Airtightness Tape for Windows: Choosing for Retrofit and Passivhaus
The best airtightness tape for a window is the one that connects the frame to the specified air barrier, bonds to the surfaces on site and fits the finishing detail. A roll chosen only by width or price can leave an awkward transition at the reveal, even when the window itself performs well.
IM Fleece Airtight Tape is designed for window connections and other building junctions. Its fleece face can receive plaster or paint, making it useful where the connection needs to disappear beneath a finish. This guide explains how to decide whether that detail suits your project; our main airtightness tape guide covers the wider range of joints and applications.
Start with the airtight line, not the window opening
Ask the designer to identify which layer carries the airtight line through the wall. It might be a specified membrane, a suitable board system or a prepared masonry surface with a liquid air barrier. The tape must connect the window frame to that layer continuously. Sticking tape to a decorative lining that is disconnected from the air barrier does not complete the connection.
Check the sill, head and both jambs together. Fixing brackets, cavity closures, sill supports and changes of material can interrupt an otherwise straightforward detail. Plan how each corner will close before the window is surrounded by insulation and finishes. Keep the frame's drainage paths and operating parts clear.
Match the adhesive to both sides of the joint
A connection has at least two bonding surfaces. A clean frame and a dusty reveal need different preparation, even though one strip of tape crosses both. IM's published documentation lists masonry, timber, insulation, steel, uPVC and aluminium window applications. Confirm compatibility with the particular coatings, boards and membranes in your build-up, and make a trial bond where the surface is uncertain.
In retrofit work, inspect the existing material beneath old sealant, paint and loose plaster. Adhesion to an unstable coating is only as reliable as that coating's bond. Repair unsound areas and confirm any primer with the technical team. A wider tape cannot compensate for a crumbling reveal.
Choose fleece when the connection will be finished over
A fleece face gives the finishing trade a surface intended to accept plaster or paint. That is useful around internal reveals where a visible strip would otherwise interrupt the finish. It also lets the tape form part of an agreed detail with a compatible liquid membrane.
Confirm the complete finishing sequence: adhesive contact, any liquid coating, curing and the specified plaster or decoration. A tape's fleece backing does not make every coating compatible. If you are combining products, use the documented system detail and arrange a sample area where needed.
Choose width from the measured detail
IM Fleece Tape is available in 75, 100, 150 and 200 mm widths, in 25 m rolls. Measure the gap and the sound bonding area available on the frame and reveal. Allow for the shape of the corner and the movement provision in the detail.
Do not treat a tape's overall width as its permitted unsupported span. A 200 mm roll is not permission to bridge a 200 mm structural opening. If the gap is large, ask for a supported or otherwise approved connection detail before ordering. Calculate quantities from actual perimeters and allow separately for joins, corners and waste.
Check vapour control as well as airtightness
Airtightness concerns air leaking through gaps; vapour control concerns moisture diffusion through materials. A product can serve the airtight line while still having a particular vapour resistance that matters to the wall design.
The IM data sheet describes a moisture-dependent Sd value. Sd means equivalent air-layer thickness for water-vapour diffusion; it does not mean standard deviation. Ask for the current declared values required by your specification and assess them as part of the complete wall. Avoid choosing an internal or external connection by colour, backing type or the word “breathable” alone.
What changes on a retrofit or Passivhaus project?
Retrofit often adds uneven surfaces, restricted access and a need to preserve inward or outward drying paths. Agree the moisture strategy before sealing, and coordinate the ventilation provision. For Passivhaus work, tape is one connection within the building's airtightness strategy: the final result also depends on the surrounding air barrier, workmanship and testing.
Arrange an inspection while connections remain accessible. Photographs of the completed corners and an intermediate pressure test make defects easier to locate before the finishing trades cover them.
Specify, install and check the whole connection
Record the tape width, substrates, preparation, finishing sequence and inspection stage on the detail. Then follow our window and door tape installation guide for the application checks.
Explore Passive Purple system products for adjoining field areas, or send the technical team your window detail. When the connection is agreed, choose your IM Fleece Tape roll or box and download the current technical documents from the product page.