The 2015 IRC/IECC now requires a post-construction blower door test to verify that each house has an acceptable air leakage rate.
A blower door is a fan that is installed in an exterior door of a house. As it draws air out of the house, it creates depressurization that pulls outdoor air into any openings in the building envelope.
A high-accuracy gauge is used during the test to measure both the amount of house depressurization (in units of Pascals) and the amount of air flowing through the fan (in cubic feet per minute - CFM).
The result of a blower door test is a measurement of CFM50: the amount of air flow, in cubic feet per minute, required to depressurize the house to 50 Pa of pressure.
Air Changes per Hour
A large house will leak at a higher CFM flow rate than a similarly constructed small house, simply because there is more surface area exposed to the exterior. Therefore, it is necessary to convert the blower door test results into a unit that is independent of house size.
The IRC/IECC sets a maximum Air Changes per Hour (ACH). ACH is the number of times each hour the full volume of air in the house is removed and replaced by outdoor air entering through leaks in the envelope.
ACH = (CFM x 60) / Volume
For compliance purposes, the 2015 IRC/IECC refers to the Air Changes per Hour (ACH) at the blower door test pressure of 50 pascals (rather than trying to estimate the leakage rate under natural house pressures).
The 2015 IRC/IECC specifies a maximum air leakage rate of 3 ACH in the Northeast (PA) Climate Zones.