Why do I keep getting black mould on the wall?

Black spot mould grows where a surface stays damp for long periods. That happens when moist indoor air repeatedly meets a surface colder than its dew point — typically an uninsulated external wall, a window reveal, a thermal bridge, or the wall behind furniture. The mould is a symptom; the cold, damp surface is the cause.

Is condensation caused by the tenant or the building?

The building. GOV.UK guidance for rented housing providers states plainly that it is unreasonable to blame damp and mould on lifestyle choices, and that the fundamental cause is building deficiencies, inadequate ventilation, inadequate heating or poor energy efficiency. Everyday cooking, washing and drying add moisture — the building's job is to remove it.

Why is condensation worse in winter?

Two reasons. External walls and windows are much colder, so more surfaces sit below the dew point for longer. And windows are opened less and heating is used intermittently, so moisture-laden air is exchanged for fresh air more slowly. Both effects push indoor humidity up and surface temperatures down at the same time.

Why do I get condensation with the windows closed but not open?

Open windows massively increase the rate at which humid indoor air is replaced with drier outdoor air. When they are closed and there is no other background ventilation — no trickle vents, no working extract fans — moisture from normal activity accumulates until it condenses. That points to a ventilation deficiency in the property.

Can new double glazing cause condensation?

It can move it. Old draughty windows leaked air continuously, providing unplanned background ventilation. Sealed replacement units without trickle vents remove that, so moisture that used to escape now stays in the room and condenses on the next-coldest surface — often the window reveal or an external wall. Since 15 June 2022, Approved Document F requires replacements not to reduce existing ventilation.

Why do I keep getting black mould on the wall?

Black spot mould grows where a surface stays damp for long periods. That happens when moist indoor air repeatedly meets a surface colder than its dew point — typically an uninsulated external wall, a window reveal, a thermal bridge, or the wall behind furniture. The mould is a symptom; the cold, damp surface is the cause.

Is condensation caused by the tenant or the building?

The building. GOV.UK guidance for rented housing providers states plainly that it is unreasonable to blame damp and mould on lifestyle choices, and that the fundamental cause is building deficiencies, inadequate ventilation, inadequate heating or poor energy efficiency. Everyday cooking, washing and drying add moisture — the building's job is to remove it.

Why is condensation worse in winter?

Two reasons. External walls and windows are much colder, so more surfaces sit below the dew point for longer. And windows are opened less and heating is used intermittently, so moisture-laden air is exchanged for fresh air more slowly. Both effects push indoor humidity up and surface temperatures down at the same time.

Why do I get condensation with the windows closed but not open?

Open windows massively increase the rate at which humid indoor air is replaced with drier outdoor air. When they are closed and there is no other background ventilation — no trickle vents, no working extract fans — moisture from normal activity accumulates until it condenses. That points to a ventilation deficiency in the property.

Can new double glazing cause condensation?

It can move it. Old draughty windows leaked air continuously, providing unplanned background ventilation. Sealed replacement units without trickle vents remove that, so moisture that used to escape now stays in the room and condenses on the next-coldest surface — often the window reveal or an external wall. Since 15 June 2022, Approved Document F requires replacements not to reduce existing ventilation.

Why do I have condensation?

Condensation and the black mould that follows it are a building problem, not a behaviour problem. Here is the mechanism, and the building factors that decide whether your home gets it.

Quick answer

You have condensation because moist indoor air is meeting surfaces colder than its dew point, faster than the building can remove that moisture. The drivers are cold surfaces (poor insulation, thermal bridging), a high moisture load, and inadequate air exchange — not normal cooking, washing or drying.

Already have visible mould? Condensation is the cause; the black growth on the wall is the symptom. Most mould in UK homes is condensation-related — deal with the growth and the moisture together. See mould removal specialists →

It is not “lifestyle”

“We are absolutely clear that it is totally unreasonable to blame damp and mould in the home on ‘lifestyle choices’... the fundamental cause of damp and mould will be due to building deficiencies, inadequate ventilation, inadequate heating and/or poor energy efficiency, not tenants’ normal domestic activities.”

GOV.UK — Understanding and addressing the health risks of damp and mould in the home (guidance for rented housing providers) ↗

The six factors that cause condensation

Air reaches its dew point: Warm air holds more water vapour than cold air. When moist indoor air touches a surface that is colder than the air's dew-point temperature, it can no longer hold that water and it condenses out as liquid — on the wall, the window, or inside the fabric of the building. Condensation is a temperature problem as much as a moisture problem.
Cold surfaces and cold spots: The colder a surface is, the more readily condensation forms on it and the longer it stays wet. Single-glazed windows, uninsulated solid walls, north-facing external walls, and the backs of wardrobes and units pushed against outside walls are the usual first places black mould appears.
Thermal bridging: A thermal bridge is a path through the structure — a concrete lintel, a steel beam, a junction between wall and floor, an un-insulated reveal around a replacement window — that conducts heat out faster than the area around it. That localised cold line or patch is where surface condensation and mould concentrate, often in an otherwise well-heated room.
The building's moisture load: Everyday living adds moisture to the air — cooking, washing, drying clothes, even breathing. This is normal domestic activity, not misuse. The question for diagnosis is whether the building can remove that moisture at the rate it is produced. Where it cannot, the moisture accumulates and condenses.
Inadequate air exchange: Moisture-laden air needs to be replaced with drier outdoor air. Draught-proofing, replacement windows without trickle vents, missing or underpowered extract fans, and blocked airbricks all cut the rate of air exchange. When ventilation falls below what the property needs, humidity climbs and stays high.
Under-heating and intermittent heating: Rooms that are heated briefly and then allowed to go cold never let the wall surfaces warm up, so those surfaces sit below the dew point for long periods. Fuel poverty and poor energy efficiency make this worse — a building that is expensive to heat will be under-heated, and under-heated buildings grow mould.

How to tell condensation from other damp

Condensation appears on cold surfaces, worsens in winter, and usually shows no tide mark and no salt staining. A tide mark low on a ground-floor wall suggests rising damp; wet patches that track rainfall suggest penetrating damp; persistent damp in a basement suggests below-ground water. The fixes are completely different, so if the signs don’t match, get an independent damp survey before spending money. Full comparison: condensation vs the other damp types →

Where it appears, and what that tells you

Bedrooms

Black spot mould in the top corners of external walls, around the window reveal, and on the ceiling line — worst in unheated or lightly-heated rooms and on north-facing walls. Moisture from overnight breathing has nowhere to go.

First check: Check for background ventilation: are there trickle vents, are they open, and is there any mechanical extract on the same floor? Take a humidity reading first thing in the morning.

Bathrooms

Mould on ceilings, along silicone seals, on window frames and in grout. Mirrors and tiles stay wet long after use. The moisture source is obvious; the question is whether it is being removed.

First check: Test the extract fan with a sheet of tissue held to the grille. Check it is actually ducted to outside (not into the loft), correctly sized for the room, and not switched off at an isolator.

Kitchens

Condensation on windows and cold external walls, mould behind and beside units on outside walls, streaming windows while cooking. High, sustained moisture production.

First check: Check for a working extract fan or cooker hood ducted to outside. Confirm whether any tumble dryer is condensing/heat-pump or vented, and where it vents.

Behind furniture and wardrobes

Mould discovered when a wardrobe, sofa or headboard is moved away from an external wall — the item blocks air movement and warmth from reaching an already-cold surface, creating a pocket that sits below the dew point.

First check: Feel the wall temperature relative to internal walls, and check whether the external wall is insulated. This pattern points to a cold-surface / thermal problem, not occupant behaviour.

Windows and reveals

Water pooling on the inside of the glass and on the sill each morning; mould on the frame, the sealant line and the plaster reveal. Common after old draughty windows are replaced with sealed double glazing and no trickle vents.

First check: Check whether the replacement windows have trickle vents and whether the reveals were insulated. Compare rooms with and without vents.

Lofts and roof voids

Damp timbers, mould on the underside of the roofing felt, and water droplets on the sarking — usually where warm moist air from the house below leaks into a poorly-ventilated loft, or where a bathroom fan discharges into the loft instead of outside.

First check: Trace any fan ducting — a fan discharging into the loft is a frequent single cause. Check eaves ventilation is not blocked by insulation.

Found the cause — now fix it

Once you know whether your problem is moisture load, air exchange, cold surfaces — or a combination — the solution follows.

PIV vs MVHR vs extract vs dehumidifier →What does it cost? →

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Causes of condensation — FAQs

Why do I keep getting black mould on the wall?: Black spot mould grows where a surface stays damp for long periods. That happens when moist indoor air repeatedly meets a surface colder than its dew point — typically an uninsulated external wall, a window reveal, a thermal bridge, or the wall behind furniture. The mould is a symptom; the cold, damp surface is the cause.
Is condensation caused by the tenant or the building?: The building. GOV.UK guidance for rented housing providers states plainly that it is unreasonable to blame damp and mould on lifestyle choices, and that the fundamental cause is building deficiencies, inadequate ventilation, inadequate heating or poor energy efficiency. Everyday cooking, washing and drying add moisture — the building's job is to remove it.
Why is condensation worse in winter?: Two reasons. External walls and windows are much colder, so more surfaces sit below the dew point for longer. And windows are opened less and heating is used intermittently, so moisture-laden air is exchanged for fresh air more slowly. Both effects push indoor humidity up and surface temperatures down at the same time.
Why do I get condensation with the windows closed but not open?: Open windows massively increase the rate at which humid indoor air is replaced with drier outdoor air. When they are closed and there is no other background ventilation — no trickle vents, no working extract fans — moisture from normal activity accumulates until it condenses. That points to a ventilation deficiency in the property.
Can new double glazing cause condensation?: It can move it. Old draughty windows leaked air continuously, providing unplanned background ventilation. Sealed replacement units without trickle vents remove that, so moisture that used to escape now stays in the room and condenses on the next-coldest surface — often the window reveal or an external wall. Since 15 June 2022, Approved Document F requires replacements not to reduce existing ventilation.