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Air leakage is defined as the flow of air through gaps and cracks in the building fabric. Uncontrolled air leakage increases the amount of heat loss as warm air is displaced through the envelope by colder air from outside. Air leakage of warm damp air through the building structure can also lead to condensation within the fabric (interstitial condensation), which reduces insulation performance and causes fabric deterioration.

Air leakage is a ‘double whammy’ in energy efficiency terms, because warmed air leaks out and cold air leaking in then needs to be heated

The airtightness of a dwelling, or its air permeability, is expressed in terms of air leakage in cubic metes per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascals (m³/(h.m²)@50Pa).

The dwelling envelope area is defined in this context as the total area of all floors, walls and ceilings bordering the dwelling, including elements adjoining other heated or unheated spaces.

Air permeability figure  are used in DEAP calculations  and poor airtightness can significantly effect the  calculated thermal performance of a building.

A reduction in air permeability (@ 50Pa) from 15 (m³/(h.m² ) to 5 (m³/(h.m²)

=  Energy savings of approximately 25%

The air permeability of a building can be determined by means of a pressure test conducted using a device referred to as a Blower door Building Regulations 2011 TGD-L (Dwellings) indicates that reasonable provision for airtightness is to achieve a pressure test result of no worse than 7m³/(h.m²)@50Pa. , however best practice is  regarded  as less than 3m³ 3m³/(h.m²)@50Pa , with Passive House  requirements being less that 1m³/(h.m²)@50Pa .

All Kingspan Century systems, with their factory manufactured applied airleakage measures offer the highest levels of airtightness, typically achieving less than 2m³/(h.m²)@50Pa and can be designed to achieve less than 1m³/(h.m²)@50Pa .

However in traditional construction it is necessary to understand that air leakage routes are complex, and subsequent works such as service penetrations can affect airtightness..

Typical air leakage paths in Traditional Construction include:

• Having perpends and mortar joints not fully sealed when using plasterboard on dabs or timber battens.
• Cracks, gaps and joints in the structure (possible due to drying shrinkage or settlement).
• Joist penetrations into the inner leaf of external walls.
• Service entries and ducts, not just in the external fabric, but also through holes drilled into joists and stud wall timbers, and especially through either a suspended floor, or through the ceiling below a ventilated roof void.
• Areas of unplastered masonry, for example behind bath panels and between joists of intermediate floors.
• Attempting to reduce a measured air leakage rate following a failed airtightness test is significantly more expensive and difficult than eliminating the causes of air leakage as the dwelling is being built.

Typical Airleakage paths in Traditional Construction

Thermographic images demonstrating the thermal effects of Airleakage paths

Standard Wall Vent	Unsealed Attic Hatch
Unsealed External Wall to Ground Floor Junction	Penetration of airleakage membrane at Service Penetration