Air permeability
Approved Document L1A Conservation of fuel and power in new dwellings (2010), defines air permeability as:
‘…the physical property used to measure airtightness of the building fabric. It is defined as air leakage rate per hour per square metre of envelope area at the test reference pressure differential of 50 pascals (50 N/m2). The envelope area, or measured part of the building, is the total area of all floors, walls and ceilings bordering the internal volume that is the subject of the pressure test. This includes walls and floors below external ground level. Overall internal dimensions are used to calculate this envelope area and no subtractions are made for the area of the junctions of internal walls, floors and ceilings with exterior walls, floors and ceilings.’
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.
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