How to Pass an Airtightness Test: The Pre-Test Checklist

If your airtightness test is booked for next week, the good news is this: most failed tests come down to a handful of predictable leak paths, and nearly all of them can be found and sealed in a day or two. This checklist walks through what the tester will measure, where air leaks hide on a typical UK build, how to find them yourself before test day, and how to fix them fast without waiting on wet trades.

What a blower door test actually measures

An airtightness test (often called a blower door test) uses a calibrated fan mounted in a doorway to pressurise and depressurise the building to 50 Pascals — roughly the load of a stiff breeze on every surface at once. The tester records how much air the fan has to move to hold that pressure, then reports the result in one of two ways:

• Air permeability — cubic metres of air per hour, per square metre of building envelope, at 50Pa (m³/h·m² @50Pa). This is the figure UK Building Regulations use, and your SAP assessment will have set a design target your result must meet. Part L sets limits for new dwellings, and missing your design figure can mean a failed SAP calculation, not just a failed test.
• Air changes per hour (ACH @50Pa) — how many times the building's entire air volume leaks out per hour at test pressure. This is the metric Passivhaus uses, with a target of 0.6 ACH @50Pa.

Know which number you are chasing before the tester arrives. A large detached house and a mid-terrace flat with the same leakage will score very differently on each metric.

Where air leaks hide

Testers see the same culprits on site after site. Walk the building with this list before you book anything:

• Joist ends and intermediate floor zones — where joists bear into masonry, air tracks around every joist through the perimeter. On a two-storey house this hidden band can be one of the biggest single leak paths.
• Window and door reveals — the gap between frame and structure is often foamed but rarely sealed continuously. Foam alone is not an air barrier.
• Service penetrations — soil pipes, boiler flues, MVHR ducts, electrical entries, waste pipes through floors. Every hole cut after the air barrier went in is a suspect.
• Wall-to-floor and wall-to-ceiling junctions — the line where blockwork meets slab, and where ceilings meet the wall airtight layer, especially behind dot-and-dab plasterboard where air moves freely in the void behind the boards.
• Loft hatches, downlights and socket boxes — small individually, significant in total.

How to find leaks before the tester does

You do not need a fan to find most leaks. On a breezy day, close up the building and run the back of your hand slowly around reveals, junctions and penetrations — moving air on skin is surprisingly easy to feel. Better still, use a smoke pencil or smoke matches: hold the smoke near a suspect junction and watch it. Still smoke means tight; smoke pulled sideways means a leak path. If your tester offers a pre-test or leak-detection visit with the fan running, take it — with the building depressurised, every leak becomes obvious with smoke.

Work methodically, room by room, and mark every leak with tape or chalk as you go. It is much faster than trying to remember them later.

How to fix leaks fast

The problem with traditional fixes a week before a test is time: a sand and cement parge or plaster repair needs to dry, and mastic beads at every junction are slow and easy to miss. Liquid applied membranes exist for exactly this situation. Passive Purple is a spray or brush applied airtight membrane that goes on as a two-coat system, bridging junctions, joist zones and blockwork in a single continuous layer — with a tested air leakage of 0.02 L/s·m² at 75Pa (ASTM E2178). Sprayed, one installer can cover up to 300m² a day, so a whole plot's problem areas can be treated well inside a working week.

For small areas and detailing work, Passive Purple Brush handles reveals, penetrations and patch repairs without spray kit. Pair it with airtight tapes for membrane laps and frame-to-wall joints, and airtight sealant for pipe penetrations and movement joints. Whatever you use, the rule is the same: the air barrier must be continuous. Trace it with a pen on your section drawing — anywhere the pen lifts is where the test fails.

The pre-test checklist

• Confirm your design air permeability target from the SAP assessment — do not guess it.
• Walk the building with a smoke pencil and mark every leak: joist ends, reveals, penetrations, junctions.
• Seal marked leaks with liquid membrane, tape and sealant — allow the two coats of membrane to be applied and dry before test day.
• Fit loft hatches, downlight covers and socket boxes properly rather than leaving them "for after".
• Before the tester arrives, close trickle vents and windows, and prepare intentional openings (extract fans, flues) as your tester instructs — testers follow set rules on what may be temporarily sealed, so ask rather than improvise.
• If you are unsure where your air barrier line runs, use our free drawing markup service — send your drawings over and we will mark the airtight line and problem details before you are on site.

For a full overview of products and where each one fits, see our airtightness solutions page.

Frequently asked questions

What happens if I fail my airtightness test?

You seal the worst leaks and retest. Most testers can point out the major leak paths while the fan is running, and liquid membranes plus tapes and sealant can usually close the gap within days. A failed first test is common and fixable — but it costs a retest fee and programme time, which is why pre-test checking pays for itself.

Can I use normal expanding foam or silicone to pass?

Foam fills gaps but is not reliably airtight long term, and general-purpose silicone does not bond well to dusty masonry. For a result that holds at the retest and for the life of the building, use products tested as air barriers: airtight membranes, tapes and sealants.

How airtight does a new build need to be?

Your specific target comes from the SAP assessment for your plot, within the limits Part L sets for new dwellings. Passivhaus projects work to 0.6 ACH @50Pa, which is far tighter and needs the air barrier designed in from day one.

How long before the test should I apply a liquid membrane?

Apply both coats and allow the membrane to fully dry before test day — check the datasheet for cure times at your site temperature, and build in a margin rather than spraying the night before.