Airtightness detailing around windows and doors (UK guide)
Windows and doors are airtightness hot-spots. However good the frame itself, there's always a perimeter gap between it and the structural opening — and if that gap isn't sealed into the building's airtight layer, it leaks. Multiply that by every opening in the building and it's often the difference between a pass and a fail on the air test.
The principle: the airtight layer must be made continuous across the opening — carried from the wall, around the reveal, and sealed onto the frame — so there's no break where the window meets the wall.
Start with the right fixing gap
A window needs a consistent gap around it for fixing, packing and sealing — not too tight to seal, not so wide it's hard to bridge. Pack and fix the frame square and plumb, keeping the perimeter gap even all the way round. That even gap is what you'll seal; a frame rammed hard against the structure in places and wide open in others is difficult to make airtight.
Seal the gap — airtight inside, weathertight outside
Window perimeters follow the same rule as the wall build-up: sealed (airtight) on the inside, weather-resistant but vapour-open on the outside. On the internal face you form the continuous airtight seal; on the external face you keep driving rain out while letting any moisture in the gap escape. Expanding foam alone is not an airtight seal — it fills the gap but doesn't reliably stop air, so it must be combined with proper sealing.
Tying into the airtight layer
This is the part people miss. The seal around the frame has to connect to the wall's airtight layer so the line is unbroken:
- Airtight tapes — airtight tapes bridge from the frame onto the reveal and the airtight layer, forming a continuous internal seal. Many are designed to be plastered or finished over.
- Airtight sealants — Passivhaus airtight sealant or The GOAT airtight flexible sealant seal the frame-to-reveal joint and accommodate the small movements windows go through.
- Liquid airtight layer — where the reveal is coated with a liquid airtight/vapour control layer such as Passive Purple, the tape or sealant ties neatly into that continuous surface.
Don't forget the reveal and the cill
Leakage often sneaks in at the bottom corners and under the cill, where several details meet. Treat the corners carefully — tapes should turn the corner without tenting or gaps — and make sure the cill detail is sealed into the same continuous line.
Common mistakes
- Relying on expanding foam as the airtight seal. It insulates and fills, but doesn't reliably stop air.
- Sealing the frame but not tying into the wall's airtight layer. The line has to be continuous.
- Sealing tight on the outside too. Keep the outer face weather-resistant but vapour-open so the gap can dry.
- Uneven fixing gaps. Hard to seal consistently — get the frame packed evenly first.
Related reading
- How to pass an air permeability test
- How to make a building airtight
- Which side of the insulation does a vapour control layer go?
Frequently asked questions
Is expanding foam enough to seal around a window?
No — foam fills and insulates the gap but is not a reliable airtight seal on its own. Use it with airtight tape and/or sealant that ties into the building's airtight layer.
Should I seal the inside or the outside of the window?
Both, differently: airtight on the inside, weather-resistant but vapour-open on the outside, so the perimeter can dry outward.
Why do my windows leak air at the bottom corners?
Corners and cills are where multiple details meet and tapes are easiest to leave gapped. Detail the corners carefully so the seal turns them continuously.
Detailing openings on a project? Our technical team can recommend the right tapes, sealants and airtight layer for your window and door perimeters.