What is a cavity closer, and why do window reveals go damp?
The cavity has to be closed where a window or door interrupts it, and how it is closed decides whether the reveal is a cold bridge. Why black mould at the edge of a window is usually a construction detail rather than a lifestyle problem.
- Difficulty
- intermediate
- Time
- 8 min
- Read
- 4 min
Short answer
At every opening the cavity is stopped so the two leaves are not open to the weather. If it is closed with masonry, the two leaves are joined and heat runs straight across — a cold bridge — and the reveal runs colder than the rest of the wall, which is where condensation and mould appear. An insulated cavity closer breaks that path, which is why they are standard now and absent in older cavity work.
Mould in a neat vertical band down the side of a window, or in the corner where the reveal meets the wall, gets blamed on drying washing and closed trickle vents. Sometimes that is fair. Very often the reveal is simply colder than everywhere else, for a reason that was built in decades ago.
Step by step
- Understand why the cavity has to be closed.An open cavity at a reveal would let wind and rain straight in and would let the cavity ventilate to the outside. Something has to seal the gap between the leaves at every opening.
- Recognise the traditional closure.Older cavity work returned the outer leaf into the reveal so the two leaves met, with a vertical damp proof course between them. Structurally fine, and thermally a direct bridge from outside to inside.
- Recognise the modern closer.A proprietary insulated cavity closer — usually an extruded plastic profile with an insulating core — that closes the cavity, carries the damp proof course and separates the leaves thermally in one component.
- Learn what a cold bridge does to a reveal.Heat takes the shortest route out. Where the leaves are joined, the inner surface at the reveal sits colder than the wall around it. Warm room air touching that colder surface reaches its dew point and condenses, and mould grows where the surface is repeatedly damp.
- Distinguish it from a leak.Condensation at a reveal is worst in cold weather, appears on the surface rather than soaking out of the wall, is often accompanied by water on the glass, and is frequently symmetrical about the window. Penetrating damp follows rain and wind direction and is usually patchier.
- Check the obvious contributors before rebuilding anything.Whether the room is heated at all, whether there is any ventilation, whether the trickle vents are closed, and whether a plastic-backed lining or furniture is trapping still air against the reveal.
- Know what can actually be improved.Insulated plasterboard or an insulating render to the reveal, done so it does not simply move the cold spot to its edge, is the usual retrofit. It is a detail worth getting designed, because a badly stopped insulation upgrade relocates the mould rather than removing it.
Tips
- New windows fitted into an old opening rarely change the closure detail, so a house that had cold reveals before very often still has them after. That surprises people who expected new windows to solve the mould.
- A cheap infrared thermometer aimed at the reveal and then at the middle of the same wall on a cold day gives you the answer in about ten seconds.
- Where the reveal is cold, warm dry air moving across it does more good than any surface treatment. Mould washes off; the cold surface does not.
Common mistakes
- Treating reveal mould only with fungicidal wash — It removes what has grown and does nothing about the surface being cold and damp again next week. It is a cleaning step, not a remedy.
- Sealing every vent to keep the room warmer — It raises the moisture content of the air in a room that already has a cold surface in it, which makes condensation worse rather than better.
- Insulating the reveal and stopping the insulation at the frame — The cold path reappears at the edge of the new insulation. The detail has to be continuous into the frame, which is precisely why it is worth designing.
If it doesn't work
A neat vertical band of black mould down the side of a window
Cause: A cold bridge at the reveal producing repeated surface condensation — Fix: Confirm it with a surface temperature reading, then improve ventilation and heating first and consider insulating the reveal as a designed detail. Clean the mould, but do not expect cleaning to be the fix.
Mould in the top corners of a room rather than at the reveal
Cause: A different cold bridge — often the junction of wall and ceiling, or a lintel — Fix: The same reasoning applies. Find the cold surface first, because the treatment depends entirely on why it is cold.
Water pooling on the window board every cold morning
Cause: Condensation on the glass running down, which is a ventilation and humidity question — Fix: Ventilate after showering, cooking and drying washing, and use the trickle vents. Persistent glass condensation on modern double glazing points at humidity in the room rather than at the window.
Questions people ask
Do I have insulated cavity closers?
Houses built to recent standards will have them. Older cavity-walled houses usually do not, and the reveal will be masonry returned across the cavity. The behaviour of the reveal in cold weather tells you more than the build date.
Is mould at a window my fault?
It is usually a combination: a cold surface that was built in, and moisture in the air that comes from living in the house. Both parts are real. Blaming either one alone is what makes these arguments unproductive.
Will external wall insulation solve it?
It can, because it warms the whole wall including the reveal, but only if the reveal detail is insulated too. Returning the insulation into the reveal is one of the details that separates a good installation from a poor one.