Retrotec USACE - Manual (Page 321)

Appendix F F27
The Canadian Building Code ■
Various applicable ASHRAE standards ■
Air Tightness Testing and Measurement Association (ATTMA) Techni- ■
cal Specifi cation 1 (United Kingdom [UK])
CGSB 149.10, Canadian air leakage standard.
■
Key differences among these standards are:
Choice of test pressures (10 Pa, 50 Pa versus 75 Pa) ■
Way of expressing results (EqLA, CFM50, ACH50, CFM/sq ft @ 75Pa) ■
Necessity and method for accounting for bias pressures (called “zero ■
fl ow pressures” for the pressure measured with zero fl ow going through
the door fan).
Necessity and method for accounting for additional parameters (baro-
■
metric pressure, humidity, temperature, elevation).
Both ASTM standards were originally intended for the testing of residential
detached housing. Under these standards, multiple test points are gathered
from 10 Pa up to 60 Pa and results are expressed in CFM @ 50 Pa or air
changes per hour (ACH) @ 50 Pa (where CFM @ 50 Pa is the fl ow rate, in
CFM, required to depressurize the house to – 50 Pa). It is also referred to as
“CFM at 50 Pa,” “CFM @ 50,” or simply “CFM50.” ACH @ 50 Pa is CFM50
divided by the house volume. It is also referred to as “Air Changes at 50 Pa” or
“ACH50”).
Both ASHRAE and the Canadian Building Code use testing points up to
75 Pa and express their results in terms of fl ow per square foot of surface area
at 75 Pa.
The preferred test method for this standard includes:
Multiple test points from 75 Pa to 40 Pa 1.
Testing in both the pressurization and the depressurization directions 2.
Taking a comprehensive bias pressure over a long time interval to deter- 3.
mine the lowest possible test pressure and to provide a more accurate
bias correction
Expressing results in terms of L/s *m
4. 2 @75Pa or cfm/sq ft @0.3 in.w.g.
of enclosure area
The higher test pressure of 75 Pa was chosen for this standard since larger
buildings are subject to higher bias pressures from wind and stack effects. Since
wind velocity increases with height above ground, higher pressures due to wind
are experienced. As height doubles, the increased bias pressures experienced
due to wind roughly double. Houses typically experience bias pressures of 2
to 5 Pa whereas larger buildings can experience 10 to 20 Pa. Taking results at
higher pressures helps achieve a more consistent result. A 75 Pa test pressure
is about as high a pressure as is practical without vastly increasing door fan
power, which would substantially increase the risk of damage due to higher
wind velocities and pressures, and which is about the maximum a well-hung