Building Compliance Testing – Air Permeability Testing
Air Tightness Definition
Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, defines design air permeability as the measure of airtightness of the building fabric. It is defined as the air leakage rate per hour per m2 of envelope area at the test reference pressure differential of 50Pa or 4Pa. Also referred to a DAP or Q50.
Design Air Permeability
The limiting air permeability is the worst allowable air permeability, known as DAP for short.
The design air permeability is the target value set at the design stage, and must always be no worse than the limiting value.
The assessed air permeability is the value used in establishing the Dwelling Emission Rate (DER) and Dwelling Fabric Energy Efficiency (DFEE) rate and is based on specific measurement of the dwelling concerned or on measurements of other dwellings of the same dwelling type.