Air Permeability is the physical property used to measure the air tightness of building fabric (known as the envelope area).
It is defined as air leakage rate per hour per square metre of envelope area at a given test reference pressure e.g. commonly 4 or 50 Pascals.
Design air permeability is a target value set at design stage to meet Part L SAP compliance or a retrofit target/ outcome (known as the air tightness strategy).