Air Tightness/Pressure Testing

What is air tightness and why is it important?

A major aspect of improving the energy efficiency of a dwelling is the reduction or elimination of heat loss, occurring due to the uncontrolled movement of air, either through the fabric of the building or through gaps and cracks that may exist around window and door frames and other components that legitimately pass through the external envelope of the dwelling.


Uncontrolled movement of air through the fabric of the building is described as “air leakage”. As the air that is leaking from a building is generally heated air, there is a direct relationship between the air tightness of a building and its energy efficiency.  Air leakage can account for substantial energy losses.  By limiting the leakage of heated air from a building it is possible to reduce energy consumption and costs.

Air leakage is not to be confused with ventilation that is designed into a building such as roof space ventilation, background ventilation provided by trickle vents, “combustion air” ventilation etc, All of these types of ventilation are designed into a property for a specific purpose and are of a predetermined quantity or are controlled in some way or other.

Air leakage through gaps and cracks is often difficult to detect by visual inspection and leakage paths through the fabric of the structure are often complex, so the only satisfactory  way to assess the air tightness of a dwelling is to measure the building as a whole rather than its individual component parts

Air  pressure testing carried out in accordance with BS EN 13829 by Test Engineers registered by the British Institute of Non Destructive Testing (BINDT) is the mandatory method by which the air tightness of a dwelling is evaluated.

Air pressure tests are carried out by installing a fan in the doorway of the property, all legitimate ventilation openings designed into the building are temporarily closed or sealed and a pressure differential of 50 Pascal is created.  Pressure readings are taken and used to measure the amount of air that leaks through the building fabric per hour.  The internal wall, floor and ceiling areas are calculated and used to find the air permeability in metres cubed per hour per square metre of building area. The air permeability measured in this way  is referred to as the “q50” and is expressed in cubic metres per hour per square metre of external building envelope (m3/h/m2@50Pa).

How air tight should a dwelling be?

Building Regulations require that all buildings are designed and built to achieve a maximum air permeability rate of 10m3/h/m2@50Pa.

The only exception to this being that small developments of one or two dwellings can opt out of undergoing an air pressure test by assuming a default air permeability rate of 15m3/h/m2@50Pa, however additional measures such as increased levels of insulation etc. will need to be incorporated into the design to compensate for the assumed increased air leakage. The cost and inconvenience of these additional measures may well make an air pressure test preferable. 

Good building practice and careful site supervision along with the implementation of Accredited Construction Details should enable an air permeability rate in the region of  6 or 7 to be achieved relatively easily, with results around the 3m3/h/m2@50Pa. level being achieved with a much more committed approach.

What are Accredited Construction Details?

Accredited Construction Details (ACD’s) have been developed to assist the construction industry achieve the performance standards required to demonstrate compliance with the energy requirements (part L) of the Building Regulations.

The details focus on the issues of insulation continuity (minimising cold bridges) and air tightness.The details contain checklists which should be used by the builder/Project Manager and the Building Control Authority to record compliance.

Accredited Construction Details (ACD’s) are found on the Planning Portal website here