Revisions to Part L for England & Wales in April 2006 made air permeability testing a requirement for:

• New dwellings (based on a sampling rate)
• New buildings other than dwellings (ie commercial buildings)
• Extensions to buildings other than dwellings (greater than 100m² total useful floor area or 25% of the existing building's floor area)

NB: There are some exceptions to the above.

Testing involves pressurising the building using fans installed into doorways. The test measures the volume of 'conditioned' air leaking through 'unplanned' ventilation across every m² of the 'envelope' area in an hour (at a internal to external pressure difference of 50Pa).

Part L states that the measured air permeability must be less than 10m³/hour/m². Although SAP or SBEM calculations may require a lower figure.

For further details about the requirements and the test process, see our Technical Information Sheet on the subject