What is an airtightness test and what result do you need?

How it works in Ireland

An airtightness test, also called a blower door test, fits a fan into an external door frame, pumps the house to a pressure difference of 50 pascals and measures how much air the fan needs to hold it there. The leakage is divided by the area of the external envelope to give the air permeability, in cubic metres per hour per square metre at 50 Pa. A lower number means a tighter house.

For new dwellings, Technical Guidance Document L sets a maximum permeability of 5 m³/(h·m²) at 50 Pa. A house that exceeds it does not comply with Part L. The BER assessor also uses the measured result in the calculation, so a tighter house scores better, and the assumed figure in your design stage BER often sits below the maximum. Many designs aim for 3 or lower, particularly with mechanical ventilation and heat recovery, which works properly only in a tight house. Ask your designer what target the energy model assumes and what the test must achieve.

The test is done near completion, when the building envelope is complete, windows and doors are fitted, service penetrations are sealed and floors, ceilings and wall linings are in place but before finishes make leaks hard to reach. Testers advise on when the house is ready. A pre-test earlier, before plastering or when the airtightness membrane is still visible, lets you find and fix leaks cheaply. Common leak points are service penetrations, the eaves and wall plate junction, around windows and doors, recessed light fittings, attic hatches and electrical sockets in external walls.

If the final test fails, you must fix the leaks and retest, which costs money and can delay the BER and the certificate of compliance on completion. The tester’s report goes to the BER assessor, and you keep a copy with the other completion records.

Choose a tester who holds a recognised qualification and uses calibrated equipment, and book early, because the test is often the last thing before the BER.

A common mistake

Leaving the first test until the house is finished. Fixing a failed result then means opening up finishes. Book a pre-test when the airtightness layer is complete but still visible.