How to stop condensation and black mould for good
No paint fixes condensation. The cause isn't the wall, it's the air. Which is annoying to hear from a decorator, but it's the only advice that actually works, and most of it is free.
This is the most common problem we're called to, and the one where we most often tell people not to book us yet.
The three ingredients
Condensation needs all three of these at once. Remove any one and it largely stops:
| Ingredient | Where it comes from | How to remove it |
|---|---|---|
| Moisture in the air | Showering, cooking, drying washing, breathing | Extract it at source |
| A cold surface | External walls, corners, window reveals | Heat, insulation, airflow |
| Still air | Furniture against walls, closed rooms | Circulation |
Notice that none of them is the paint. Which is exactly why painting over mould fails: you've changed the colour of the surface, not the reason water lands on it. Our guide on mould on walls covers what happens next.
Why it's always the same three spots
Mould appears where warm moist air meets the coldest surface with the least airflow. In a British house that's almost always these three, in this order:
- The corner where two external walls meet. Two cold faces meeting means the coldest point in the room, and the geometry means air barely moves there. This is why it starts in the corner and spreads outward, not the other way round.
- Behind a wardrobe on an outside wall. The furniture insulates the wall from your heating while blocking all air movement. You've made a cold, still pocket and sealed it. People discover this when they move house.
- The reveal around a window. The narrow strip of wall between frame and room is thin, poorly insulated and often has a cold bridge straight through it.
If your mould is in one of those three places, it's condensation. If it's a tide mark rising from the skirting, or a patch that's wet to the touch and worse after rain, that's a different problem entirely and a decorator isn't the answer.
How much water your house actually makes
This is the number that changes people's minds, because nobody realises the scale.
| Activity | Water released |
|---|---|
| Two adults, breathing, one night | ~1 litre |
| One shower | ~0.5 litres |
| Cooking a meal | ~1 litre |
| Drying one load of washing indoors | ~2 litres |
| Unvented tumble dryer | ~2 litres |
| A family of four, per day | Often 10+ litres |
Indicative figures, but the order of magnitude is right and it's the point. You are putting a bucket of water into your air every day. It has to go somewhere. If it can't leave the building, it condenses on the coldest surface it can find, and that's your bedroom corner.
The free fixes that actually work
In order of how much difference they make:
- Run the extractor longer. Most people switch it off with the light. Moisture hangs around for 15 to 20 minutes after a shower. Run the fan on for 20 minutes after, and if it's wired to the light switch, that's the thing worth changing.
- Close the door, open the window. When showering or cooking, contain the moisture in that room and vent it out. Leave the bathroom door open and you've just sent two litres of water to condense on a cold bedroom wall down the landing.
- Move furniture off external walls. Five centimetres is enough. It sounds trivial and it's one of the most effective things you can do, because it turns a still cold pocket back into part of the room.
- Dry washing outside, or vented. Even in winter, an hour outside removes more than a day on a radiator.
- Open the trickle vents. Those little sliders on your window frames. They exist for exactly this, and a remarkable number of people tape them shut because of draughts.
- Ten minutes of windows in the morning. Bedrooms especially. You've just spent eight hours putting a litre of water into that room.
- Low and steady beats bursts. A house heated to 22°C for two hours and then left cold cycles through condensation every single day. A constant low background temperature keeps wall surfaces above the point where moisture forms.
That list costs nothing and fixes most condensation in most houses. It's also why we won't take a mould job without having this conversation first: we'd be taking money to paint a wall that will be black again by February.
What's worth spending money on
| Fix | Rough cost | Worth it? |
|---|---|---|
| Extractor fan, properly rated | £150–£350 fitted | Yes. The highest-value fix there is |
| Fan with humidity sensor + overrun | £200–£400 | Yes. Removes the human factor entirely |
| Vented or condenser tumble dryer | £250+ | Yes if you dry indoors |
| Dehumidifier | £120–£250 | Situational. See below |
| PIV unit (whole-house ventilation) | £400–£900 fitted | Effective for persistent whole-house cases |
| Insulation / cold bridge work | £££ | The real fix, if it's affordable |
| Repainting the wall | £300–£500 | No. Not until the above is done |
Read that last row again. We are a decorating company telling you that decorating is the worst value item on this list until the ventilation is sorted. That's not modesty, it's arithmetic: you'd be paying us twice.
On dehumidifiers
They treat the symptom, not the cause, and they cost electricity to run. But they're genuinely the right answer in specific cases: a flat with no openable bathroom window, a household that has to dry washing indoors, or a property standing cold and empty. What they aren't is a substitute for a working fan. Running a dehumidifier because your extractor is broken is paying an electricity bill to avoid a £200 repair.
When it isn't your fault
Condensation advice has a habit of blaming the occupant, and that's often unfair. Sometimes the building is the problem:
- No extractor at all in a bathroom or kitchen. No amount of window-opening compensates properly.
- An extractor that doesn't work. Hold a sheet of paper to it while it runs. If it doesn't hold, it isn't extracting. Very common, and very often nobody has checked in years.
- A fan venting into the loft rather than outside. Genuinely common in older conversions, and it just moves your problem into the roof.
- No trickle vents in modern sealed double glazing.
- Single-skin or uninsulated walls. A cold bridge you cannot ventilate your way out of.
- A property that can't be heated affordably. Being told to heat a house you can't afford to heat isn't advice.
Anti-condensation paint, honestly
It's a real product and it does a real thing. Anti-condensation paint contains insulating microspheres that keep the surface of the wall marginally warmer than the wall behind it. A warmer surface means moisture is less likely to condense on it.
So it works, narrowly. On a specific cold corner, it can buy you years. What it does not do is remove water from your air. If nothing else changes, the moisture simply finds the next coldest surface: the corner beside the one you treated, or behind the wardrobe, or the window reveal.
We use it where it's warranted, usually alongside a stain block and a fungicidal treatment. We'd never sell it as a solution on its own, because it isn't one, and anyone who does is selling you a delay.
What we'll actually tell you
If you call us about black mould in a bedroom corner, expect these questions before a price: does the bathroom fan work and how long does it run, is there a wardrobe against that wall, and where does the washing dry.
Most of the time the answers are the whole story, and the honest advice is free. If the ventilation is genuinely fine and you just need the damage put right properly, that's a real job and we'll treat, seal and repaint it correctly.
What we won't do is roll emulsion over a black corner and take your money. It'd be back before the year is out, and it'd be our name on it.
Ventilation sorted? Get it put right properly
Treatment, sealing and repaint itemised so you can see exactly what you're paying for.
See My Price Instantly →Frequently asked questions
How do I stop condensation on my walls permanently?
Condensation needs three things: moisture in the air, a cold surface, and still air. Remove any one and it largely stops. In practice that means extracting moisture at source by running bathroom and kitchen fans during and for twenty minutes after use, keeping furniture a few centimetres off external walls so air can circulate, drying washing outside or in a vented tumble dryer, and heating the house at a low steady temperature rather than in short bursts. No paint fixes condensation, because the cause is the air, not the wall.
Why does mould always appear in the same places?
Because mould grows where warm moist air meets the coldest surface with the least airflow. That's almost always the corner where two external walls meet, the wall behind a wardrobe pushed tight against an outside wall, and the reveal around a window. Those three spots are colder than the rest of the room and have the least air movement, so they reach the point where moisture condenses first.
Does opening windows stop condensation?
It helps, but timing and location matter more than duration. Opening a bedroom window for ten minutes in the morning removes the moisture that built up overnight from breathing. Opening a window in the bathroom during and after a shower, with the door closed, removes moisture at source before it can travel to a cold bedroom wall. Leaving a window open all day in a cold house is less effective and costs heating.
Should I buy a dehumidifier?
A dehumidifier treats the symptom rather than the cause, but it's genuinely useful in specific cases: a flat with no openable bathroom window, a room where washing has to be dried indoors, or a property that stands cold and empty. It isn't a substitute for extraction and airflow, and running one in a house where the extractor fan doesn't work is paying electricity to fix a mechanical problem.
Is condensation the tenant or the landlord problem?
Usually both, which is why it causes disputes. Tenants control ventilation habits such as running the fan, opening windows and drying washing. Landlords control the building: whether there's a working extractor of adequate rating, whether trickle vents exist, and whether insulation and heating are adequate. If the fan doesn't work or there's no extraction in a bathroom, that isn't a habits problem and no amount of ventilation advice will fix it.
Does anti-condensation paint work?
Anti-condensation paint contains insulating microspheres that keep the wall surface slightly warmer, which does genuinely reduce condensation forming on that specific surface. It buys time and helps in a cold corner, but it doesn't remove moisture from the air. If nothing else changes, the condensation simply forms on the next coldest surface instead.