You can insulate 95% of a wall perfectly and still end up with mould — because the 15cm around each window is where wall insulation succeeds or fails. Reveal and sill detailing appears as its own line on 33 of our real quotes, and this guide explains why we treat it as non-negotiable in both external and internal wall insulation.
Why the strip around the window decides the whole job
A window opening interrupts the insulation layer. The main wall can carry 100mm of board, but at the opening the wall turns through 90 degrees and runs — bare masonry — to the window frame. If the insulation stops at the corner instead of turning into that reveal, you have left a continuous ribbon of uninsulated wall wrapped around every window and door in the house.
Before insulation, that ribbon was no colder than the rest of the wall. After insulation it is the coldest surface in the room by a wide margin, because heat that used to leave through the whole wall now funnels out through the one path left open. Insulating a wall and skipping the reveals does not leave a small gap in the benefit — it actively concentrates the old problem into a visible frame around your windows. This is cold bridging in its purest form; the mechanism is covered in our cold bridging guide.
The physics: surface temperature and dew point
Condensation is a surface-temperature problem. Warm air holds more moisture than cold air; when room air touches a surface below its dew point, the excess moisture condenses there. In an averagely heated, averagely ventilated room, the dew point sits a few degrees below room temperature — so any surface cold enough crosses the line, and the reveal beside a window is the prime candidate.
The fix does not require matching the main wall’s insulation thickness. Even 20–30mm of high-performance board in the reveal lifts the surface temperature above dew point in normal conditions. That is the whole game: not maximum insulation everywhere, but no surface left below the line. It is also why reveal boards are thin and why “there was no room for insulation in the reveal” is an excuse, not a reason — there is nearly always room for the board that matters.
Reveals in external wall insulation
On an EWI job the main boards stop at the corner of each opening, and thinner boards — typically 20–30mm — are bonded into the reveal, running from the face of the new insulation back to the window frame. The junction with the frame is closed with a flexible sealing bead, not a rigid fillet of render: frames move with temperature, and a rigid joint cracks and lets water in behind the system.
An installer’s tell worth knowing: reveal boards must be cut so the main wall’s mesh and basecoat wrap the corner in one continuous detail, with a corner bead protecting the arris. If the reveal is rendered as an afterthought — a separate patch, no bead — the corner chips and hairline-cracks within a couple of years. On a survey we can read the quality of someone else’s EWI job from the window reveals faster than from anywhere else on the building. The full external system is explained in our EWI quote guide and on the EWI service page.
Sills: the horizontal part everyone forgets
Thicken a wall by 100mm and the existing sill no longer reaches the face of the building. Rain that lands on the sill then discharges straight onto the new render below the window — the fastest way to stain and saturate an EWI system. So sills are extended or replaced: a new sill (aluminium, or a matching material on period properties) that projects past the finished wall face, with end caps where it meets the reveals and a drip groove or drip edge on the underside so water lets go of the sill instead of tracking back to the wall.
That drip detail is millimetres of metalwork, and it is the difference between a facade that stays clean and one with a grey moustache under every window within two winters. Internally, IWI has its own sill consequence: the window board must be replaced wider because the reveal is now deeper.
Reveals in internal wall insulation
IWI has exactly the same reveal problem in mirror image. Boarding the room face of the wall and stopping at the window opening leaves the internal reveal as bare, cold plaster on solid masonry — inches from the warm, moist air of the room. Internal reveals get thin insulated board returns, sealed at the frame, with the vapour control layer carried continuously around the corner. A break in the vapour layer at the reveal lets moist air behind the insulation, which is the hidden version of the same failure — covered in our airtightness and vapour membranes guide.
Because internal reveals are visible joinery — linings, beads, window board — this is finishing-trade work, of a piece with the skirtings and plaster covered in why good IWI is a finishing trade. It is fiddly, it takes time, and it is on the quote as its own item because it cannot be honestly absorbed into a per-m² board rate.
The mould line of a skipped reveal
The signature is unmistakable once you know it: a dark line or band of mould running around the window opening — tight to the frame, on the reveal faces, often heaviest across the top — on a wall that is otherwise clean and dry. General condensation from lifestyle spreads diffusely and favours corners and behind furniture; a skipped reveal draws a neat frame around the window. If insulation was fitted recently and that frame has appeared, the reveals were not done.
The honest caveat: not every damp patch near a window is a reveal failure. A leaking sill joint, a failed sealant bead around the frame, or blocked window trickle vents in an unventilated room can each produce moisture in the same neighbourhood — and if the whole house lacks extract ventilation, treating the reveals alone will not stop condensation; see ventilation after insulating. A proper survey separates these before anyone sells you remedial work. If you want ours, get a free, fixed, itemised quote — reveal and sill detailing will be on it as a line you can read.
Reveals and sills FAQs
What is a window reveal in insulation work?
The reveal is the short return of wall between the face of the wall and the window frame — the sides and top of the opening. In wall insulation it is the strip that most often gets skipped, because it is fiddly: it needs thin insulation boards cut to fit a space usually only a few centimetres deep between the wall face and the frame.
Why do windows get mould around them after wall insulation?
Because insulating the main wall while leaving the reveals bare makes the reveals the coldest surfaces in the room — colder, relatively, than they were before. Warm indoor air condenses on the coldest surface available, so moisture that used to spread across a cold wall now concentrates in a neat frame around the window, and mould follows it.
Do sills need to be changed for external wall insulation?
Almost always. EWI thickens the wall by 100mm or more, so the existing sill no longer overhangs the face of the building — rainwater would run off the sill straight down the new render. Sills are extended or replaced so they project past the new wall face with a drip groove, which is why reveal and sill works appear as their own line on our quotes.
How thick is the insulation used in reveals?
Much thinner than the main wall — typically 20–30mm high-performance board, because the space between wall face and window frame is limited. Thin board in the reveal does not match the main wall’s insulation value, and does not need to: its job is to lift the surface temperature of the reveal above the dew point so condensation cannot form there.
Is reveal detailing included in a normal insulation quote?
It should be, but often is not — reveals and sills appear on 33 of our quotes as explicit lines. When you compare quotes, look for the words “reveals” and “sills”. If they are absent, the price is lower for a reason, and the missing work is exactly the part that determines whether the windows stay condensation-free.