The short answer: Most black mould comes from condensation, but a patch that keeps coming back after cleaning, sits low on a wall, or grows near pipework can point to a hidden leak. The clue is the moisture source. Condensation follows cold surfaces and humid air, while a leak feeds the same spot whatever the weather.
You wipe the wall, the mould comes back. You buy a spray, paint over the patch, open a window, and within a few weeks the dark bloom returns in exactly the same place. It is one of the most common frustrations we hear about across Devon homes, and it usually means the real cause has not been dealt with. Black mould is a symptom, not the problem itself. To get rid of it for good, you have to find out where the moisture is coming from.
In the majority of cases that moisture is condensation, which is a lifestyle and ventilation issue rather than a plumbing one. But not always. Sometimes a slow, hidden leak behind a wall, under a floor, or above a ceiling is quietly feeding the same patch day after day. Telling the two apart is the key to fixing it, and it is also where a lot of homeowners go wrong.
Condensation or a leak? Why the difference matters
Condensation is the most common cause of black mould in UK homes. It happens when warm, moisture-laden air meets a cold surface and the water vapour turns back into liquid. Cooking, showering, drying washing indoors and even breathing all add moisture to the air. When that air settles on a cold window, an outside-facing wall or a chilly corner, it leaves a film of water that mould loves. This is why condensation mould tends to be worst in winter, in bathrooms and bedrooms, and in spots where air does not circulate well, such as behind wardrobes or in the corners of a room.
A leak behaves very differently. A leak supplies water from a fixed source, so it keeps feeding the same area regardless of how you heat or ventilate the room. You might improve the airflow, run a dehumidifier and keep the heating steady, and the patch still will not budge. That is a strong hint that you are not dealing with condensation at all. The risk is treating the wrong cause. A homeowner can spend months wiping windows and buying dehumidifiers while a dripping pipe behind the bath panel keeps the wall cavity wet the whole time.
Signs your mould might be a hidden leak
No single clue is proof on its own, but when several of these appear together it is worth taking a closer look at the plumbing rather than the ventilation.
Signs that point towards a leak rather than condensation:
- The patch keeps returning in the same place after cleaning, no matter how well you ventilate.
- Mould or a damp patch low down on a wall, near skirting, or directly behind a bath, shower, sink, toilet or radiator.
- A well-defined damp patch with a brown or yellow tide mark, rather than a fine, even film.
- The wall feels cold and genuinely wet to the touch, not just clammy.
- Damp that appears or worsens in dry spells, when condensation should ease.
- An unexplained rise in your water bill, or a water meter that ticks over when nothing is running.
Condensation mould, by contrast, usually appears higher up, in corners and around cold window reveals, and tends to be a speckled or dusty growth across a broader area rather than one concentrated wet patch. It also responds to changes in how you use the room. If better airflow and steadier heat make it noticeably better, condensation is the likelier culprit.
Where hidden leaks tend to hide
The leaks that cause stubborn mould are rarely dramatic. A burst pipe announces itself. The troublesome ones are slow weeps that release a small amount of water continuously, which is just enough to keep plaster, timber and masonry damp without ever showing a visible puddle.
Common culprits include pipework buried in walls or running under suspended floors, the supply and waste connections behind a bath panel, failed sealant or grout around a shower tray that lets water track into the wall, leaking radiator valves and heating pipes, and a slow drip from a pipe joint in the ceiling void above a downstairs room. Penetrating damp from outside can mimic a plumbing leak too, where wind-driven rain finds its way through a cracked render, a failed seal around a window, or a fault in the roof or guttering. In every one of these cases the moisture arrives from a single source and concentrates in one place, which is exactly why the mould keeps coming back.
A quick word on damp and mould in the home
Beyond the look of it, persistent damp and mould are worth dealing with promptly. Left alone, ongoing moisture can damage plaster, decoration, skirting and structural timber, and a long-running leak quietly wastes water you are paying for. Mould in a living space is also something most households would simply rather not live alongside. None of this is medical advice, and if you have health concerns it is always best to speak to a professional. The practical point is straightforward: the sooner you find and stop the moisture source, the less damage it can do.
How to find and fix the moisture source
Start with the simple checks you can do yourself. Improve ventilation, use extractor fans when cooking and showering, keep a little background heat on in cold weather, and pull furniture a few centimetres off cold external walls. Give it a couple of weeks. If the mould clearly improves, condensation was the main driver and you can manage it from there. If it stubbornly returns in the same spot regardless, treat that as a signal to investigate the moisture source properly.
This is where professional leak detection earns its place. Rather than guessing and pulling up floors or hacking off plaster, a specialist uses non-destructive methods to pinpoint the source first. Moisture meters confirm how wet a wall really is and map the spread. Thermal imaging cameras pick up the temperature differences that hidden heating and hot-water leaks leave behind walls and floors. Cold-water leaks give off little heat, so for those an engineer turns to acoustic listening equipment, which detects the faint sound of water escaping a pressurised pipe, and to tracer gas where needed. Used together, these techniques can locate a leak to within a small area before any wall is opened.
The advantage is obvious. You repair the right thing in the right place, with the least mess, and you only cut into a wall once the source has been confirmed. Once the leak is stopped and the area has dried out, the mould loses its food supply, and treating the surface finally works for good. If you suspect a hidden leak is behind your mould, our water leak detection service is the place to start, and you can read more practical guides on our articles page. We cover homes right across Devon.
Frequently Asked Questions
No. Most black mould in UK homes comes from condensation, which is about humid air meeting cold surfaces rather than a plumbing fault. A leak becomes the likely cause when the mould keeps returning in one spot, sits low on a wall or near pipework, and does not improve when you ventilate and heat the room.
Condensation tends to appear high up, in corners and around cold windows, and eases with better airflow and heating. A leak feeds the same patch whatever you do, often shows a defined wet patch or tide mark, and may sit near a bath, shower, sink or radiator. If improving ventilation makes no difference, suspect a leak.
Yes. Professional leak detection uses non-destructive methods such as moisture meters, thermal imaging cameras and acoustic listening equipment to pinpoint the source before anything is opened up. This means a repair can be made in the right place with the least disruption, rather than cutting into walls to go looking.
Cleaning removes the mould but not its water supply. If a hidden leak or ongoing condensation keeps the surface damp, the mould simply grows back. The lasting fix is to find and stop the moisture source first, let the area dry out, and only then treat the surface.
It can. A continuous leak on a mains supply pipe often shows as an unexplained rise in usage, and a water meter that keeps ticking over when nothing is running is a useful clue. Not every leak shows clearly, though, so it is worth checking with your supplier or South West Water if you are unsure.
Mould that keeps coming back? Let us find the source
If a hidden leak could be feeding the mould on your walls, our Devon engineers can pinpoint it without tearing your home apart. Get in touch for a clear answer.