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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 | 
