The short simple version: A blower door is a diagnostic tool used to figure out how leaky a house is and where the leaks are. It is a powerful fan that mounts into an exterior door and pulls air out of the house, simulating a strong wind on all sides of a house. Anyone doing an audit of your house or sizing a furnace or air-conditioner should use one. Unfortunately, most contractors don’t. You should look for contractors that do.
The slightly longer version: The blower door measures the air tightness (or conversely, the leakiness) of a house and to help find where leaks are. The blower door consists of three primary components. First is the fan, calibrated to measure air flowing through it. Second is a fabric or rigid shroud that fits in an opening of the house, generally an exterior door and seals the fan body into the envelope of the house. And third, is a manometer, a device to measure the pressure created by the fans airflow. The combination of the fan’s flow and the pressure difference it creates between the inside and the outside—giving us a standardized measure of air tightness.
We typically measure leakage rates at a standard number of Pascals (a small unit of pressure). Leakage is measured in cubic feet per minute (U.S.) and usually at 50 Pascals. (The number is expressed as XXX CFM50.)
Often this number is converted to air-changes per hour, how many times the air in a house turns over each hour.
What you need to know is that your contractor should use a blower door to measure air-tightness before and after any major project that impacts the energy use in your home. Ask for it.
If you dying to know more, Home Energy Magazine has an excellent overview article in its archives.
I’ll talk about IR scans (infrared thermography) another time.