Measuring House Air Tightness
Protecting Your Home
Despite the growing importance of house air tightness, few homeowners
actually know how tight their homes are. Unfortunately, it is virtually
impossible to accurately estimate the tightness level of a home by visual
inspection alone. And without knowing house air tightness, it is difficult to
assess the need for, or to design an effective approach to duct, air sealing
& indoor air quality.
The easiest way to measure house air tightness is with a diagnostic tool
called a blower door. The blower door consists of a powerful, calibrated fan
that is temporarily sealed into an exterior doorway. The fan blows air out
of the house to create a slight pressure difference between inside and
outside. This pressure difference forces air through all holes and
penetrations in the exterior envelope. Blower door tests are typically
performed at a pressure difference of 50 Pa (0.2 inches of water column).
By simultaneously measuring the airflow through the fan and its effect on
the air pressure in the house, the blower door system measures the
air tightness of the entire building envelope. The tighter the building (e.g.
fewer holes), the less air you need from the blower door fan to create a
change in house pressure.
Air tightness measurements are presented in a number of different formats including:
square inches of leakage
air flow needed to generate 50 Pa of pressure difference (CFM50)
air changes per hour at 50 Pa of pressure difference (ACH50)
It takes about 20-40 minutes to set-up a blower door and do a test to document the air tightness of a house. In addition to assessing the overall air tightness level of the building envelope, the blower door can be used to estimate the amount of leakage between the conditioned space of the building and attached structural components such as garages, attics and crawlspaces. It can also be used to estimate the amount of outside leakage in forced air duct systems. And because the blower door forces air through all holes and penetrations, these problem spots are easier to find using chemical smoke, an infrared camera or simply feeling with your hand. The air tightness measurement can also help you assess the potential for back drafting of natural draft appliances by exhaust fans and other mechanical devices.