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For Airtight Buildings - Mind the Gap

Steve Richardson | November 30, 2017

Air leakage in the form of a draught within a building’s fabric is usually easy to detect. A shiver-inducing light gust is normally solved with the closing of a door or window, or with the strategic placement of a gap-filling excluder. For a property to achieve Passive House standards for air tightness, however, requires sealing the building against air leakage which isn’t felt or immediately apparent.

Securing good levels of air tightness is not only beneficial for the building’s owner in terms of reduced energy usage and lower fuel bills. Since 2006, UK Building Regulations have included compulsory air leakage testing of new buildings, requiring developers to prove the air tightness of a sample of new buildings on a new residential housing estate, for example.

Air leakage or air permeability, which refers to escaping or penetrating a building, is generally seen in the following areas:

·         at external wall and floor junctions

·         around windows and doors

·         around pipe work including those generally boxed-in

·         behind fitted units or behind bath and shower panels

·         at socket points and around electricity units.


Air assessment and APR

During an air test, assessors will fit a temporary airtight screen at the entrance door of a building, whilst all other areas, such as water traps and vents, are temporarily blocked or closed. A fan then blows air into or out of the building to create a pressure difference between inside and outside of approximately 50 Pa. Air tightness is calculated by measuring the rate of airflow through the fan whilst a range of pressure differences between the inside and outside of the house are sustained.

To pass an air leakage test a building must achieve an air permeability result (APR) of 10 m3/(h.m2). However, some targets are even more stringent when defined at design stage. A test that doesn’t achieve a Building Regulations minimum performance requirement would be classed as a fail. Should tests fail to achieve the necessary performance level, the building may require remedial work and retesting.

This is where a good test engineer will often be able to identify the leakage points and provide corrective advice. Ignore these areas of escape at your peril.