Airtightness Tape: Choosing, Sizing and Installing It
Airtightness tape seals a junction between parts of a building’s air barrier. It can connect a window frame to a reveal, join compatible membrane laps or close a board joint. Its value is in the connection: the tape must bond to both surfaces and continue into the rest of the airtight layer.
For a plasterable window and junction detail, IM Fleece Airtightness Tape is available in 75mm, 100mm, 150mm and 200mm widths, all on 25m rolls. Start with the junction drawing, then choose the width, surface preparation and compatible adjoining system. A wider tape cannot compensate for a dusty substrate or an unsupported gap.
What does airtightness tape do?
The tape closes paths for uncontrolled air movement across a join. It supports the air barrier, while planned ventilation supplies fresh air. A successful building depends on both; sealing a junction does not replace ventilation design.
Vapour diffusion is a separate property. Two airtight tapes can have different resistance to water vapour. IM Fleece Tape has a modified polyamide film and nonwoven fleece construction with moisture-dependent vapour resistance. Check the manufacturer’s data and the moisture design of the whole build-up rather than assuming every airtight tape is a vapour barrier.
Choose the detail before the roll
- Window and door perimeters: identify the air-barrier layer in the reveal and the suitable bonding area on the frame. Keep drainage and operating parts clear.
- Board joints: confirm the board forms part of the designed airtight layer and that the tape bonds to its actual surface finish.
- Membrane laps: use the tape and overlap specified for that membrane system. Not every adhesive is compatible with every sheet.
- Changes of material: allow for movement between timber, steel, masonry and frames. Detail corners without putting the adhesive under constant tension.
- Service penetrations: select a collar, grommet, tape or liquid detail that closes around the actual shape. Avoid channels between grouped cables or irregular pipework.
For frame selection, use our guide to airtight tape for windows and retrofit. For a site sequence, go to installing fleece tape around windows and doors.
How to choose the tape width
Measure the path the tape must follow, including returns around the reveal. Allow enough width for the specified bonded area on both sides plus any movement allowance. Use the manufacturer’s junction detail to decide the width; the gap alone is not the measurement.
| Available width | What to check before selecting |
|---|---|
| 75mm | Whether the narrow detail leaves sufficient bond on each surface. |
| 100mm | Whether a wider return or extra bonding area is needed. |
| 150mm | The full path across a broad reveal or change of plane. |
| 200mm | Support, movement and bonding requirements across the widest detail. |
These are the available sizes, not a rule that a particular width suits every window or gap. Check the IM Fleece Tape data sheet or send the detail to the technical team if the surfaces are uneven or the movement is uncertain.
Fleece, foil and butyl: compare the whole tape
Fleece and foil describe facings or layers; butyl describes an adhesive or sealing material. The terms are not mutually exclusive. Some tapes combine a fleece face with a butyl adhesive. Compare the complete product’s substrate compatibility, vapour properties, movement capacity, exposure limits and permitted finish.
A fleece surface may suit a plastered reveal, but confirm the finish system. Do not assume all foil tapes are unsuitable for a particular finish or all fleece tapes have the same performance. Our fleece, butyl and foil tape comparison explains what to check.
Installation checks that prevent avoidable leaks
- Prepare a sound surface. Remove loose material, dust, grease and contamination. Repair weak areas and confirm whether the surface needs a compatible primer.
- Check dryness and temperature. The IM Fleece Tape data sheet gives an application range of +5°C to +40°C and excludes wet or damp surfaces. Its service-temperature resistance is not permission to install outside that range.
- Plan joins and corners. Position the tape without tension, keep movement allowance where required and overlap tape-to-tape joins by at least 50mm.
- Apply pressure. Work progressively from the release liner and use a roller to press the bonded area. An adhesive layer needs contact with the substrate; a loose-looking tape edge is a reason to investigate.
- Connect the adjacent system. Follow the specified tape-to-membrane detail, preparation and curing requirements. Do not improvise by sticking self-adhesive tape onto a wet coating.
- Inspect before covering. Check for lifted edges, wrinkles, gaps at corners and unsealed ends. Photograph the detail and arrange the project’s required testing while repairs remain accessible.
For exposed work, the stated 12-month UV stability is a maximum exposure limit before protection, not a reason to leave a finished junction uncovered. Read the full installation guide before work starts.
When tape and liquid membrane work together
A tape can bridge a suitable linear connection while a liquid membrane covers the adjoining wall or board surface. Passive Purple is one option for a specified internal airtight and vapour-control layer. The interface still needs compatible materials and the stated coating coverage.
Use our tape and liquid membrane guide to plan the division of work. For lifting edges or a failed adhesion test, follow the tape troubleshooting guide before applying another layer over the problem.
Frequently asked questions
How many rolls do I need?
Measure the actual taped routes, then add tape-to-tape overlaps, corner pieces and a cutting allowance based on the detail. For example, a measured 40m route plus 4m of allowances needs 44m, so two 25m rolls of the selected width provide 50m. That is a worked example, not a fixed allowance for every project.
Can I plaster over IM Fleece Tape?
The fleece face is intended to accept compatible plaster or paint. Confirm the proposed finish, substrate and installation sequence. The tape must already be correctly bonded before the finish is applied.
Does airtightness tape provide radon protection?
Not automatically. A radon barrier needs suitable materials and validated joints and penetrations. Do not infer gas resistance from airtightness or the word fleece. Read our radon and service-penetration guide before specifying a gas-control detail.
Does tape guarantee a Passivhaus result?
No single product guarantees a whole-building result. The design, all connections, workmanship and pressure test determine performance. Tape contributes at the junctions included in the specified system.
Choose the right tape and detail together. View IM Fleece Tape sizes and pack options, or ask the technical team to check an unusual substrate or junction.
Technical source: IM Fleece Tape product data and installation guide, checked 16 September 2026. Use the current manufacturer instructions and project specification.