Air leakage is measured as...

The rate of leakage per square metre of external envelope per hour at an artificial pressure differential through the envelope of 50Pa
(m³/hr/m² @ 50Pa).

Two parameters are used: -

All wall, roof and floor areas that are exposed to the external environment are included within the calculated area. Floor slabs in direct contact with the ground are not included, on the basis that they will not allow any significant levels of leakage to occur.

All wall, roof and floor areas that are exposed to the external environment are included within the calculated area. This is the parameter used to define the leakage standards in Part L (Conservation of Fuel and Power) of the Building Regulations 2006.

Prior to the introduction of maximum permissible standards within the Building Regulations, the typical level of leakage index for a non-domestic building in the UK was around 20 m³/hr/m² @ 50Pa.

For an industrial unit this index figure could increase to somewhere in the region of 25.0m³/hr/m² @ 50Pa to 30.0m³/hr/m² @ 50Pa.