Air leakage can increase heating and cooling costs by over 30%, contributing to comfort, health, and safety problems. Finding hidden air leakage sites, bypasses, can be difficult without a blower door. This diagnostic equipment uses a fan to depressurize (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 maintain pressure. The amount of air the fan blows, measured in cubic feet per minute (CFM), is used to determine ACH.