A blower door is a machine used to measure the air tightness of buildings. It can also be used to measure airflow between building zones, to test duct-work air tightness and to help physically locate air leakage sites in the building envelope.
There are three primary components to a blower door:
   (1) a calibrated, variable-speed fan, capable of inducing a range of air flows sufficient to pressurize and depressurize a variety of building sizes,
   (2) a pressure measurement instrument, called a manometer, to simultaneously measure the pressure differential induced across the face of the fan and across the building envelope, as a result of fan airflow, and
   (3) a mounting system, used to mount the fan in a building opening, such as a door or a window.
A variety of blower door air tightness metrics can be produced using the combination of building-to-outside pressure and fan airflow measurements. These metrics differ in their measurement methods, calculation and uses. Blower door tests are used by building researchers, weatherization crews, home performance contractors, and others in efforts to assess the construction quality of the building envelope, locate air leakage pathways, assess how much ventilation is supplied by the air leakage, assess the energy losses resulting from that air leakage, determine if the building is too tight or too loose, determine if the building needs mechanical ventilation and to assess compliance with ATTMA building performance standards.

Blower door metrics:
Depending on how a blower door test is performed, a wide variety of air tightness and building airflow metrics can be derived from the gathered data. Some of the most common metrics and their variations are discussed below.
Airflow at a specified building pressure:
One metric is to measure airflow at a given building-to-outside reference pressure, typically at 50 Pascal (Q50). This standardized single-point test allows for comparison between homes measured at the same reference pressure. A suitably high pressure, such as 50 Pa, is chosen so that the effects of stack-induced airflow and wind-driven airflow are minimized. This increases the repeatability of the measurement, essentially controlling for the different temperature and wind conditions under which a building can be tested. Buildings of different sizes and similar envelope quality will have different results in this test.

Airflow per unit surface area or floor area:
Often, an effort is made to control for building size and layout by normalizing the airflow at a specified building pressure to either the building’s floor area or to its total surface area. These values are generated by taking the airflow rate through the fan and dividing by the area. These metrics are most used to assess construction and building envelope quality, because they normalize the total building leakage area to the total amount of area through which that leakage could occur. In other words, how much leakage occurs per unit area of wall, floor, ceiling, etc.

Uses of blower-door testing:
Blower doors can be used in a variety of types of testing. These include (but are not limited to):
Testing residential and commercial buildings for air tightness
Testing buildings for compliance with standards for energy efficiency - ATTMA
Testing building envelopes and window frames for water tightness and rain penetration
Duct leakage testing of forced air heating/cooling systems - both supply (vents) ducts and return ducts can be tested to determine if and how much they leak air.