required
balanced and unbalanced ventilation, either, and no ventilation required.
These measures can be found in the following graph, along with a box-and-whisker
plot of the MIT ICF data.
Figure 2. Normalized Leakage
As can be seen in the figure, the median of the data is similar to the range found by
Persily for new, conventional homes. In addition, it is in a vague zone in terms of
ventilation for both standards.
Values of Air Changes per Hour
There are many standards in air tightness that are based on the concept of
infiltration. The following table contains these, in units of air changes per hour at 50
Pascals of pressure.
Standard
Passivehaus
Building America Program
EPA Energy Star
German Building Code
Swedish Building Code
Czech Republic Building
Code
IECC 2009
ASHRAE 90.2 modeling
value
Value (ACH@50)
.6
3
3 or 5 (climate
dependent)
3
3
4.1
Source
<passivehouse.us>
Lstiburek, 2010
Lstiburek, 2010
7
10*
IECC 2009
ASHRAE 90.2
Erhorn, 2008
Lstiburek, 2010
Erhorn, 2008
Table 3. ACH Standards
*This value was given as .5 ACH@4 Pa. Based on [Sherman, 1998] this can be
converted to ACH@50 Pa using the following equation: ACH 50=20*ACH4
These values can be seen on the following chart, overlaid onto the MIT ICF data as a
box and whisker plot.
Figure 3. Air Changes Per Hour
Although the ICF homes can be considered tight, they are similar to newer,
European standards.
Appendix X. Air Leakage Calculation
This is a brief description of the process in ASTM standard E779-2010
Methodology
 Seal the house and create a uniform pressure throughout
 Install air moving equipment
 Increase the pressure from 10 to 60 Pa, in steps of 5 or 10 Pa
 Determine the airflow to maintain the pressure
 Measure indoor and outdoor temperature, and elevation
Calculation
 Correct the pressure based on the average zero-flow measurement
 Correct the flow based on the indoor and outdoor densities
 Calculate the natural logarithm of the correct pressures and flows
 Find the variances and the covariances of these natural logarithm values
 Calculate n and C in the following equation
Q = C∆Pn
Q = flow, m3/s
Covar iance
n=
Variance(ln( P))
C  exp(ln( Q)  n * ln( P))



1.458 *10 6 (T  273).5

, kg/(msK.5)
110.4

1
T  273
µ0 = µ(T = 20 C)
 .0065 * 600 5.2553 293 
  1.20411 
 


 T  273 
293
T = indoor or outdoor temperature, C


Then find the corrected C, C0, using the following equation
  2n 1  1n
C0  C   
 0   0 


Finally, the air leakage can be calculated
  2n 1  1n
AL  C0    , m2
 0   0 
The confidence limits of n, C and AL are found using the variance of these
values and the two-sided T statistic

ADD THIS INFORMATION AFTER CONFIRMING THAT THE EQUATIONS ARE
CORRECT (ESPECIALLY SINCE THEY REFER TO VARIANCE BUT USE THE SYMBOL
FOR STANDARD DEVIATION)
Appendix X
NOTES – NOT PART OF THE REPORT
Air Leakage/Infiltration
Standards
3ach@50 Pa – Swedish building code
Source: Lstiburek, “Just right and airtight” ASHRAE Journal.
3ach@50 Pa – Building America Program
Source: Lstiburek, “Just right and airtight” ASHRAE Journal.
3ach and 5ach@50 Pa – EPA Energy Star. Depends on climate.
Source: Lstiburek, “Just right and airtight” ASHRAE Journal.
7ach@50 Pa – IECC 2009
ASHRAE 90.2 – must meet ASHRAE 119. For modeling, there are three options but
the important value is .5 ACH = 10 ACH@50 based on [Sherman, 1998]
Please see [Erhorn, 2008]
Passivehaus test buildings = average .37 ACH (2000) to .46 ACH (2002) at 50 Pa
Czech Republic – 4.1 ACH@50 Pa
Germany – 3 ACH@50 Pa or 7.8 m3/h /m2 floor area or 3 m3/h/m2 surface area
Denmark – 1.5 l/s per m2 floor area
Norway – 3 l/h
Airtight building shell ≤ 0.6 ACH @ 50 pascal pressure
(http://www.passivehouse.us/passiveHouse/PassiveHouseInfo.html)
ASHRAE standard 119 -1988 – in Chicago, less than NL = .57
Measured Values
Sherman, 1998; Chan 2003/2005; Persily combined these into
Normalized Leakage
Year Built
Floor area < 148.6
Floor area > 148.6
Before 1940
1.29
.58
1940-1969
1.03
.49
1970-1989
.65
.36
1990 and newer
.31
.24
[Sherman, 2002] – average NL for new, conventional homes = .55
Mechanical Ventilation
Persily 2006 – Used 6.1 L/s/m2 (balanced) for any home with a central system
[ASHRAE 1989]
ASHRAE 119 Normalized Leakage
Mechanical Ventilation
Class A – C
Less than .2
Required
Class D-F
.2 < NL < .57
Unknown
Class G-J
Greater than .57
Not required
ASHRAE standard 62.2 – 2010
ASHRAE 90.2
Minimum 50 cfm outdoor air (when summer design infiltration rate is less than 0.35
ACH) - calculate according to equation 6-1
Please see [Erhorn, 2008]
Czech Republic – 1.5 ACH w/o recuperation, 1 ACH with recuperation
Germany – 1.5 ACH@50 Pa or 3.9 m3/h /m2 floor area or 3 m3/h/m2 surface area
Denmark – 1.5 l/s per m2 floor area
Norway – 3 l/h
[Sherman, 1998]
NL<.14 – Balanced
.14<NL<.28 - Either
.28<NL<.8 - Unbalanced
NL>.8 - None
ASTM Standard E779, 2010, "Standard Test Method for Determining Air Leakage
Rate by Fan Pressurization," ASTM International, West Conshohocken, PA, 2003,
DOI: 10.1520/E0779-10, www.astm.org.
ii Persily A, Musser A, Leber D. “A collection of homes to represent the U.S. housing
stock.” National Institute of Standards and Technology, NISTIR 7330; 2006.
i