Air permeability
Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, defines air permeability as:
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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.
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The limiting air permeability is the worst allowable air permeability.
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.
The BSRIA guide to 'Commissioning Air Systems' (BG 49/2024) by Keith Barker defines air permeability as: 'The amount of air leakage (sometimes referred to as air tightness or Building Leakage Rate) from the building as tested prior to the start of functional testing.' See also CIBSE TM23:2022 'Testing Buildings for Air Leakage' for further guidance.
[edit] Related articles on Designing Buildings
- Air permeability in isolation rooms.
- Air permeability testing.
- Air tightness in buildings.
- Approved documents.
- Building emission rate.
- Dwelling emission rate.
- Dwelling type.
- Energy performance certificates.
- Permeability.
- Standard assessment procedure.
- Target emission rate.
- The history of non-domestic air tightness testing.
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