Sealing Window and Door Frames Airtight with Fleece Tape
Windows, doors and board joints are some of the most common places air leaks out of a building. They sit at junctions between different materials, they move, and they are fiddly to seal — which is exactly where a good airtight tape earns its place in the system.
Why frames and joints leak
The gap between a window or door frame and the surrounding structure is a natural weak point, as are the joints between sheathing boards. Left unsealed, these gaps let air, draughts and moisture straight through the fabric — and they show up clearly on a pressure test. With Approved Document L now requiring every new dwelling to be air-tested (max 8 m³/(h·m²) at 50 Pa, with most targeting around 5), getting these details right is no longer optional.
The solution: a fleece-backed airtight tape
IM Fleece Airtight Tape is a flexible, fleece-backed tape that bonds strongly to timber, brick, concrete and membranes. Its fleece face takes plaster or paint, so it disappears into the finish, and it seals frames and joints reliably as part of a complete airtight system.
Where tape fits the wider system
Tape isn’t a rival to liquid membrane — it’s a teammate. Use tape for the precise, linear details (window and door frames, board joints), Passive Purple Brush for junctions and penetrations, and liquid Passive Purple for the large areas. Right tool for each part of the build.
How to apply it
- Make sure surfaces are clean, sound and dry
- Apply the tape across the frame-to-structure gap or board joint
- Press it down firmly — a foil applicator gun gives faster, crease-free results
- Plaster or paint over the fleece face as part of the finish
Benefits
- Strong adhesion to common substrates
- Fleece face — plaster or paint straight over it
- Flexible and durable for corners and joints
- Smoke-tight as well as airtight
Frequently asked questions
Tape or liquid membrane — which should I use? Both. Tape excels at linear frame and board details; liquid covers large areas and complex shapes.
Can I plaster over it? Yes — the fleece backing is designed to take plaster and paint.
See how it all fits together in How to Make a Building Airtight, or ask our team.