Black Mould Around Windows: Why It Forms and How to Stop It
Mould on the reveals, the sill and the bottom of the curtains is almost never a hygiene failure. It is the predictable result of warm, humid room air meeting a glass surface that is colder than the dew point of that air.
Why the reveal goes black before the glass does
Single glazing is the coldest surface in a room, but the plaster reveal beside it is a close second — it is a thermal bridge running straight through the wall. Water condenses first on the coldest surface, then wicks into porous plaster and paint, which is exactly the food source mould spores need. Cleaning removes the colony but not the condition, so it returns in the same place within weeks.
- Cold surface, not dirty surface, is the root cause
- Plaster and emulsion hold moisture longer than glass
- Recurring mould in one spot points to a thermal bridge
The two numbers that decide whether you get mould
Mould needs a surface relative humidity above roughly 80% for a sustained period. That depends on room humidity and surface temperature. Cooking, drying laundry indoors and showering raise the first; a 4 mm single pane in a South Yorkshire winter drives the second down towards outside temperature. Raise the surface temperature by a few degrees and the same room air no longer condenses.
What secondary glazing changes
A room-side pane with a sealed perimeter puts a still-air cavity between you and the cold original. The room-side glass then sits far closer to room temperature, typically 4–6°C warmer, which is usually enough to move the whole window assembly out of the condensation risk band. Because the cavity is vented outward to the primary window rather than into the room, residual moisture escapes rather than pooling on the sill.
- Warmer room-side glass and reveal
- Sealed room-side, vented outward
- Trickle vents available where Part F background ventilation is needed
What to fix alongside it
Glazing alone will not solve a genuinely over-humid house. We will always tell you if the honest answer is extraction first: a working bathroom fan on a humidistat, a kitchen extractor vented outside rather than recirculating, and somewhere other than a bedroom radiator to dry washing. Fix those and the window upgrade holds.
Frequently asked questions
In most cases yes, because it removes the cold surface the moisture was condensing on. If the house is very humid — indoor laundry drying, no working extraction — you need ventilation improvements alongside it, and we will say so at survey rather than sell you glazing that will not solve it.
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South Yorkshire's specialists in secondary glazing — survey, measure, manufacture and install.