Facade & Window Membranes
Sealing the Junction Between Windows and the Wall Build-Up
Facade and window membranes seal the joint where a window or door frame meets the surrounding wall construction, one of the most common places for air leakage and water penetration to occur in an otherwise well-built wall. These membranes wrap around the frame perimeter, tying into the airtightness and weatherproofing layers already in the wall, so the window reveal doesn't become the weak point that undoes the performance of everything else around it. On a lot of jobs, window junctions are actually where air tests fail, not the main wall area.
Where Facade and Window Membranes Are Used
These membranes come up on most window and door installations where airtightness and weather performance both matter:
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New build window and door reveal sealing
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Retrofit window replacement in existing walls
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Timber frame and masonry construction
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Curtain walling and facade glazing junctions
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Render and cladding interfaces around openings
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Passivhaus and low-energy window installations
Internally, the membrane usually needs to manage vapour control, while externally it needs to shed water and allow the wall to breathe, so the two sides of the same reveal often use different membrane types entirely.
Internal Versus External Membrane Requirements
This is where people sometimes get caught out. The internal reveal membrane needs to restrict vapour movement into the wall, similar to a vapour control layer, while the external membrane needs to be weatherproof but still vapour permeable, similar to a breather membrane. Using the same product on both sides, or mixing them up, can undermine the whole junction rather than improve it.
Worth checking before ordering:
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Whether the membrane is designed for internal or external use, since the two aren't interchangeable
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Vapour permeability rating suited to its position in the build-up
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Compatibility with the airtightness tape being used at laps and corners
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UV resistance if the external membrane will be exposed before cladding or render is applied
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Flexibility and stretch, since window reveals often have awkward corners and angles
Getting Corners and Junctions Right
Flat runs of membranes rarely cause problems. It's the corners, particularly at reveal returns and cill junctions, where gaps and creases tend to appear if the membrane isn't cut and dressed properly. Pre-formed corner pieces exist for exactly this reason, and using them instead of trying to fold standard membrane around a tight corner usually gives a far more reliable seal.
Frequently Asked Questions
- Why do windows need a separate membrane from the rest of the wall?
Window reveals are a common weak point for air and water leakage because multiple materials meet in a small, awkward area. A dedicated membrane ties the window into the wall's airtightness and weatherproofing layers properly. - Do I need different membranes for the inside and outside of a window?
Yes, generally. The internal side needs vapour control, while the external side needs to be weatherproof yet breathable, so using a single membrane on both sides often doesn't perform correctly on either. - What causes air leakage around windows even after installation?
Poorly sealed corners, gaps at the cill, or membrane that hasn't been properly taped and dressed into the reveal are the most common causes, more so than the window unit itself.