Condensation & damp explained
Why the mould keeps coming back after you clean it
Because cleaning removes the mould and changes nothing about the conditions that grew it. You did not do it wrong — you treated the thing you could see. The water that fed it is still arriving on the same cold surface every night, and mould is what happens next. Here is what is actually going on, and how to tell which of the three damps you have.
Mould needs three things: spores, something to eat, and water. The spray you used dealt with none of them — it killed the growth that was there on the day and bleached the stain out of the paint.
Spores are not the variable. They are in the air of every building on earth, they arrive through every open door, and no amount of cleaning changes that. Food is not the variable either: plaster, emulsion, wallpaper paste, dust and timber are all perfectly good food, and you are not going to remove them from your own walls.
Water is the only one of the three you can take away. That is the entire argument. Leave the water arriving on that surface and you have a wall that is spores, food and water — which is a wall that grows mould, on whatever schedule the weather sets. Six weeks is typical. Through a mild wet winter it can be three.
It comes back in the same shape, and that is the diagnosis
You can do this yourself, tonight, for nothing. Look at where it returns.
Mould does not choose randomly. It grows where the surface is coldest and stays damp longest, so when it returns it returns in almost exactly the outline it had before — because that outline is a map of the cold part of your wall, and the cold part of your wall has not moved.
Which means the shape tells you what is wrong:
- A wedge in the corner where two outside walls meet, or where an outside wall meets the ceiling. Corners lose heat in two directions at once, so they run colder than anything around them. This is the classic condensation signature and it is the most common single pattern we see.
- A rectangle behind the wardrobe or the headboard. Furniture against an external wall stops room air reaching the plaster, so that patch never warms and never dries. Pull the furniture two inches off the wall and this one improves on its own.
- Down the window reveal, or along the top of the frame. The reveal is a thin bridge of masonry between inside and outside. On a lot of houses it is the coldest strip of wall in the room.
- A band across the ceiling in straight lines. That is usually the joists: the timber is warmer than the plasterboard between it, or the insulation between the joists stops short. You are looking at the structure through the mould.
- The ceiling above the shower, or the wall opposite it. Not a cold-surface problem so much as sheer quantity of water, and it points at the extract fan rather than the fabric.
None of those patterns is caused by dirt, and none of them is caused by anything you failed to wipe. They are caused by geometry and temperature, which is why the outline survives being scrubbed off.
Why painting over it makes things worse, not better
This is the step most people reach after the spray stops working, and it is the one that costs the most for the least.
It removes your only warning light
The mould was telling you where the damp surface is. Cover it and the water carries on arriving, the plaster carries on absorbing it, and you find out how far it has gone when it comes back through — usually further out than it was before, because it has had a few undisturbed months.
Anti-mould paint runs out
It works by carrying a biocide in the film, and that biocide depletes. On a wall that is genuinely wet it is being asked to work continuously, and it does not last. It is a reasonable finish on a wall where the moisture problem has actually been solved. On a wall where it has not, it buys you a season.
A sealed surface dries more slowly
Several coats of impermeable paint on a cold external wall make it harder for that wall to give moisture back to the room when conditions improve. You can end up with the water sitting in the plaster rather than on it, which is a worse problem and a more expensive one.
If you want to clean it anyway — and you should
Removing the growth is still worth doing before the real work happens, and it is worth doing properly rather than aggressively. What matters is what you are breathing while you do it.
- Do not dry brush it or hoover it with an ordinary vacuum. Both put a large quantity of spores into the air of a room you are standing in. Wipe it with a damp cloth so it stays stuck to the cloth.
- Open the window and shut the door while you work, so what you disturb goes outside rather than round the house.
- Bin the cloths rather than washing them through with everything else.
- Wear gloves and a mask if there is any quantity of it, and get somebody else to do it if anyone in the house has asthma or a respiratory condition.
- Do not mix bleach with anything else. Particularly not with anything containing ammonia, which includes some household cleaners.
UKHSA and the then DLUHC published guidance in September 2023 setting out that damp and mould in the home can cause or worsen respiratory problems and allergies and affect the immune system, that children are among those at greater risk, and that the effect on mental health should not be underestimated. The same guidance is explicit that this is not to be dismissed as a lifestyle problem. That is worth knowing when deciding how urgent this is.
When it is not condensation, and we will tell you
There are three damps and they look alike on a wall. Fitting ventilation to the wrong one is money for nothing, so this is worth ruling out before anybody quotes you.
It carries on through the summer
Condensation is a cold-weather problem, so it should ease off markedly from about April and be gone by June. Damp that persists through a warm dry August is very unlikely to be condensation, and something else is putting water into that wall.
It is a defined patch that tracks a defect
Penetrating damp comes in from outside and it usually has an obvious partner on the other side of the wall: a leaking gutter or downpipe, cracked render, a slipped tile, a blocked gully, a bridged cavity, failed pointing. Go outside and look at the wall behind it when it is raining.
It stops at a level line, low down
A tide mark roughly a metre up, sometimes with salt staining, is the classic description of rising damp — which is genuinely much rarer than the volume of advertising for injected damp courses would suggest. Far more often that pattern turns out to be a raised external ground level, a blocked airbrick, or a path bridging the damp course.
This is why a survey measures rather than looks. Surface temperatures, air readings, moisture in the fabric, and an airflow reading at whatever extract already exists — then a written answer saying which of the three it is and why, with dated photographs. If it turns out not to be a ventilation problem we will say so and tell you what trade you actually need. That is a better outcome for us than selling you a system that does not fix your wall.
Start with what you can actually see
Most people arrive here with a symptom rather than a diagnosis, and the symptom is usually enough to narrow it down. Each of these explains what is happening and what genuinely fixes it — no form, no email address, just the answer.
My windows are wet every morning
Water running down the glass from October to March, and pooling on the sill. What it means, when it is ordinary and when it is not — and why the glass is the only surface in the room honest enough to show you.
Read the explanationThe mould keeps coming back
You cleaned it, you sprayed it, you painted over it, and six weeks later it returned in the same shape. Why that shape is the diagnosis, why anti-mould paint runs out, and how to tell which of the three damps you have.
Read the explanationThere is mould in the bathroom, and a fan
A fan running is not a fan working. The eight things that go wrong — starting with the one you can check yourself today with a torch and a loft hatch, which is the worst of them and the most common.
Read the explanationIt started after we had work done
New windows, insulation, a loft conversion, a new kitchen. Not a coincidence and usually not a botched job — every trade did their own work correctly, and nobody was responsible for the air.
Read the explanationQuestions people ask about mould coming back
On a hard non-porous surface like tile or glass, yes, and it removes the stain as well. On plaster, painted plasterboard, wallpaper or timber it is far less convincing, because those are porous and the growth has roots into them — bleach is mostly water, the water soaks in, and the bleaching agent stays near the surface. So the visible black goes and the colony does not. But the more important point is that it does not matter much either way: killing what is there does nothing about the water that is still arriving on that surface tonight. You are removing the symptom on a wall that is about to grow it again.
"Toxic black mould" is a phrase from the internet rather than a medical category, and identifying the species would not change what you do about it. What the evidence does say is worth taking seriously without alarm. The World Health Organization guidelines on dampness and mould from 2009 found that occupants of damp or mouldy buildings are at increased risk of respiratory symptoms, respiratory infections and worsening of asthma. UKHSA and DLUHC guidance published in September 2023 says damp and mould can cause or worsen respiratory problems and allergies and can affect the immune system, that children are among those at greater risk, and that the effect on mental health should not be underestimated. The remedy is the same whatever colour it is and whatever it turns out to be: remove the moisture, and it has nothing to grow on. We are a ventilation contractor and not a medical service — if anyone in the house is unwell, that is a conversation for your GP.
Because the spot is a map of the coldest, slowest-drying part of that surface, and cleaning does not move it. Corners where two external walls meet lose heat in two directions and run colder than anything around them. A wall behind a wardrobe never gets room air across it. A window reveal is a thin bridge of masonry between inside and outside. Straight lines across a ceiling are usually the joists showing through. The outline returning unchanged is genuinely useful information rather than a nuisance: it is the building telling you where its cold surfaces are, and it is what a survey then measures rather than guesses.
It has a place, and the place is after the moisture problem has been dealt with, as a finish that gives you some insurance. Used instead of fixing the cause it buys a season or two. It works by holding a biocide in the paint film, and on a wall that is genuinely wet that biocide is being asked to work continuously and it depletes. There is also a less obvious cost: several coats of impermeable paint on a cold external wall make it harder for the wall to give moisture back to the room when things improve, so the water sits in the plaster instead of on it. That is a more expensive problem than the one you started with.
Around six weeks is typical through the winter, and through a mild wet spell it can be three. If yours is coming back within days, the surface is not drying at all between wettings and there is usually a lot of water in that room — a shower with no working extract, washing dried on a radiator, or an unvented tumble dryer. If it takes most of a winter to return, you are closer to the line than you think and a fairly modest change may be enough. Either way the interval is a rough measure of how bad the balance is, which is worth mentioning when you ring anybody.
You can, and on a wall that has been wet for a long time it may well need doing eventually. It is not a fix on its own. New plaster on a cold wall in a room that still produces more moisture than it can get rid of is a new surface for the same process, and it usually looks perfect for one winter. The order that works is: establish which damp it is, deal with the water, let the fabric dry, then make good. Re-plastering first is the most common way to spend a four-figure sum and still have the problem, and it is the reason we would rather survey before anybody starts cutting plaster off.
Almost always the landlord, and the law does not turn on who caused it. Under the Homes (Fitness for Human Habitation) Act 2018 a rented home has to be fit to live in for the whole tenancy, and damp and mould is a category 1 hazard under the HHSRS. Section 11 of the Landlord and Tenant Act 1985 puts the structure and installations on the landlord too. "The tenant dries washing indoors" is not a defence, because a household of four puts ten to fifteen litres of water a day into any home just by living in it, and the question the law asks is whether the property can deal with that. If you are the landlord or the agent rather than the tenant, we have written the whole thing up properly on our page for landlords and letting agents.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, held by this business. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.