Black Mould Around Windows: The Cause and the Lasting Fix

You wiped it off with bleach in November. By the middle of December the black speckling is back in the top corners of the reveal and along the bottom rail, and there is a tidemark on the paint where the water has been running off the glass. You are now wondering whether the window is leaking.

It almost certainly is not. Mould in those two places is a temperature problem, and until the temperature of those surfaces changes it will keep coming back however often you clean it.

Mould does not need water. It needs a cold surface

This is the point everything else follows from. Mould germinates on a surface that sits at high relative humidity, not one that is visibly wet. The threshold used in building physics is around 80% relative humidity at the surface, held for long enough. Liquid water is not required.

That distinction has a practical consequence. Take a room at 20°C and 55% relative humidity, which is entirely ordinary in a London flat in January. Condensation on a surface in that room begins at about 10.7°C. But mould conditions begin at about 14.1°C. There is a band of roughly three degrees where a surface will never run with water and will still grow mould happily. That is why the reveals go black while the plaster next to them looks dry, and it is why "there's no condensation on that wall" is not an argument.

Two other things have to be in place, and in a plaster reveal they always are.

Something to eat. Mould does not eat plaster or glass. It eats what is on the surface: the binders in emulsion, dust, skin cells, cooking residue. A freshly painted reveal takes a little longer to colonise than a grubby one, but only a little.

Still air. Air movement carries moisture away from a surface and evens out its temperature. Corners are the worst place in a room for air movement, and behind a curtain or blind is the second worst. That is exactly the pattern you see: corners first, then along the bottom rail where the curtain sits against the glass.

The numbers behind your window

Dew point is the temperature at which the air in your room becomes saturated. Anything colder than that gets wet.

Now the other half. On a cold London night, say 2°C outside and 20°C in, the inside surface temperature of your glazing works out roughly like this:

Read those two tables together and the whole thing falls out. Single glazing in a normally heated London room is below the dew point on any cold night, because keeping a home below 44% relative humidity in winter is neither achievable nor desirable. Water condenses on the glass, runs down, and collects on the bottom rail, which is timber and holds it. That is why the bottom rail rots before anything else, why the cill takes the damage, and why the gap between single and double glazing shows up as damp long before it shows up on a heating bill.

The reveal is a separate calculation. A solid nine-inch brick wall runs at around 2.1 W/(m²·K), and a corner is thermally worse than a flat surface because the air barely moves there. In the same conditions a reveal corner sits at roughly 10 to 11°C, which is above the dew point but comfortably inside mould territory. Hence: corners black, glass wet, wall between them apparently fine.

What the pattern is telling you

That last row is the important one. Condensation is seasonal and it dries out. If a patch is damp in July, stop reading about ventilation and start looking for water coming in.

Ruling out the leaks that look like condensation

Four failures on a sash window let water in and produce staining that people misread as condensation.

Failed putty. The fillet cracks away from the glass or the rebate and water runs down the inside face of the pane into the timber. The tell is a stain starting at the bottom of the glazing rebate rather than at the bottom rail, and putty you can lift with a fingernail.

A blocked drip groove. Under the front edge of the cill there should be a groove that breaks the flow of water so it drips clear. Fill it with paint, which happens after about four redecorations, and water tracks back along the underside to the brickwork. The tell is damp on the wall below the window, worst after driving rain.

Defective pointing at the reveal. Where the box frame meets the brickwork there should be a mastic or mortar seal. Open joints there let wind-driven rain into the reveal. The tell is damp on one elevation only, the one that takes the weather, and only after wind and rain together.

A gutter or hopper above. Overflowing at a joint puts water down the face of the wall in a stripe. The tell is obvious once you go out and look during rain, which almost nobody does.

If none of those apply, and the problem is worst on cold still nights and gone in a heatwave, it is condensation. There is more diagnostic detail in the guide to why condensation forms on the inside of windows.

Cleaning it off so it stays off

Bleach is the wrong product, and it is worth knowing why. Household bleach is mostly water with a surface-acting oxidiser in it. On glass or gloss paint it works fine. On plaster, emulsion or bare timber the oxidiser stays on the surface while the water carries the growth deeper into the pores, and you have added moisture to a surface that already had too much.

1. Ventilate the room and open the window. Wear disposable gloves and a mask, and keep anyone with asthma or a respiratory condition out of the room while you work.
2. Wipe the growth off with a damp cloth rather than brushing or hoovering it. Dry brushing throws spores into the air.
3. Apply a proper fungicidal wash and leave it the full contact time on the label. This is the step people skip. The contact time is what kills what is below the surface.
4. Let the surface dry completely. Days, not hours, on plaster.
5. Redecorate if you need to, and consider a fungicidal emulsion on the reveals. Understand what it does: it puts a biocide in the paint film so colonisation is slower. It does not warm the wall.
6. Bin the cloths. Do not put them in the wash.

Government guidance on damp and mould is explicit that this stage is not the answer on its own: removing surface mould will not prevent it reappearing, and larger areas should be dealt with by someone qualified. Cleaning buys you a season. The next section is what buys you a decade.

The fixes that last, in the order they are worth doing

Prices vary by access, condition and borough, and are before VAT. Work down that list rather than jumping to the bottom of it. Most of the mould we get called to look at in London flats is fixed somewhere in the first four rows.

On extract, Approved Document F gives the rates a fan is meant to achieve: 30 litres per second in a kitchen where a cooker hood vents outside, 60 where it does not, 15 in a bathroom and 6 in a WC. A tired 1990s axial fan on a long duct with two bends will not be moving anything like that, and holding a sheet of loo roll to the grille is not a test.

Household moisture output is the other half of the sum. A family of four puts something in the order of 10 to 15 litres of water into the air every day through cooking, washing, bathing and breathing, and one load of washing dried indoors adds a couple of litres on its own. That water has to leave the building. If the only route out is the coldest surface in the room, the coldest surface in the room is where it goes.

The draught-proofing paradox, resolved

People are told draught-proofing causes mould. Done badly, it can. Done properly, it is one of the better things you can do about it, for a reason that is not obvious.

An unsealed sash leaks cold air along its full perimeter. That air runs down the reveal and across the wall below, chilling exactly the surfaces where mould appears. It also makes the room feel colder than the thermostat says, so people either turn the heating up and pay for it or let the room run cold. Neither helps.

Sealing the sash with routed carrier and pile seals raises the surface temperature of the surrounding plaster and makes the heating do what you paid for. The condition is that the house keeps a controlled route for moisture to leave: working extract fans in the wet rooms, trickle ventilation, an open flue or a passive vent. That is a design decision, not an accident, and it is the difference between the two outcomes. Where the DIY versions go wrong is covered in how to draught proof a sash window and in the comparison of which draught excluders actually work.

One piece of good news: draught stripping supplied and installed in a home is zero-rated for VAT as an energy-saving material until 31 March 2027, after which it drops to the 5% reduced rate. Secondary and double glazing get no such relief and stay at 20%.

If you rent

Damp and mould are assessed as a hazard under the Housing Health and Safety Rating System, and a home must be free of category 1 hazards. Government guidance to housing providers is unusually blunt about where responsibility sits: it says it is "totally unreasonable to blame damp and mould in the home on 'lifestyle choices'", on the basis that cooking, washing and breathing are not optional and that the underlying causes are building deficiencies, inadequate ventilation and inadequate heating.

If you are a social tenant in England, the Hazards in Social Housing (Prescribed Requirements) (England) Regulations 2025, generally known as Awaab's Law, came into force on 27 October 2025 and set timescales for social landlords to investigate and act on damp and mould. Those regulations cover the social rented sector rather than private landlords or owner-occupiers, though private landlords remain subject to the HHSRS and to their repairing obligations. Report it in writing, keep the dates, and photograph the affected area with something in shot for scale.

What to do next

Buy a cheap hygrometer and put it in the worst room. If it reads above 60% on a winter evening, you have a moisture and ventilation problem before you have a window problem, and no amount of glazing will fix that. If it reads 45 to 55% and the glass is still streaming, the surface temperature is the issue and glazing or secondary glazing is the answer.

Then check the bottom rails and cills with a screwdriver point. Mould on the plaster is often the visible half of a problem whose expensive half is wet rot in the sash itself, and if the timber is soft that changes the order of work.

Where the windows are sound and the room is simply cold and leaky, our draught-proofing service is usually the intervention that pays for itself, and the wider sash window repair service covers the London boroughs where solid walls and single glazing make this so common.