How airtightness testing works

Sealing the OSB airtight layer

When a building is being constructed one of the requirements from the Building Standards is that it should achieve a reasonable provision for airtightness. This blog post explains how it works.

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

This air leakage is the flow of air through cracks and gaps in building fabric. Excessive air loss can add to the energy consumption of the building (as the lost air within the envelope will have been heated and replaced with colder air, which then needs heating, with associated costs and added carbon emissions).

Air Pressure Test

The air permeability of a building can be determined by a pressure test. This test is usually conducted when the building nears completion, when all windows, doors and service penetrations are installed and air sealed.

To carry out the test, first all the external doors and windows are closed and intentional ventilation turned off and vents sealed. All internal doors are opened and drainage traps must contain water (eg the U-bends in plumbing); a blower door with fan is connected (typically into the front door opening). The air pressure is then either raised or lowered to a pre-set level and the rate of return to normal level measured in order to test the air leakage of the building.

The objective is to measure the volume of conditioned air escaping through the building envelope via uncontrolled ventilation at an induced pressure difference of 50Pa. If the building fails, an attempt is typically made to identify and quantify the  air leakage and the infiltration / exfiltration  paths.

Standards Required

The Part L building regulations indicate that the pressure test result should be no worse than 10m3/(h.m2)@50Pa, however for energy efficient buildings a lower number may be required. The exact requirement depends on what standard the building is being designed and constructed to; for example the Passivhaus standard requires an air permeability of less than 1m3/(h.m2)@50Pa.

The consequences of failing to achieve a required air tightness can be significant, with remedial measures being harder  once a dwelling is essentially complete and further air tightness testing is likely to then be required together with additional testing of other properties. It is therefore essential to consider how best to achieve a good air tightness level at the design stage and attention to detail during construction is required.

Ventilation

Adequate ventilation needs to be achieved alongside good airtightness and needs to be sufficient to ensure a comfortable and healthy environment as ventilation removes or dilutes pollutants that accumulate in the building as well as additional moisture from occupants and their activities.

Ideally that extracted air can be recovered via a heat exchanger, to pre-warm the fresh air brought in to replace it (this works best in a dwelling achieving a low air-tightness as the majority of the intentional ventilation is thus subject to heat recovery).  The Kingspan TEK building system inherently achieves a good level of airtightness and works best in combination with such a heat recovery system.

A good level of air permeability if combined with good insulation levels, good detailing at junctions (thermal bridging) and good openings performance to achieve a good overall building fabric, which reduces the energy demand of a dwelling, making it cheaper to run and cutting emissions from building.

For more information about the Kingspan TEK system visit our TEK website.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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