Architectural Masonry

Public Sector

Air Leakage Testing

Since 1985 U-values for the external elements in dwellings have undergone continual improvement. In the main, this has been driven by the Government in a bid to reduce carbon emissions in excess of the figures required by the Kyoto agreement.

For external walls, 1985 saw the introduction of elemental U-values of 0.60 W/m2K, improving to 0.45 W/m2K in1990. Further amendments to Part L in 1995 saw the introduction of the Governments SAP procedure which, for the first time, began to assess the overall performance of the external fabric of the building and its services in relation to heat loss and carbon emission.

Part L1 2006 now assesses the actual Carbon Emission generated as a result of the dwelling and its services. The building as designed is compared to a notional building and the carbon emissions compared. If the building as designed has a lower carbon emission rate (DER) than the notional (TER), then a "pass" under the requirements of Part L is achieved.

Generally, this means that improvements to the heat loss through the external fabric will become necessary. In particular, U-values of external walls may require a design specification suitable of achieving 0.30 W/m2K.

The law of diminishing return makes it increasingly difficult to arrest heat loss where U-Values are as low as those required by Part L1 2006. The next step is to ensure that the heat is retained for as long as possible within the dwelling by reducing air leakage paths. The buildings external envelope should therefore be constructed to a so as to achieve an acceptable rate of air permeability, or air leakage, when tested.

Air leakage assessment of newly constructed dwellings has become a mandatory requirement with the introduction of L1A 2006.
The regulation requires that the maximum acceptable air permeability or leakage rate for dwellings is (10 /m3 / hm2 (@ 50 Pa). In some circumstances a lower air leakage rate may be required for the DER (dwellings emission rate) to be less than the TER (target emission rate) in order to achieve a "Pass".

Improvements to the air leakage rate can be used as a tool to reduce the DER and subsequently enable u values in excess of 0.3wm2k to be incorporated.
Our trained assessors will be able to offer advice on this subject. It is likely that detached houses and bungalows will have the greatest difficulty in meeting the required Carbon Emission Rate, and will almost certainly require a lower air permeability specification.

Part L 2006 requires that the testing be undertaken by suitably qualified persons. Our comprehensive technical backup service now offers a fully accredited Sap rating service together with an On Site Air Leakage testing unit. Our fully qualified and accredited technicians utilise "state of the art" computer controlled equipment to provide highly accurate and consistent test results.


On each site, an air leakage test should be carried out on a unit of each dwelling type selected by the Building Control. As a fully accredited test facility, we are able to make the selection of plots for test without the need to refer Building Control. It is desirable to commence tests at an early stage in order to establish the robustness of the proposed design and construction at the earliest possible opportunity.

For the purpose of L1, a dwelling type means a dwelling of the same generic form:

Semi detached.
Mid terrace.
End terrace.
Mid floor, ground floor and top floor flats.

Different dwelling types are not defined by their floor area or their amount of bedrooms. Different forms of wall or roof construction define a dwelling type. Where a housing development has a mix of construction forms, a higher number of tests will be required. Blocks of flats should be treated separately irrespective of the number of blocks on the site.


Where accredited construction details have been adopted, an air leakage test should be carried out on one unit of each dwelling type on the site. For information on accredited construction details visit www.planningportal.gov.uk Then select Building Regulations, then Part L. The drawings illustrating the accredited construction details may be down loaded.

Where accredited construction details have not been adopted the required frequency of testing will be as below.

Number of instances of the dwelling

Number of tests to be completed on the dwelling type

4 or less One test for each dwelling type
More than 4, but equal or less than 40

Two tests for each dwelling type

More than 40 At least 5% of the dwelling type, unless the first 5 units of the type that are tested achieve the design permeability, when the sampling frequency can be subsequently reduced to 2%



Where no more than two dwellings are to be erected, an alternative approach to pressure testing would be either:

  1. To demonstrate that during the preceding 12 month period a dwelling of the same dwelling type constructed by the same builder had been pressure tested in accordance with the standards required by the Building Regulations and had achieved the specified air permeability.
  2. Avoid the need for any pressure testing by using a value of 15m3 / (h.m2) for the air permeability at 50 Pa when calculating the DER (Dwellings Emission Rate).

For option 2, higher standards of insulation and boiler efficiency will be required to compensate.


Besblock Limited are authorised to complete air pressure testing as required by L1A 2006. We employ specially trained personnel and the latest computerised measuring equipment. All tests will include official test data and certification required for Building Control.

As authorized SAP assessors we are highly qualified to assist and advise customers in compliance with Part L1A 2006. This service includes full SAP and NHER assessments, and the production of all documentation required for Building Regulation submissions.

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Besblock Limited - Heslop, Halesfield 21, Telford, Shropshire, TF7 4NF
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