You usually spot it on a cold morning in October. There's water running down the inside of the bedroom window, and when you pull the wardrobe away from the wall to have a look, there it is: a spread of black speckles creeping up the corner. Wipe it off and, within a fortnight, it's back.
That's the frustrating thing about black mould. Getting rid of it on the surface is the easy half of the job. Stopping it returning is the half most people skip.
You're not alone with it, either. The 2024–25 English Housing Survey found around 1.4 million homes in England (5%) had a damp problem, and 7.7 million households (31%) said they'd had an issue with condensation, damp or mould. Government guidance from the UK Health Security Agency and the Department of Health and Social Care links damp and mould in English homes to roughly 5,000 asthma cases and 8,500 lower respiratory infections a year. It's a health issue as much as a decorating one.
This guide covers what black mould actually is, how to find the moisture feeding it, how to clean mould off walls properly, and what to put on the wall afterwards so you're not doing the same job again next winter.
Why black mould keeps coming back
Mould spores are in the air in every home in Britain, all year round. There's no way to get rid of them and no need to. They only become a problem when they land somewhere that gives them three things: moisture, a surface cool enough to hold that moisture, and something to feed on. Dust, emulsion, wallpaper paste and plaster all do nicely.
Take away the moisture and the mould dies. Leave the moisture there and you can scrub the wall every month for the rest of your life.
That's why mould on walls turns up in such predictable places:
-
Behind wardrobes, sofas and headboards pushed flat against an outside wall
-
Corners where two external walls meet, and where walls meet the ceiling
-
Window reveals and around frames
-
North-facing rooms and unheated spare bedrooms
-
Bathrooms, kitchens and utility rooms
-
Behind kitchen units on an outside wall
What these have in common is cold surfaces with poor air movement. A room at 20°C and 60% relative humidity has a dew point of about 12°C, meaning any surface colder than 12°C will start collecting moisture. A solid brick wall behind a wardrobe on a January night is comfortably below that.
Step one: find out what's feeding it
Before you buy anything, work out which type of damp you've got. It changes the whole approach, and it's the step people skip most often.
Condensation is the most common by far. The English Housing Survey found severe condensation in 3% of English homes, compared with 2% for penetrating damp and 1% for rising damp. The tell-tale signs are misted windows, water pooling on sills, mould in corners and behind furniture, and a problem that's noticeably worse from October through to March. The wall feels cold but isn't soaking wet.
Penetrating damp comes in from outside. Look for patches that darken after heavy rain, blistered or peeling paint in one concentrated area, and a stain that doesn't move with the seasons. Then go outside and look up: blocked gutters, a cracked downpipe, failed pointing, a slipped tile, a leaking overflow pipe. Nine times out of ten the answer is out there rather than in here.
Rising damp is far rarer than the internet suggests. It stays within about a metre of the floor, follows a fairly level tide mark across the wall, and often leaves a white powdery salt deposit. If your damp patch is at head height, it isn't rising damp.
A cheap hygrometer, around a tenner, is one of the most useful things you can buy. Aim to keep indoor relative humidity between 40% and 60%. If yours is sitting at 70% every evening, you've found your problem.
Renting? Report it in writing and keep a copy. Since 27 October 2025, the first phase of Awaab's Law has required social landlords in England to investigate significant damp and mould hazards within 10 working days, give the tenant a written summary within 3 working days of that investigation ending, and begin safety works within 5 working days where a significant hazard is found. Emergency hazards must be made safe within 24 hours.
Before you touch it: get the safety bit right
This is where a lot of well-meaning DIY goes wrong.
Never dry-brush or hoover mould. A stiff brush on a dry wall fires spores straight into the air you're breathing, and a standard domestic vacuum blows most of them out the other end. Council and NHS guidance across the UK is consistent on this point.
Wear proper kit. Nitrile gloves, wraparound goggles and an FFP2 or FFP3 mask. A paper dust mask does nothing useful here.
Open a window and close the door to the rest of the house so you're not moving spores from room to room.
Sheet up the floor and any furniture you can't move out, and bag the sheeting afterwards.
Don't paint over live mould. It will come back through, usually within weeks, and you'll have wasted both the paint and the afternoon.
If anyone in the house has asthma, COPD, a suppressed immune system, or is very young or elderly, keep them out of the room while you work and for a few hours afterwards.
How to get rid of mould on walls, step by step
Here's the sequence that actually works. Removing wall mould is not complicated, but the order matters.
1. Ventilate and prep. Window open, door shut, floor covered, PPE on.
2. Wet it down first. Lightly mist the affected area with water or your treatment before you touch it. Damp spores stay put. Dry ones don't.
3. Apply a fungicidal wash. Use a proper mould treatment that carries a Health and Safety Executive approval number, and follow the label. Treat at least a foot beyond the edge of what you can see, because the growth extends further than the staining does.
4. Respect the dwell time. Most fungicidal washes need 10 to 20 minutes on the surface to kill the growth rather than just wet it. Wiping straight away smears living mould around and leaves the roots in place. This is the single most common mistake.
5. Wipe, don't scrub. Use disposable cloths and work in one direction, off the wall rather than in circles. Bin each cloth as it loads up. Seal them in a bag and put them straight in the outside bin.
6. Rinse if the product says to, then dry it out properly. Warm the room and keep air moving for a day or two. Painting onto a wall that's still holding moisture is asking for trouble.
7. Repair what's failed. Rake out and replace mouldy silicone around baths and shower trays rather than trying to clean it. Fill cracks, scrape back flaking paint, and cut out any plaster that's blown or crumbling.
8. Treat a second time if it's stubborn. On porous plaster, a second application a day later is often what makes the difference between gone and nearly gone.
Cleaning mould off walls: surface by surface
Not every wall responds the same way, and knowing which one you're dealing with saves a lot of pointless effort.
Painted plaster is the easiest case. A fungicidal wash, correct dwell time and a clean wipe will usually take it back to bare, sound paintwork.
Wallpaper rarely survives. Mould gets behind the paper and into the paste, so cleaning the face of it is cosmetic at best. If mould has taken hold, strip it back, treat the plaster underneath and start again.
Bare or blown plaster holds mould deep in the surface. Sound plaster can be treated and sealed. Plaster that sounds hollow when you tap it has failed and needs hacking off and replacing. If you do end up with fresh plaster, get the first coat right, because a poor mist coat causes months of grief later. Our guide to common mist coat mistakes covers the errors worth avoiding.
Silicone sealant is a replace job, not a clean job. Once mould is in the sealant, it's in the sealant.
Textured coatings and Artex trap spores in every ridge. Expect two treatments and a lot of patience, and check for asbestos before you consider sanding anything installed before the mid-1980s.
Outside walls and render are a different problem again. Green and black growth on masonry is usually algae and lichen rather than indoor mould, and it needs an exterior treatment such as a moss and algae remover followed by a breathable coating like exterior masonry paint.
Redecorating so it doesn't come straight back
Once the wall is clean, dry and sound, the coating you choose does a lot of the heavy lifting. Standard emulsion offers no resistance at all. Specialist interior coatings do, provided you match the product to the problem.
For damp staining that keeps bleeding through, a stain-blocking sealer such as Ronseal One Coat Damp Seal stops old marks ghosting up through fresh paint. Worth being clear about what it does, though: it blocks the stain, it doesn't fix the moisture.
For walls affected by penetrating damp, Coo-Var Anti-Damp Paint forms a breathable, moisture-resistant barrier with biocidal protection in the dry film. Take it at least 30cm past the edge of the damp patch, not just over the visible mark.
For cold walls where condensation is the culprit, Coo-Var Anti-Condensation Paint is the one to look at. It's formulated with insulating microspheres that keep the surface fractionally warmer, which reduces the amount of moisture that condenses out onto it, plus fungicidal additives to inhibit regrowth. You'll find it alongside similar products in our anti-condensation paint range.
Where water is getting through masonry, Zinsser WaterTite is built for basements, cellars and below-ground walls.
For bathrooms and kitchens that need wiping down regularly, a tough washable finish like Johnstone's Cleanable Matt stands up to cleaning far better than standard vinyl matt.
One point worth knowing that catches people out: if you overcoat a specialist coating with ordinary emulsion, you can lose the benefit you paid for. Some products are designed to be overcoated and some aren't, so check the manufacturer's data sheet before you decorate over the top. Our anti-damp and anti-mould interior paint guide goes through the differences in more detail, and the full interior paints and interior sealers ranges are worth a look if you're not sure which route suits your wall.
Stopping it coming back for good
This is the unglamorous part, and it's the part that works.
Ventilate properly. A bathroom extractor should shift at least 15 litres per second and needs a run-on timer so it keeps going after you've left. Kitchen extraction wants to be directly over the hob. Check your trickle vents are open, because a surprising number of people close them and forget. Opening windows wide for 15 minutes in the morning clears more moisture than leaving one on the latch all day, and loses far less heat.
Cut the moisture at source. Lids on pans. Bathroom door shut while you shower, with the fan running. Dry washing outside where you can, and if you can't, use a vented or condensing tumble dryer rather than draping it over radiators. A single load of wet washing puts around two litres of water into the air.
Heat low and steady. Short blasts of heat in a cold house create exactly the temperature swings that produce condensation. Gentle background warmth in rooms you use costs less than you'd think and keeps wall surfaces above dew point.
Give walls some air. Leave a 50mm gap between furniture and external walls, and don't cram wardrobes tight into cold corners.
Maintain the outside. Clear the gutters twice a year, check downpipes, look at the pointing, and deal with any leaking overflow. Most penetrating damp starts with a five-minute job that got left for three years.
Consider mechanical help if the house needs it. For persistent whole-house condensation, a positive input ventilation unit fitted in the loft typically runs £400 to £600 installed and shifts the problem in a way no amount of scrubbing will.
When to call someone in
Handle it yourself if it's surface mould over a small area, you know where the moisture is coming from, and the plaster underneath is sound.
Get professional advice if the affected area is bigger than about a square metre, if it comes back within weeks of a proper treatment, if the plaster is blown or the skirtings are soft, if you suspect a leak inside a wall or under a floor, or if anyone in the household has a respiratory condition. A damp survey costs far less than repeatedly redecorating a wall that was never going to stay clean.
