A blower door is a calibrated fan (or blower) in a portable expanding frame, typically installed in an exterior door opening. Blower doors are used to determine the leakiness of a building envelope. A blower door measures the total air flow rate through all of the openings in a house by extracting air from (or blowing it into) the house and measuring the air pressure drop across the house. From this information, the area of all the house cracks and openings can be calculated. A duct tester is a smaller fan and is used to measure the leakiness of ductwork.

A blower door can also be used to determine how much air must be added to change a building from operating under a negative air pressure to operating under a positive air pressure, or vice versa.