Last edited 09 Sep 2020

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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 method for testing required by the building regulations is set out in ATTMA TSL1 (for dwellings) and ATTMA TSL2 (for non-dwellings).

ATTMA is the Air Tightness Testing & Measurement Association. ATTMA was authorised on behalf of the government to create the testing standards.

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).

The ‘design air permeability’ is the target value used to calculate the TER. The ‘assessed air permeability’ is the measured value used to establish the BER (or DER).

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.

In compartmentalised buildings where pressure testing the whole unit may be impractical, testing may be carried out on a representative area.

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.

[edit] Related articles on Designing Buildings Wiki

[edit] External references

--ATTMA 13:12, 24 February 2015 (UTC)


The Independent Air Tightness Testing Scheme (iATS) was authorised by DCLG on 18 April 2015 to operate a competent person scheme for air tightness testing. The iATS scheme is bound by the same Conditions of Authorisation as the ATTMA scheme.

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