An airtightness test, or blower door test, uses a fan mounted in an external doorway to pressurise or depressurise the house and measure the air flow required to maintain a pressure difference of 50 pascals. The result gives the air permeability of the building envelope.
During the test all intentional openings such as trickle vents and extract fans are temporarily sealed, so that what is measured is unintended leakage. The fan flow rate needed to hold the pressure difference is divided by the envelope area to give the air permeability result.
Testing is most useful when carried out twice: a preliminary test once the airtightness layer is complete but still accessible, so leaks can be found and sealed with smoke pencils or thermal imaging, and a final test for certification once the house is finished.