The blower door test is the most practical way to evaluate the air movement in a home as well as the effectiveness of any air sealing measures.

Air leakage can increase heating and cooling costs over 30% and contribute to comfort, health
and safety problems. Finding hidden airleakage sites, called bypasses, can be difficult without the use of a blower door. This diagnostic equipment uses a fan to pressurize (force air into) or de-pressurize (force air out of)a building. When the fan operates, it is easy to feel the effects of infiltration – air leaking through cracks in the building envelope.

Blower doors have gauges which can measure the relative leakiness of a building. One measure of a home’s leakage rate is air changes per hour (ACH), which estimates how many times in one hour the entire volume of air inside the building leaks to the outside. Leakier houses have higher ACH’s, therefore higher heating and cooling costs, and greater potential for moisture, comfort, and health problems.

To determine ACH, the blower door creates a pressure difference of 50 Pascals between inside and outside. Fifty Pascals is approximately equivalent to a 20 m.p.h. wind blowing against all surfaces of a building. The leakier the house, the harder the fan must work to maintain the pressure. The amount of air the fan moves, measured in cubic feet per minute (CFM), is used to determine ACH.

To see some nice video footage of the blower door take a look at the following link at the
California Energy Commission