A method of quantifying how much air leaks into or out of an enclosure. EN 13829 gives a standard test method for buildings. Several standards apply to ductwork systems (see also “How is the ductwork airtightness quantified?“).
Building airtightness levels can be measured by using a fan, temporarily installed in the building envelope (a blower door) to pressurize the building. Air flow through the fan creates an internal, uniform, static pressure within the building. The aim of this type of measurement is to relate the pressure differential across the envelope to the air flow rate required to produce it. Generally, the higher the flow rate required to produce a given pressure difference, the less airtight the building .
 M. Limb, “Technical note AIVC 36- Air Infiltration and Ventilation Glossary,” International Energy Agency energy conservation in buildings and community systems programme, 1992.