Air leakage (also known as air permeability or air infiltration) is the air tightness of a dwelling through uncontrolled means such as cracks and gaps in the building envelope. Any ventilation system installed in a building is classified as a source of controlled air flow and is therefore not considered as air leakage. In ‘laymans’ terms, air leakage can be described as unwanted draughts, which will lower internal temperatures and may cause discomfort to building occupants. Air pressure testing is a method of measuring and quantifying the air leakage of a building. Under Part L1A of the Building Regulations, Air Tightness Testing is referred to as Pressure Testing and is the mandatory method by which developers measure the air tightness of their residential developments. Testing shows how well properties will retain heat and in turn reduce carbon emissions, making them more efficient and cheaper to maintain.

An Energy Performance Certificate (EPC) is another compulsory requirement under Part L1A of the Building Regulations for all newly built residential units and these can only be issued when air pressure testing results are available for the specific dwelling or one of the same design.

How does it work?

Air pressure tests are carried out by increasing the air pressure within your properties and then measuring the rate by which the pressure returns to normal. This is carried out using large fans inserted in the entrance doorway. The number of units you will have to test will depend on the number of unit types on your development and the assumptions your SAP assessor has made with regards the SAP calculations for the units being assessed.

There are options for not testing all units on a site, however, the reality is that in practice ALL units will have to be tested (to be sure, check with your SAP assessor, do NOT take your Building Control officers word on this issue as we often see them supplying incorrect advice on this subject).

The Design Air Permeability for the building will be determined initially by the developer/architect or suggested by the SAP assessor. The maximum design air permeability allowable is 10 m³/hr. per m²  @50Pa. A reduced Design Air Permeability (i.e. a high level of air-tightness) will serve to further reduce carbon emissions and make the Target Emission Rate (TER) easier to achieve. It may also mitigate the cost of more expensive carbon emission reduction strategies. The majority of dwellings that Blewburton assess target a result of 5m³/hr. per m² @50Pa or below.

The issues listed below are common causes of uncontrolled air leakage. Attention should be paid to these during construction; Intermediate floors, behind skirting boards, boxed in pipes & soil stacks, behind bath panels, kitchen units, badly installed trickle vents (testing only allows for trickle vents to be shut and not sealed), windows and doors, recess lights & loft hatches.

Dwelling types failing an air pressure test will require remedial work and retesting, along with one additional test on the same dwelling type. Spending time and attention at the outset to avoid this eventuality will save costs and hassles in the long-run.

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