How do I stop mould growing on my window reveals?

Why the strip of plaster beside and above a window runs colder than the rest of the wall, and the three fixes that actually raise its temperature.

Difficulty: beginner
Time: 40 min
Read: 4 min
Safety: caution

Short answer

The reveal is a cold bridge. Where the wall turns into the window opening the cavity has to be closed, and in most houses it is closed with solid masonry that conducts heat straight through. Cleaning and repainting changes nothing. You need to raise the surface temperature: insulated reveal board, an insulated cavity closer when the window is replaced, or more warmth and less moisture in the room.

Mould that grows in a neat vertical strip up the side of a window and along the head above it is telling you something very precise. It is not following the water, it is following the temperature, and the temperature drops there because the insulation in the wall stops at the opening and solid material takes over.

Safety

Wear a fitted FFP2 or FFP3 mask, gloves and eye protection when cleaning mould, open the window and keep the door shut. Never sand or dry-brush it. Anyone with asthma or a respiratory condition should not do this job. If the reveal is plastered onto an old textured coating or the window was replaced before 2000, do not sand or abrade the surface — pre-2000 artex and some board products can contain asbestos and must be assessed by a licensed surveyor before being disturbed.

What you'll need

FFP2 or FFP3 mask, gloves, eye protection
Fungicidal wash
Disposable cloths
Insulated plasterboard or aerogel board (optional)

The cold bridge at a window reveal, in plan

Step by step

Confirm it is the reveal and not the frame.Mould in the sealant joint and on the frame is condensation running off the glass. Mould on the plaster of the reveal itself is the cold bridge. They often occur together and have slightly different fixes.
Clean it properly.Mask, gloves, window open, door shut. Fungicidal wash on a disposable cloth, working inwards from the edge of the patch. No brushing, no sanding.
Deal with the water running off the glass.Wipe the glass and the frame in the morning, and clear the drainage slots in the bottom of the frame so water that collects there drains outside rather than sitting against the reveal.
Improve the airflow past the reveal.Blinds and heavy curtains hanging right against the reveal cut it off from the room's warmth. Curtains that clear the reveal, or a blind mounted outside the recess, make a measurable difference.
Get some heat to it.A radiator under the window is not decoration — it puts a rising warm air curtain across the coldest surfaces in the room. If the radiator has been removed or is boxed in, that is part of the cause.
Insulate the reveal if the problem persists.A thin insulated plasterboard or an aerogel-backed board bonded to the reveal and skimmed raises its surface temperature above dew point. It loses a little of the opening width and it is the real fix.
Specify an insulated cavity closer if you are replacing the window.This is the one moment it is cheap. An insulated closer breaks the masonry path across the cavity at the opening. Ask for it by name at quote stage.
Reduce the moisture in the room as well.The reveal condenses because the room air's dew point has risen to meet it. Extract at source, air the room short and wide each morning, and do not dry washing in that room.

Tips

If mould appears on the reveals of one room only, look at what is different about that room: no radiator, an unheated room, drying washing, a shut door, or a bathroom next door with no fan.
Trickle vents in the window head are there to keep a small continuous air change going and they are directly relevant here. Closing them in winter is why reveals often start growing mould in December.
Anti-condensation paint on a reveal is a partial measure — it slightly raises surface temperature and delays condensation, but it does not defeat a solid cold bridge.

Common mistakes

Repainting the reveal each year with mould-resistant paint — The biocide is consumed within a few years and the surface temperature is unchanged, so the mould returns in exactly the same strip.
Sealing the trickle vents to keep the room warmer — It raises the room's humidity, which raises the dew point, which makes the cold reveal condense sooner and for longer.

Questions people ask

Why did this start after I had new windows fitted?

Two reasons. The new units are far better insulated than the old ones, so the glass is no longer the coldest surface — the reveal is, and the moisture that used to run down the glass now condenses on the plaster instead. And the old windows leaked air continuously, which ventilated the room by accident.

Is an insulated reveal board a big job?

For one window it is an afternoon plus decorating. The board is bonded on, the edges are made good, and the window boards and architraves may need packing out. It is well within reach of a competent DIYer.

The mould is only on the top of the reveal, not the sides. Why?

The lintel over the opening is usually the worst cold bridge of all, particularly a concrete or steel one, because it is a single dense element spanning from outside to inside.