To decide between condensation or leak, look at the pattern. Condensation mould is speckled, sits on cold surfaces such as corners, window reveals and behind furniture, and eases when you ventilate and heat. A leak makes one patch that stays wet, spreads outward and ignores the weather. Fix condensation with ventilation and warmth; find a leak with a moisture and thermal survey.
Every November it starts again. Black speckles in the top corner of the bedroom, a grey bloom behind the wardrobe, a dark line along the bottom of the window. You wipe it off with a mould spray, it fades, and by February it is back. Coastal Cornwall is good at this. Mild, wet air off the sea, old solid walls that never quite warm through, and houses that were built long before anyone worried about airtightness. Most winter mould here is condensation, and it is fixable. Some of it, though, is a wall being fed by a leak, and no amount of ventilation will cure that. This guide gives you a simple way to tell which you have.
What condensation mould looks like
Warm indoor air holds moisture. When it meets a surface colder than its dew point, the moisture drops out onto that surface as water. Mould follows wherever the surface stays damp long enough. So condensation mould turns up in predictable places:
- Cold corners where two outside walls meet and the wall is thickest and coldest.
- Window reveals and frames, especially single glazing or old aluminium frames.
- Behind furniture on outside walls, where air cannot move and the wall stays cold.
- Ceilings under a poorly insulated loft, often in a spotted pattern that follows the joists.
- Bathrooms and kitchens where steam is produced and the extractor is weak or unused.
The mould itself is usually black or dark grey, speckled rather than solid, and sits on the surface of the paint. Wipe it and the wall behind is dry, or only slightly damp. It appears in autumn, peaks in the coldest months and fades or disappears in summer. In a granite or cob cottage near Penzance or St Ives, it will also track the wettest weather, because damp air outside makes the walls colder and the indoor air more humid at the same time.
What a leak looks like
A leak does not care about the season or the weather. Water arrives from a pipe, a joint or a fitting at a fixed point and spreads from there. The signs are different:
- One patch, not many. A leak produces a single area of damp with a definite centre, often shaped like a tide mark, rather than speckles scattered across cold spots.
- It is wet, not just mouldy. Press a hand on it. A leak patch feels cold and damp, and paint or plaster may be soft, blistered or salty.
- It grows. Mark the edge with a pencil. A leak patch spreads outward over weeks. Condensation mould stays where the cold surfaces are.
- It sits somewhere warm or odd. Mould halfway up an internal wall, at the base of a wall next to a bathroom, or on a ceiling under a pipe run is not where condensation forms.
- It does not respond to ventilation. Open the windows and run the heating for a fortnight. Condensation mould eases. A leak patch is unchanged or worse.
- Something else is wrong too. A boiler that keeps losing pressure, a water meter that moves with everything off, or a musty smell that gets stronger when the heating is on.
The pattern test
Put the two side by side and most cases sort themselves out.
| What you notice | Condensation | Leak |
|---|---|---|
| Where it is | Corners, reveals, behind furniture, cold outside walls | One area, anywhere, often near plumbing or below a bathroom |
| Shape | Speckled, scattered, follows the cold | Solid patch with a centre and a tide mark |
| Feel | Surface mould, wall dry beneath | Wet, soft or blistered, cold to the touch |
| Season | Autumn to spring, gone in summer | All year, grows over time |
| Weather | Worse in cold, damp spells | Unchanged by weather |
| Ventilation and heat | Improves | No effect |
| Other clues | Streaming windows, high humidity readings | Falling boiler pressure, moving meter, musty smell |
There is a third possibility, water getting in through the building itself: failed pointing, a cracked render, a blocked gutter, a slipped slate or high ground outside. That behaves like a leak in shape and feel, but follows the rain rather than staying constant. It matters for insurance too. Escape of water from a pipe is usually covered; rain through poor maintenance usually is not, and condensation never is.
If it is condensation: the fixes that work
Condensation has three ingredients: moisture, cold surfaces and still air. Take away any one and the mould stops.
Cut the moisture
- Use the extractor fan every time you shower or cook, and leave it running for twenty minutes afterwards. If there is no fan, or it is feeble, fitting a proper humidity-sensing one is the single best improvement.
- Keep lids on pans and dry washing outdoors or in a room with the door closed and the window open, or use a vented or condenser tumble dryer.
- Close the bathroom door while showering so the steam does not travel into cold bedrooms.
Warm the surfaces
- Keep a low, steady background heat rather than blasting the heating twice a day. A wall that never gets cold never reaches the dew point.
- Top up loft insulation, and consider insulating the coldest outside wall internally if it is a persistent problem.
- Pull furniture a hand’s width off outside walls so air can move behind it.
Move the air
- Open trickle vents and keep them open. Many are painted shut.
- Open windows for ten minutes morning and evening in winter, even when it is cold. The heat loss is small; the moisture loss is large.
- A dehumidifier in the worst room during the coldest months helps in a house that cannot be ventilated well.
Clean existing mould with a fungicidal wash rather than bleach, let the wall dry fully and repaint with an anti-mould paint if you like. Then fix the cause, or it will be back next winter.
If it is a leak: finding it
A wet patch that fails the pattern test needs its source found, and guessing is expensive. The plaster you strip to look behind is rarely where the pipe has failed, because water travels along pipes, joists and the back of plasterboard before it shows. This is the work we do with thermal imaging leak detection: a camera that shows the wet area as a temperature difference and maps the pipe runs, followed by a moisture meter to confirm the extent and acoustic listening or tracer gas to pinpoint the pipe failure. One hole in the right place instead of several in the wrong ones.
Two checks before you book. Watch the water meter with every tap off for ten minutes; movement means the cold supply is leaking. Note the boiler pressure gauge cold and check it each morning for a week; a steady fall means the heating circuit is leaking. Our guide to a damp patch on a wall that will not dry goes into the wall-specific clues, and our article on rising damp or a leak in a Cornish cottage covers the older-property case where all three causes can overlap.
If a leak is confirmed, most buildings policies cover the resulting damage as escape of water, and many include trace and access cover to pay towards finding it and making good afterwards. The survey report is what the insurer relies on to accept that the cause was a leak rather than condensation. Our guide to what trace and access insurance covers sets out the detail.
Holiday lets and empty houses
A house that stands empty through winter gets both problems at once. Unheated rooms are cold enough to condense whatever moisture is in the air, and a small leak has months to soak a wall before anyone sees it. If you let or own a property that is closed out of season, leave a low background heat on, leave internal doors open, and have someone visit after each cold or wet spell to look at the outside walls, the boiler gauge and the meter. A guest’s review mentioning a damp smell is a poor way to find out.
We carry out leak detection across Cornwall, and the question in the title of this article is one of the most common we are asked in the first cold weeks of the year.
Frequently asked questions
How can I tell if black mould is from condensation or a leak?
Condensation mould is speckled, sits on cold surfaces such as corners and window reveals, appears in winter and eases with ventilation and heating. A leak makes one solid patch that is wet to the touch, spreads over time and ignores the weather. Check the boiler pressure and water meter for confirmation.
Why does the mould come back every winter even though I clean it?
Cleaning removes the mould but not the moisture that feeds it. In winter, warm indoor air meets cold outside walls and condenses. Unless you reduce moisture, warm the surfaces or improve ventilation, the same corners will reach the dew point again the following autumn.
Can a leak cause mould in a room with no water pipes?
Yes. Water travels along joists, pipe runs and the back of plasterboard, so a leak from a bathroom above or a heating pipe in the next room can wet a wall some distance away. A solid patch that stays wet all year in an odd position deserves a survey.
Does home insurance cover condensation mould?
No. Condensation is treated as a maintenance and ventilation issue and is not an insured event. Damage from a leaking pipe usually is covered as escape of water, which is why a survey report confirming the cause matters when a wet wall turns out to be a leak.
Will a dehumidifier fix mould caused by a leak?
It will dry the room air and slow the mould, but the wall will stay wet because the leak keeps feeding it. If a dehumidifier is collecting a lot of water for weeks with no obvious source of steam, that itself points to a leak, and it needs finding rather than managing.
Wet wall that will not dry?
If the pattern test points to a leak, we can scan the wall and floor, confirm whether it is wet from a pipe or from condensation, and pinpoint the source, with a written report your insurer will accept.
