In this article 7 sections
Black mould on windows and walls usually starts to show in autumn, when the weather turns colder, windows stay shut and the heating comes back on. The cause is condensation, and that’s why wiping the mould away doesn’t stop it returning. Removing surface mould will not prevent it from coming back.
Cleaning still matters. Mould on a window reveal or a canteen wall should be cleaned off safely and promptly. But the clean only lasts if someone also deals with the moisture, and that usually means ventilation, heating or a fault in the building.
Where condensation and black mould start
Air always carries some water as invisible vapour. Warm air holds more than cold air, and when warm, moist air touches a cold surface it cools, and the vapour turns back into water. The temperature at which that happens is called the dew point.
In a room at 20°C with 60% relative humidity, the dew point is about 12°C. Any surface colder than that collects water.
Glass is usually the coldest surface in the room. Working from standard figures for how quickly glazing loses heat, on a 0°C day the inside face of single glazing sits at about 5°C, older double glazing at about 13°C and modern low energy double glazing at about 17°C. Single glazing streams with water. Older double glazing sits right on the line, which is why the edges of the pane and the metal frame, which run colder still, are where the water gathers first.
Why it happens in offices and staff rooms
People produce moisture just by being in a room. The British Standard on moisture in buildings, BS 5250, puts a seated person at about 70 grams of water vapour an hour. Twenty people at their desks for an eight hour day add around 11 litres of water to the air.
Then there’s the kitchen. BS 5250 gives about 2 kilograms a day for cooking on an electric hob and 400 grams for washing up, and 1.5 kilograms for drying clothes indoors. The same standard expects offices and shops to run at 35 to 50% humidity, and kitchens and canteens much higher, at 60 to 70%.
In summer, open windows carry that moisture away. In autumn, the windows close, the heating goes on, and the same moisture stays in the building until it finds a cold surface.
Why black mould follows condensation
Mould spores are in the air everywhere. They don’t need water running down a wall to grow; they need the surface to stay damp. BS 5250 notes that many moulds can start growing once the relative humidity at a surface goes above 80%.
That surface figure is the one that matters. A cold external wall runs about ten percentage points more humid at its face than the air in the middle of the room. BS 5250 says that if a room stays above 70% humidity for several days, its external walls will be above 80%, and mould has what it needs.
That’s why mould appears in the same places every year:
- the corners of rooms, especially on outside walls
- window reveals, sills and the frame around the glass
- behind furniture and cupboards pushed against outside walls
- where the floor meets the wall, and on north facing walls
These are the coldest surfaces in the room, or the places where air doesn’t move.
Ventilation is what stops it
Condensation and black mould are, at root, a ventilation problem. The Building Regulations guidance for non domestic buildings, Approved Document F, requires extract ventilation in toilets, washrooms and kitchens. For a kitchenette with just a microwave and a kettle, it sets an extract rate of 15 litres a second, rising to 30 litres a second next to a hob. In rooms without an opening window, the fan should run while the room is in use and for at least 15 minutes after people leave.
GOV.UK lists blocked, damaged or switched off ventilation among the causes to check. In practice, that covers:
- extract fans switched off at the isolator because they’re noisy
- trickle vents in the window frames closed, painted over or blocked
- fan grilles clogged with dust
- furniture pushed tight against outside walls, so air can’t circulate behind it
The simplest fixes cost nothing. Open the trickle vents and leave them open, keep fans switched on, and leave a gap between furniture and outside walls. Wiping condensation off windows and sills first thing in the morning, as Liverpool City Council and other councils advise, stops water soaking into the frame and the plaster around it. Wring the cloth out into a sink, not on a radiator, or the water goes straight back into the air.
Cleaning off black mould
GOV.UK guidance says small amounts of mould on hard surfaces can be cleaned off by the people looking after the building, and anything more extensive should be left to specialists. Its advice for the clean itself:
- Wear a mask, gloves and goggles, whatever the size of the patch.
- Close the door to stop spores spreading, and open the window while you work and afterwards.
- Use a product made for mould, which the guidance prefers to bleach.
- Let the surface dry completely afterwards.
- Bag the cloths and any removed material, and wash or dispose of the protective kit.
Council guidance adds two points. Use a fungicidal wash that carries an HSE approval number on the label, and follow the label exactly. And don’t brush mould or vacuum it, because both throw spores into the air.
Household rubber gloves and disposable cloths such as Handy Wipes cover the basics, and the cloths can be bagged up and thrown away after use. For the eyes, GOV.UK specifies goggles, which seal around the eyes better than ordinary safety glasses.
Porous materials are harder. GOV.UK notes that mould in carpets, soft furnishings and ceiling tiles may mean they have to be thrown away, and mould that has grown into silicone sealant usually means the sealant needs replacing. A musty smell with no visible mould can mean growth behind panelling, wallpaper or ceiling tiles, and that’s a job for a specialist.
Painting over it
Painting over mould without dealing with the moisture just gives it a fresh surface. Fungicidal and anti condensation paints can slow regrowth, but council guidance warns they stop working once they’re painted over with ordinary paint or wallpapered.
What to report
Check an area again at least six weeks after any work, to see whether the mould has come back. If it has, the cause hasn’t been fixed. The useful thing a cleaning team can do is tell whoever looks after the building, with specifics: which room, which wall or window, how big the patch is, and anything noticed nearby, such as a fan that’s off, closed trickle vents or a leaking gutter outside.
