The Coldest 150mm in the House: Window Reveals, Insulation and Old Walls

Nobody books a plasterer because they're excited about a window reveal. They book one because the wallpaper is off, the sash is out on the drive, and there's a narrow strip of wall either side of the opening that feels cold to the back of the hand. That strip is where a lot of old-house work succeeds or quietly fails.

The smallest surface in the room, doing the most work

A reveal is a thin band of plaster wrapped around an opening. It has almost no area compared with the wall, so it gets skipped in the thinking. But it sits between a cold pane of glass, a timber frame and a masonry wall, often with a radiator nowhere near it and a curtain only centimetres away. Warm, moist room air finds it. The surface is the coldest thing in the room apart from the glass, so that's where condensation forms first, and where black mould appears a winter later.

People then blame the plaster, or the paint, or the fact that lime was used. Nine times out of ten the plaster is doing exactly what it should. The reveal is simply colder than the dew point allows.

Where the cold gets in

Look at the junction properly. The outer leaf, the inner leaf, the lintel, the frame, and the gap between frame and masonry. In a Victorian terrace the gap may hold horsehair, mortar crumbs and a century of draught. In a 1930s semi it may be a finger's width of nothing. Cold air moves through that gap, and the reveal plaster is fixed to the coldest part of the wall.

There's also a thermal bridge through the frame itself and, in a lot of old walls, through a stone or timber lintel. None of this is fixed by a thicker skim coat.

Depth is the whole argument

How far the window sits from the wall face decides almost everything. In a thick stone wall in Cornwall or the Cotswolds the reveal might be 400mm deep, which gives room for insulation and a bit of thermal separation. In a 1930s brick wall the frame can sit close to the outer face and the reveal is a shallow, uninsulated fin running straight from cold masonry into the room. That fin is your cold bridge.

If the window is being replaced, push the frame back towards the middle of the wall if the wall construction allows it — you gain reveal depth and lose the fin. If it isn't, work with what's there: thin wood fibre, cork board, or an insulated backing plaster on the reveal face, followed by a compatible top coat. In a listed building this is often a non-starter, and the answer is to accept a cold surface and manage moisture instead.

Lime, clay and the timber question

Both materials can work. Clay is lovely in dry, warm rooms and buffers moisture well, but it isn't an insulator and it doesn't enjoy being permanently damp. Lime is the safer bet on a cold, occasionally damp reveal: it tolerates alkaline masonry, dries out, and has a long history in these junctions. Mix it with sand and hair, not with a dash of gypsum for speed.

The rule at the timber is simple: nothing that traps moisture against the frame. No cement, no acrylic sealant bridging the gap, no foil-backed board pressed tight against a sash that moves in the sun. Use a flexible airtightness tape or a suitable compressible seal, keep the plaster slightly back from the timber, and let the frame breathe and move.

Order of work

Fix the draughts at the frame first, then insulate, then plaster. Doing it the other way round means pulling good plaster off to chase a leak of cold air. Tape the frame-to-wall junction; seal the gap with something flexible rather than expanding foam, which can bow a frame. Form the arris — a rounded one is kinder in a small room and less likely to chip. Mesh the internal corners. Then plaster, and leave it to dry properly before painting. A breathable finish over a cold reveal is better than a vinyl emulsion that traps moisture behind it.

Heritage constraints

If the building is listed or in a conservation area, talk to the conservation officer before you touch the reveal. They will usually accept lime and clay where they are reversible and in keeping; they will usually push back on foam and cement. A small sample panel is often the fastest way to agreement.

Ventilation and the honest caveat

You cannot seal a house and skip ventilation. Trickle vents, extract fans and a bit of background air movement matter more on a cold reveal than any choice of finish. A tight house with no ventilation will grow mould on the coldest surface it can find, and that's usually the window reveal.

A quick checklist

• Insulate the reveal if depth allows; if not, accept it and manage moisture.
• Seal the frame junction flexibly, not rigidly.
• Keep gypsum away from cold reveals and damp masonry.
• Leave a small gap at the timber and let the frame move.
• Dry the plaster before painting, and choose a breathable finish.
• Ventilate — the reveal is where poor ventilation shows up first.

The reveal is a small job with a long memory. Get the depth and the junction right and the finish coat almost looks after itself; get them wrong and no amount of careful trowelling will stop the cold coming through.