Diagnosis
Condensation or penetrating damp? How to tell the difference
The two most common causes of damp and mould in UK homes look similar on a wall and are treated completely differently. Condensation is moisture from inside the home settling on cold surfaces. Penetrating damp is water from outside getting through the building fabric. Read the pattern, the timing and the location before anyone sells you a treatment.
In short
Condensation shows as black spotted mould on cold surfaces (corners, behind furniture, window reveals), is worst in winter and after steamy activities, and often comes with wet windows. Penetrating damp shows as a defined damp patch or tide mark on an external wall, chimney breast or ceiling, gets worse after rain, and usually lines up with a defect outside such as a blocked gutter, failed pointing or a cracked sill. Fix condensation with heat, ventilation and insulation; fix penetrating damp by repairing the defect outside.
Who this applies to
- Tenants, homeowners and landlords trying to identify a damp patch or recurring mould
- Anyone quoted for damp-proofing work without a written diagnosis
Key steps
- Note the timing: cold weather and steamy rooms, or rainfall
- Note the location: cold internal surfaces, or a patch that lines up with something outside
- Look outside for defects above or beside the patch
- Photograph the patch (wide, medium, close-up) and the suspected defect outside
- Ask for the cause in writing before agreeing to treatment
Jurisdiction
UK-wide building science. Responsibility for the fix depends on tenure: landlords are generally responsible for the structure and exterior in every UK nation.
Important limitations
- The patterns overlap: a cold, wet external wall can suffer both
- Photographs do not measure moisture; a meter, a leak trace or a survey may be needed
- A chimney breast, a parapet or a solid wall can behave differently from the generic cases here
Official sources
First published . Last substantively reviewed .
Side by side
| Condensation | Penetrating damp |
|---|
| What it looks like | Black or dark green spotted mould, spreading over an area, often with a fuzzy edge | A defined damp patch or tide mark; plaster may blister or stain brown; mould only if it stays wet |
| Where | Cold surfaces: external corners, behind wardrobes, window reveals, ceilings under cold roofs, bathrooms | External walls, chimney breasts, around windows and doors, ceilings under roofs and parapets |
| When | Winter; mornings; after showers, cooking, drying clothes | After rain or driven wind; can stay damp in wet spells and dry out in summer |
| Other signs | Wet windows, damp bedding, musty air, high indoor humidity | Staining outside, green algae on the wall, a defect visible above the patch |
| Feels | Surface mould wipes off; wall underneath is usually dry | Wall feels cold and wet; salts may appear as the patch dries |
Condensation: moisture from inside, settling on cold surfaces
Warm indoor air carries moisture. When it meets a surface colder than its dew point, the moisture condenses. Every home produces moisture from breathing, washing, cooking and drying clothes; the question is whether the building lets it out. Cold surfaces (uninsulated walls, cold bridges at windows and corners, rooms that are not heated) and poor ventilation (broken extractors, closed trickle vents, no background ventilation) are the two halves of the problem.
Because it is about surface temperature, condensation mould loves the places air does not move: behind furniture against external walls, in the top corners of rooms, around window frames. It does not usually come with a defect you can point at outside.
Penetrating damp: water from outside, through the fabric
Rain or ground water gets through the wall, the roof or a junction because something has failed: a blocked or leaking gutter, a downpipe discharging against the wall, cracked render or pointing, a failed window sill or seal, damaged flashing at a chimney or roof edge, or a parapet that no longer sheds water. The patch appears inside more or less where the water gets in, and it responds to weather rather than season.
Solid-walled older homes are more prone to it than cavity-walled homes, and a cavity that has been filled or bridged can carry water across.
Simple tests that separate them
- The rain test: does the patch get worse within a day or two of heavy rain? That is penetrating damp.
- The season test: does the mould appear in autumn and fade in late spring regardless of rain? That is condensation.
- The outside test: stand outside and look at the wall above and beside the patch. A stained gutter joint, a cracked sill, green algae or a downpipe that empties onto the wall is your answer.
- The window test: wet glass most mornings means the air is carrying too much moisture and the home is not venting it.
- The hygrometer test: an inexpensive hygrometer that reads high overnight, especially in bedrooms, points to condensation.
What to photograph
Inside: a wide shot of the whole wall, a medium shot of the patch and a close-up. Outside: the wall above and around the patch, and any defect you can see. Date everything. If the patch changes with the weather, photograph it wet and dry from the same spot. The pairing of the inside patch with the outside defect is the evidence that turns a vague "damp" complaint into a repair.
Who fixes what
In every UK nation, landlords are generally responsible for the structure and exterior of the home, which covers the causes of penetrating damp, and for keeping the home fit to live in, which covers a building that cannot be ventilated or heated well enough to avoid condensation. Tenants are responsible for using the ventilation and heating that works. The argument about condensation being lifestyle usually ends when the extractor is shown not to work or the wall is shown to be a cold bridge.
When it is both
A cold external wall that also gets wet from outside will show both patterns: a tide-marked patch after rain with spotted mould in the cold corner beside it. Treat the defect outside first, then reassess the condensation once the wall has dried, which can take months in a solid wall.
Frequently asked questions
- Can condensation cause a damp patch, not just mould?
- Yes, on very cold surfaces condensation can run and leave a damp patch, typically low on the wall behind furniture or under a window. The giveaway is that it appears in cold weather without rain and the wall dries when the room is heated and ventilated.
- Does a damp meter tell the difference?
- Not on its own. A meter reading shows the surface is conductive, which can be moisture or salts. It is one signal; the pattern, the timing and the outside inspection are the diagnosis.
- Is penetrating damp dangerous?
- Persistent wet plaster and timber can decay, and wet materials grow mould. The urgency depends on how much water is getting in and whether electrics or structure are affected; a ceiling that sags or a wall near a socket that is wet needs prompt attention.
- What about rising damp?
- A separate, rarer pattern: a band of damp and salts along the bottom of ground-floor walls. Our guide on condensation vs rising damp covers it.
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