Airtightness Tape: How to Choose and Install It Correctly
If your airtightness test is looming, or you've just watched smoke pour through a window reveal during a blower-door test, you already know: the tape you choose — and how you apply it — can make or break your air permeability score.
This guide covers what airtightness tape actually does, how to choose the right one for each junction, and the installation mistakes we see on site over and over again.
What is airtightness tape?
Airtightness tape is a specialist adhesive tape that seals the joints, junctions and penetrations in a building's air barrier. Unlike ordinary builder's tape, it's engineered to stay airtight for the life of the building — through thermal movement, humidity swings and decades of service.
Our Airtight Tape is fleece-backed, which means it bonds aggressively to the tape but also mechanically keys into wet-applied products — so you can plaster or paint directly over it. That's what makes it work as a system with liquid membranes like Passive Purple.
Where you need it
The air barrier is only as good as its weakest junction. The typical leak points:
Window and door reveals. The junction between frame and structural opening is the single most common failure point in airtightness tests. Tape the frame to the reveal before plastering or before applying a liquid membrane.
Wall-to-floor and wall-to-ceiling junctions. Especially in timber frame and where joists penetrate the air barrier.
Service penetrations. Soil pipes, flues, cable and duct penetrations — anywhere something passes through the air barrier.
Membrane laps and board joints. Sheet VCLs need every lap taped; airtight boards need every joint sealed.
Steel and awkward details. Where a steel beam meets blockwork, rigid tape alone often can't follow the geometry — this is where a fleece-backed tape plus liquid membrane combination outperforms tape alone.
How to choose
Width. 75mm covers most frame-to-reveal work. Choose 100mm when bridging bigger gaps or uneven substrates, 150mm for wide junctions, laps and movement joints, and 200mm for the widest details — steel flanges, wide laps and heavily uneven substrates. (Our tape comes in 75mm, 100mm, 150mm and 200mm × 25m rolls.)
Backing. Fleece-backed tape can be plastered and painted over — essential if your airtight layer is parge coat or a liquid membrane. Foil and film tapes can't take a wet finish and rely entirely on their adhesive edge.
Substrate compatibility. Masonry, OSB, concrete and membranes all hold adhesive differently. Dusty blockwork needs priming — no tape sticks to dust.
Flexibility. Junctions move. A tape that can't accommodate movement will peel or tear within a few seasons, long after your test but well within the building's life.
Installing it right: the five failure points
- Dusty or damp substrate. The number one cause of tape failure. Brush down, and prime porous or dusty surfaces first.
- No pressure applied. Airtight adhesives are pressure-activated. Roll or firmly rub every centimetre — laying it on is not enough.
- Tension in the tape. Stretched tape creeps back. Lay it relaxed, especially into corners.
- Fish mouths at corners. Cut and overlap at internal corners rather than folding a single piece through the angle.
- Stopping short. A 90% taped junction is not 90% airtight — the smoke finds the gap. Continuity is everything.
Tape + liquid membrane: the belt-and-braces detail
On real sites, tape alone struggles with complex geometry — steelwork, uneven masonry, triple junctions. The most robust approach we know:
- Tape the junction with fleece-backed airtight tape.
- Coat over the tape and surrounding area with a liquid air barrier such as Passive Purple or Airtight White.
The fleece keys into the wet membrane and the liquid seals any micro-gaps at the tape edge. This is how Passivhaus projects hit 0.6 ACH and better — and why we designed the tape and the paint to work as one system.
FAQ
Is airtightness tape the same as vapour barrier tape?
They overlap. Most airtightness tapes are also vapour-resistant, but check the declared Sd value if the tape forms part of your vapour control layer.
Can you plaster over airtightness tape?
Over fleece-backed tape, yes — the fleece gives the plaster a mechanical key. Over foil or film tapes, no.
How much do I need?
Measure the linear metres of window/door perimeter, membrane laps and junctions, then add 10% for overlaps and corners. One 25m roll typically does one to two window openings including reveals.
Does it need to be UV-stable?
Internally, no. Externally, use a tape rated for UV exposure or cover it promptly.
Passivhaus-certified airtightness starts with the details. Browse our Airtight Tape range or talk to us about speccing your air barrier system.