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Last edited 26 Aug 2019
Air permeability testing
Approved document F, Ventilation, defines airtightness as ‘…a general descriptive term for the resistance of the building envelope to infiltration with ventilators closed. The greater the airtightness at a given pressure difference across the envelope, the lower the infiltration.’
It suggests that air permeability is ‘…the physical property used to measure the airtightness of the building fabric. It is defined as air leakage rate per hour per square metre of envelope area at a test reference pressure differential across the building envelope of 50 Pascal (50N/m2). The design air permeability is the target value set at the design stage.
In April 2002 the UK government introduced legislation to enforce standards of building air tightness. This was intended to lower running costs; verify the standards of materials, components and workmanship; prevent uncomfortable drafts and avoid condensation problems.
This is achieved by air permeability testing (air tightness, air infiltration or blower door testing), which measures the air leakage rate per hour per square metre of building envelope area at a test reference pressure differential across the building envelope of 50 Pascal (50 N/m2).
Generally, this involves mounting a door, incorporating a fan, into the frame of an external door opening. The fan is used to depressurise the building by extracting air, and the resulting difference between the external and internal pressure can be used to calculate the permeability of the building envelope (given that the area of envelope is known). This permeability is an indicator of how well sealed the building is, and whether there are openings in the envelope. Testing requires that fires are put out, doors and windows are closed and ventilators are shut or sealed.
The regulations set a limiting air permeability of 10 [email protected] (metres cubed, per hour, per metre squared of external building envelope area) and require that the Building Emission Rate (BER, or Dwelling Emission Rate (DER) for dwellings) calculated using the assessed air permeability does not exceed the Target Emission Rate (TER).
Testing is required on all residential developments (this may be a sample of units) and non-dwellings. Buildings with a useful floor area less than 500m2 MAY be exempt. Where testing is not carried out, an assessed air permeability of 15m3/h.m2 must be used in calculations.
On large developments, where a specific dwelling is not tested, the assessed air permeability is the average test result obtained from other dwellings of the same dwelling type on the development, increased by 2.0m3/h.m2.
Air permeability testing can be carried out as an isolated study, or may be provided as part of a complete service to demonstrate Part L compliance (including Standard Assessment Procedure (SAP) calculations and Energy Performance Certificates (EPCs)) or may be part of an overall energy audit. Ideally, it should not be carried out simply as a compliance test, but should include an assessment of details, and diagnostic tests such as infrared thermography and if necessary smoke tests, to pinpoint air leakage paths.
See also: Air tightness in buildings.
 Related articles on Designing Buildings Wiki
- Air change rates.
- Air infiltration.
- Air permeability in isolation rooms.
- Air tightness in buildings.
- Approved documents.
- Building emission rate.
- BREEAM Testing and inspecting building fabric.
- Building services compliance with the building regulations.
- Domestic ventilation systems performance.
- Draughts in buildings.
- Dwelling emission rate.
- Dwelling type.
- Energy audit.
- Energy performance certificates.
- Infrared thermography.
- Performance gap.
- Standard assessment procedure.
- Target emission rate.
- The history of non-domestic air tightness testing.
 External references
- ATTMA (Air Tightness Testing and Measurement Association).
- iATS (the Independent Air tightness Testing Association)
- ATTMA TSL1 (ATTMA Technical Standard for dwelling testing)
- ATTMA TSL2 (ATTMA Technical Standard for non-dwelling testing)
- BINDT (British Institute of Non-Destructive Testing)
- USA government, blower door tests.
--ATTMA 13:12, 24 February 2015 (UTC)
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