How to Deal With Black Mould on Bedroom Walls in a Damp London Flat

It always starts in the same place. Behind the headboard, or in the corner where the external wall meets the ceiling, or on the skirting behind a wardrobe nobody has moved since 2019. A grey bloom first, so faint you think it’s a shadow. Then the black speckling, the smell that gets into the pillowcases, and the phone call.

I clean flats across Greater London and I’ve been doing it long enough to have opinions about this. Here’s the first one, and it’s the unpopular one: most black mould on London bedroom walls is made by the people sleeping in the room. The damp-proofing firm who quotes you £4,000 for a chemical injection course has a commercial reason to tell you the building is at fault, and in maybe one job in eight they’re right. The other seven are condensation, and condensation comes from breath, washing, showers, kettles, and a window that hasn’t been opened since the clocks went back. Which nobody wants to hear.

That doesn’t get landlords off the hook, and I’ll come back to that, because a flat that can’t be ventilated without freezing is a defective flat. But if you want the wall clean and staying clean, you have to start with what is growing there and why it chose your bedroom.

Why does black mould pick the bedroom first?

Because you spend eight hours a night breathing into it with the door shut.

An adult puts out somewhere around a pint of water vapour overnight just by existing. Two adults, double it. Add a partly-dried clothes airer in the corner because the flat has no outside space and the launderette on the high street costs more than the electricity, and you’ve got a sealed box being pumped with moisture for a third of every day. The air holds it while it’s warm. Then the heating clicks off at eleven, the room drops five degrees by three in the morning, and all that vapour has to go somewhere. It goes to the coldest surface it can find, which is the north-facing external wall, usually within about 30cm of a corner where the masonry meets a floor slab or a ceiling joist.

That’s the cold bridge. It sits two or three degrees below the rest of the room and it condenses first, every night, all winter. Mould doesn’t need a puddle. Aspergillus and Cladosporium will colonise anything sitting above roughly 80% surface humidity for long enough, and emulsion over plaster is a fine dinner.

What the cold corners are telling you

Look at where the growth stops. Condensation mould has soft, gradient edges and follows temperature – heaviest at the corner, thinning outward, often stopping dead at the line where a radiator’s convection current warms the wall. It’ll ghost the pattern of the joists behind the plasterboard. It’ll draw a neat rectangle around where a picture frame sat.

Rising damp, which is rare in London flats above ground level and doesn’t exist at all on a third floor, gives you a tide mark at a fixed height with salt bloom below it. Penetrating damp gives you a wet patch with a source – a cracked hopper head, a failed pointing joint, a bathroom on the other side of the wall. If your mould is a shape, look for a leak. If your mould is a gradient, look at yourself.

What kind of damp have you actually got?

Get a cheap hygrometer. £12, any hardware shop, and it will tell you more than a surveyor with a Protimeter poking at your skirting for twenty minutes.

Put it in the bedroom, not the hall. Read it at seven in the morning, before you open anything. If you’re sitting above 65% relative humidity at 17 degrees on a February morning, the wall is condensing and no amount of scrubbing will hold. If you’re at 45% and still growing mould, something is leaking and you have a building problem worth escalating.

I did a flat off Bellenden Road in Peckham a couple of winters back – SE15, a Victorian terrace conversion, top floor, tenant convinced the roof was gone. Black across the whole ceiling-to-wall junction on the back bedroom. The roof was fine. What she had was a bathroom extractor fan wired to the light switch, which she never used because the fan was loud and her flatmate slept badly, plus a tumble dryer venting through a hose that had come off the window bracket and was dumping its entire load straight into the hallway. The ceiling was catching the lot. We cleaned it, her landlord fitted a humidistat fan that ran on its own, and I went back eighteen months later for something else and the ceiling was still white.

The dryer hose is the one I see most. Second is drying washing in the bedroom with the door shut.

The tissue test for a fan that’s lying to you

Hold a square of loo roll flat against the extractor grille with the fan running. If it doesn’t stick there on its own, the fan is moving essentially no air and you’re heating the room to no purpose. Half the fans in London flats fail this. They spin, they make noise, the grille is furred with grey felt an inch deep, and the ducting behind it has been crushed by a joist or terminates in a loft void where it’s been quietly soaking the insulation for a decade.

What actually kills black mould on a painted wall?

Not bleach. Household bleach bleaches – it takes the pigment out of the hyphae so the wall looks clean while the colony sits there alive, and it’s mostly water, so you’ve just fed it. Two weeks later it’s back darker and the tenant thinks the flat is cursed.

You want a proper fungicidal wash, the sort with benzalkonium chloride in it, and you want to apply it and leave it. Don’t scrub first. Scrubbing dry mould aerosolises the spores into the room you sleep in, which is the single worst thing you can do with a wall like this.

The sequence I use, and it’s boring on purpose:

Ventilate – window wide, door shut to the rest of the flat. FFP3 mask, not the blue surgical thing, and nitrile gloves. Mist the growth with the fungicidal solution and let it sit for the contact time on the label, usually fifteen minutes or so. Wipe off with disposable cloths, one pass per cloth, straight into a bin bag. Never a sponge you’re going to rinse – you’d be painting spore soup back onto the wall. Second application, this one left to dry on the surface. Then leave the room shut and the window open for the rest of the day.

If the plaster is blown, or the emulsion peels off in sheets when you wipe, cleaning is finished as a strategy. That’s a job for a plasterer.

Why the smell survives the clean

The wall goes white and the room still smells of cellar. Those are microbial VOCs – geosmin and its relatives – and they’ll have gone into the plaster, the carpet underlay, the curtains, and every soft thing in a fitted wardrobe. Wash the fabrics at 60 if the label allows it. Steam the carpet edge. If a mattress has been against that wall through a whole winter, look at the seam side facing the plaster before you decide it’s fine, because I’ve seen mattresses I wouldn’t put in a skip without gloves.

How do you stop it coming back once the wall is clean?

Move the furniture. That is the highest-value thing on this list, and it costs nothing.

A wardrobe flat against a cold external wall creates a dead air pocket that runs two or three degrees colder than the room, and mould will grow behind it in perfect silence for years. Fifty millimetres of gap changes the whole calculation. I have solved whole jobs – a mansion block off Fordwych Road in NW2, since you ask – by pulling a chest of drawers 5cm forward and telling the tenant not to push it back.

After that: heat low and constant beats blasting it hot for two hours in the evening. The old pensioner logic of a cold bedroom being healthy was written for houses with chimneys and gaps under the doors. A modern London flat with new windows in a 1890s shell has the airtightness of a Tupperware and the thermal performance of a tent, which is the worst combination available.

Dry clothes anywhere but the bedroom, and if that means the bathroom with the door shut and the fan on, fine. Trickle vents open, all of them, all winter – yes, even the one over your bed. If there’s a PIV unit in the loft, leave it alone; people switch them off because of the draught and then ring me in March.

The anti-mould paint question

It buys you eighteen months and it does nothing about the reason the wall is wet. Fine as the last coat after a fix. Useless as the fix.

When should you stop cleaning and start writing to your landlord?

When the hygrometer says you’re doing everything right and the mould comes back anyway. When there’s no extractor in the bathroom, or the one there fails the tissue test. When the growth is bigger than about a square metre, keeps returning after two proper treatments, or anyone in the flat has asthma.

Awaab’s Law now puts social landlords on fixed statutory clocks for investigating and fixing damp and mould hazards, and the Renters’ Rights Act has built the framework to bring something similar to private tenancies – the private-sector end of it hasn’t fully landed yet, so check where that stands before you quote timescales at anyone. Even without it, the Homes (Fitness for Human Habitation) Act 2018 already covers you, and government guidance is explicit that landlords shouldn’t wave a damp report away as tenant lifestyle without investigating. Which, going by the letters I’ve read, nobody wants to hear either.

What to photograph before you send the email

Date-stamped shots of the growth with something for scale next to it. The hygrometer reading, in shot, in the room. The extractor grille. The tissue test, on video, five seconds. Every previous email in a numbered list with dates. Councils and ombudsmen respond to chronology and they ignore adjectives.

Then keep the wall clean while you wait, because whatever the landlord eventually does, you’re the one sleeping next to it tonight.