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Air Tightness: 5 reasons you should know more

Achieving a reasonable level of airtightness in new buildings is a mandatory requirement of Building Regulations Part L and a key aspect of the passivehaus standard; and in addition, excessive air infiltration can have serious implications on the operational energy use of a building, and can be the cause of fabric decay due to dampness.

Air leakage is driven by differential pressures across the building envelope and can be defined as the resistance to air leakage, either inward or outward, through unintended leakage points through gaps in the fabric or components in the building envelope such as doors and windows.

  1. You might not notice it directly, but achieving an air tightness figure of 10 m3/(h.m2) @ 50Pa is roughly equivalent of having a hole the size of a 20p piece in every m2 of envelope area. That’s massive when you consider the area of the whole building envelope. Therefore, you should be aiming for a figure much lower to achieve energy efficiency.
  2. Any warm moist air from inside the building that passes through the external envelope may cause interstitial (space between structures or objects) condensation as it reaches a cold surface – this can lead to fabric decay. Increasing the air tightness of your build will reduce the likelihood of this fabric decay occurring.
  3. A building can never be too airtight – but it can be under ventilated – so always remember ‘Build Tight – Ventilate Right’. A common mistake is to focus solely on reducing unintended air leakage, but it is important that there is adequate and controlled air circulating the building to prevent the build-up of moisture and airborne pollutants present as a result of living/occupying a space.
  4. When detailing the air barrier, it is important to ensure that it is on the warm side of the insulation. This is to avoid condensation in the fabric caused by the interaction of the warm moist air and a cold surface.
  5. Finally, it is always a good idea to have an air leakage audit when doing an airtightness test, even if your building has passed. This way you will see where the building is leaking, and you can improve on these details for your next building project.