Approved document FVentilation, 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 (50 N/m2). The design air permeability is the target value set at the design stage.

The Air Tightness Testing and Measurement Association (ATTMA) defines ‘air leakage’ as the ‘…uncontrolled flow of air through gaps and cracks in the fabric of a building. It is sometimes known as infiltration or draughts. Air leakage is not to be confused with ventilation, which is controlled airflow in and out of a building‘.

Approved Document E suggests that infiltration ‘…is the uncontrolled exchange of air between inside a building and outside through cracks, porosity and other unintentional openings in a building, caused by pressure difference effects of the wind and/or stack effect.’

Wherever infiltration occurs, there is a corresponding exfiltration somewhere else in the building. During the summer, infiltration can bring humid, outdoor air into buildings. In winter, exfiltration can result in moist indoor air moving into cold wall cavities and can result in condensation and ultimately mould or rot.

While air infiltration is not desirable, it is important for buildings to have sufficient purpose-provided ventilation. According to the Building Services Research and Information Association (BSRIA), ‘Project teams should design and construct the building fabric to be reasonably airtight, and also provide natural or mechanical ventilation systems that maintain good indoor air quality while minimising energy use. In other words: build tight, ventilate right.’ Ref BSRIA Topic Guide – Airtightness.

The benefits of air-tight buildings include:

As well as building tighter, the rate of air infiltration is constantly changing depending on:

A significant amount of air leakage resulting in heat loss occurs in all buildings but much less in air-tight buildings. According to the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), the energy impact is “in the order of 10 kWh per m2 of floor area per year for the heating needs in a moderately cold region”. REHVA also suggests there are a growing number of studies indicating that there is considerable impact on buildings in mild and hot climates.

ATTMA standards TSL1 and TSL2 include the following benchmarks for normal levels of building air permeability (m3/(h.m2)@50Pa):


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