Air Tightness Testing

Why You Need Air Tightness Testing

Building Regulations (Part L) requires all new residential & commercial buildings to be tested for air leakage.

Assessors carry the test when a property is nearing completion and is necessary for Building Control to provide the final Building Certificate.

A good Air Tightness Test result will give a development a better environmental rating, improving its saleability.

What you need to know

The regulations require that all new dwellings achieve an air leakage of less than 10m3/hm2.

This is the air leakage rate per hour per square meter of envelope area.

If we are being precise, there are a few caveats around SAP Calculations and dwelling types which can affect this, but broadly this is the rule.

The headline figure determines if the building will ‘Pass or Fail’. The test demonstrates how much air is being sucked into the building through ‘leakage’.


Whilst testing may take place at any time in the build process, it is common for air testing to take place in the final throes of a project prior to issuing of final SAP Calculations and an EPC.

In a residential build, the test result will be passed to the SAP assessor who will then update the calculations, establish that a pass has been achieved, and issue final reports and an EPC.

Effects on SAP

Importantly, the SAP rating will be significantly affected by the air leakage rate, as high levels of uncontrolled air leakage will reduce the energy performance of the building. A SAP Assessor will generally set a design air permeability target of between 5-10m3/hm2.

This is an achievable performance although in some cases these may need to be set lower.

If a dwelling is struggling to meet its emissions targets, a low air leakage may compensate for other areas, either arising from poor design, or factors beyond the developer’s control.

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