Most black mould in a London home is condensation, not damp. Condensation shows as spots of black mould on cold surfaces in winter, in corners, behind furniture and around windows, with no tide mark. Penetrating damp is a patch that gets worse after rain. Rising damp is a tide mark with salty staining at the base of a ground floor wall, rarely above a metre. Each has a different cause and a different fix, and treating the wrong one wastes money.
Key takeaways
- Condensation: black spot mould on cold surfaces, worst in winter, in corners, reveals and behind wardrobes. No tide mark, no change after rain.
- Penetrating damp: a patch or stain that grows after rain, often high on a wall, under a window or beside a downpipe.
- Rising damp: a horizontal tide mark with white salts at the bottom of a ground floor wall, stopping around a metre up.
- Condensation needs three things at once: a cold surface, moisture in the air and not enough ventilation. Remove any one and it stops.
- Insulation cures condensation on cold walls and keeps rain off a solid wall; it does not cure rising damp and must not be put over a leak.
- Fans £200 and air bricks £85 fitted from our quote book; PIV unit £600 to £1,000; rising damp treatment £1,000 to £3,000 per wall run. 20% VAT; insulation is 0%.
The three kinds of damp and where each comes from
Condensation
Water that was already in the air inside the house. Cooking, showers, breathing and clothes drying on radiators put litres of moisture into the air every day. When that air touches a surface cold enough, the moisture condenses onto it, and if the surface stays damp, black mould grows. It is by far the most common cause of damp complaints across London and the South-East.
Penetrating damp
Rain that has come through the wall from outside: an overflowing gutter, a cracked downpipe, failed pointing, blown render, a leaking flashing or a cavity bridged by debris or failed cavity fill. The wall is wet because water is getting in, not because of anything happening indoors.
Rising damp
Ground water drawn up through the base of a wall where the damp proof course has failed, is missing, or has been bridged by a raised path or flower bed. It carries ground salts with it, which is why it leaves a mark. It is the rarest of the three and the most often misdiagnosed.
How to tell which one you have
| Sign | Condensation | Penetrating damp | Rising damp |
|---|---|---|---|
| Where on the wall | Corners, window reveals, behind furniture, ceiling edges, north-facing rooms | Anywhere rain can reach: under windows, beside downpipes, chimney breasts, high on the wall | Bottom of a ground floor wall only, up to about 1 m |
| Pattern | Scattered black spots of mould | A defined patch or stain, sometimes with a damp smell | A horizontal tide mark with white fluffy salts |
| When it is worst | Winter, mornings, after showers or cooking | During and after heavy rain | Constant, may worsen in wet months |
Two quick checks at home. Tape a square of kitchen foil tightly over the damp patch for two days: water on the room side is condensation, water behind it came through the wall. And look outside after rain. If the patch lines up with a gutter joint, a downpipe or a crack, you have found the source.
Cold surface, moisture, ventilation: the condensation model
Condensation needs three conditions together: a surface cold enough for the air touching it to reach its dew point, enough moisture in the air to condense, and too little ventilation to carry that moisture out before it finds the cold surface. Take away any one and the mould stops. The best fix usually deals with two.
- Cold surface. Solid brick walls, uninsulated 1930s semis, concrete ex-council flats, window reveals and the wall to ceiling junction are colder than the rest of the room. Mould marks the cold spots; see cold bridging explained.
- Moisture. A family of four produces several litres of water vapour a day. Washing dried indoors and an unvented bathroom are the usual sources.
- Ventilation. New windows without trickle vents, a kitchen fan that was never wired in and blocked air bricks trap the moisture inside.
This is why a house that never had mould can get it after new windows: the draughts that carried the moisture away have gone. See ventilation after insulation.
What fixes condensation, penetrating damp and rising damp
| Problem | The fix | Indicative cost | Who does it |
|---|---|---|---|
| Condensation, one or two rooms | Humidistat extractor fan in the kitchen and bathroom; trickle vents; even heating | £200 per fan fitted, from our quote book | Our own team |
| Condensation, whole house | Positive input ventilation (PIV) unit plus working extract fans | £600 to £1,000 installed; ventilation packages from £500 | Our own team |
| Condensation on a cold wall | Internal or external wall insulation to warm the surface | IWI single room from £1,800; EWI £110 to £140 per m² | Our own team, 0% VAT |
| Penetrating damp | Clear gutters, repoint, repair render and flashings; external wall insulation keeps rain off a solid wall | Priced per repair | Our own team (render, EWI) |
| Failed cavity wall insulation | Extract the wet fill, let the wall dry, then refill or insulate externally | £20 to £40 per m² extraction | Arranged by Stay Warm; PAS 2030 cavity wall and TrustMark installer |
| Rising damp | New damp proof course, lower ground levels, replaster | £1,000 to £3,000 per wall run | Arranged and project-managed by Stay Warm; accredited specialist contractor |
Ventilation and damp works are 20% VAT because they are not energy-saving materials. Wall, loft and floor insulation is 0% VAT until 31 March 2027, so where a fix mixes the two, the quote shows the split; see 0% VAT on energy-saving materials. One rule applies to all three: find the cause before anything is sold. If it is a gutter, we will tell you it is a gutter.
When is insulation the answer to damp?
Insulation is the right fix when the problem is a cold surface. Once a solid wall is insulated, its inside face sits close to room temperature, so moist air no longer condenses on it. Internal wall insulation does this room by room. External wall insulation does it for the whole house and, because the new render sheds rain, also stops penetrating damp through a tired solid wall.
Insulation is the wrong first fix over a wall that is wet from a leak or a bridged cavity, because it traps the water; over rising damp, because the water keeps rising behind the new lining; and in a house with no working extract fans, because a tighter house holds more moisture and the condensation moves to the next coldest surface. PAS 2035 requires ventilation to be assessed with every insulation measure, so we fit the fans and vents as part of the job. See insulation, condensation, damp and mould and breathable wall insulation.
What our free damp survey does
We measure moisture in the wall and the surface temperature, walk the outside of the house for gutters, pointing, render, ground levels and the damp proof course, and check the fans and vents. You get a plain written diagnosis naming which kind of damp it is and why, with a fixed quote. Where the answer is a specialist damp proofing job, we arrange and project-manage it. From October to April the survey can include thermal imaging; see what a thermal imaging survey shows and the damp and condensation service page.
Frequently asked questions
Is black mould condensation or damp?
Almost always condensation. Black spot mould grows where moist indoor air meets a cold surface: corners, window reveals, behind wardrobes and along the top of an external wall. Penetrating and rising damp leave stains, tide marks and crumbling plaster rather than scattered black spots.
How high does rising damp go?
Rarely above a metre, because capillary action cannot lift water much further in brick. Damp at first floor level or high on a ground floor wall is not rising damp, whatever a meter says; it is condensation or penetrating damp.
Will insulation stop condensation?
On a cold wall, yes. Insulating the wall keeps its inside surface warm so moisture no longer condenses on it. It must be done with working ventilation, or the moisture moves to the next coldest surface. It does not cure rising damp and should not be put over a leak.
Who fixes the damp?
Our own team fits fans, PIV units, air bricks and the insulation that removes a cold surface. Rising damp treatment and structural damp proofing are arranged and project-managed by Stay Warm and carried out by accredited specialist contractors.