Nearly every wet patch in a British home gets called damp and most of them are not. This is how to tell condensation from penetrating damp and rising damp, why the difference decides what you should spend and why the person you ask first matters as much as the wall.
There is a black patch in the corner of the bedroom, or a stain spreading up the wall behind the sofa; the word everybody reaches for is damp.
The trouble is that damp is three different problems wearing the same coat. They have different causes, different cures and wildly different costs. Treating one as another is how people end up paying four figures for work that was never going to fix anything, while the actual cause carries on quietly.
So the first job is not to fix it. It is to work out which one you have.
The short version
- Condensation is by far the most common and it is the cheapest to deal with
- Penetrating damp comes from a defect outside and the fix is the defect, not the wall
- Rising damp is genuinely rare and is frequently diagnosed when it is not there
- The pattern, the height and the time of year tell you most of what you need
- Mould is a symptom, so cleaning it without fixing the cause just buys a few weeks
- Ask someone who is not selling the treatment
The three things people call damp
| Type | What it looks like | Where it appears | When it is worst |
|---|---|---|---|
| Condensation | Water on glass, black spot mould, musty smell | Cold corners, window reveals, behind furniture on outside walls, north facing rooms | Winter and after showering, drying washing or cooking |
| Penetrating damp | A defined patch, often darker in the middle, sometimes with staining | Any height, usually on an outside wall, often near a gutter, pipe or window | Worse during and after heavy rain |
| Rising damp | A tide mark with a fairly even top edge, salt deposits, blown plaster | Ground floor only and rarely more than about a metre up | Fairly constant, rather than tied to weather |
The two useful questions are how high and when. Damp that stops at a consistent height on a ground floor wall behaves differently from a patch that appears at first floor level after a storm, which behaves differently again from mould that comes back every November in the same cold corner.
Condensation, which is nearly always the answer
Warm air holds more moisture than cold air. When warm, moist indoor air touches a cold surface, it cools and lets go of that moisture as water. That is condensation and it is the cause of the overwhelming majority of damp complaints in British homes.
A modern home produces a surprising amount of moisture: showering, cooking, drying clothes indoors, even breathing. A household of four can put several litres of water into the air every day. In summer it ventilates away without anyone noticing. In winter, with the windows shut and the walls cold, it lands on the coldest surface it can find.
That is why condensation shows up where it does: window reveals, the corners of external walls, behind wardrobes pushed flat against an outside wall and in rooms that are heated less than the rest of the house. The black spotted mould that follows is a consequence, not a cause.
It is also why condensation responds to things that cost very little. More ventilation, slightly more consistent heating and stopping moisture at the source do more than any treatment applied to the wall.
Penetrating damp
This is water getting in from outside and it is a building defect rather than a lifestyle problem.
The usual culprits are dull and findable: a blocked or leaking gutter, a cracked downpipe, failed pointing, damaged render, a slipped tile, a blocked drain, a leaking internal pipe, or a window seal that has given up. Cavity wall insulation that has slumped or been badly installed can also bridge a cavity and carry water across it.
The tell is that penetrating damp is localised and responds to the weather. A patch that darkens during a wet week and dries back afterwards is pointing at something outside and the position of the patch usually points at what.
Go outside and look during heavy rain. It feels unscientific and it is remarkably effective: an overflowing gutter or a downpipe splashing against a wall is obvious when it is actually happening and invisible three days later.
Rising damp and why it is rarer than you think
Rising damp is groundwater drawn up through masonry by capillary action. It is real and it does happen, particularly in older buildings where a damp proof course has failed, was never fitted, or has been bridged by a raised path, a flower bed or a rendered plinth.
It is also diagnosed far more often than it occurs.
Two things drive that. The first is that the symptoms overlap with condensation and penetrating damp closely enough that a quick look cannot separate them. The second is that the handheld meters commonly used to diagnose it measure electrical conductivity; salts in old plaster conduct. A meter can therefore read high on a wall that is perfectly dry, which is a well known limitation and not a reason to inject anything.
Genuine rising damp has a fairly consistent tide line, rarely climbs much above a metre, is confined to the ground floor and is usually accompanied by salt deposits and blown plaster. If what you have does not look like that, be sceptical.
What actually helps with condensation
In rough order of how much difference they make for the effort involved.
- Use the extractor fans and check they work. Hold a sheet of paper to the grille: it should hold there. Many do not, because the filter or the ducting is blocked. Run them during and for a while after showering and cooking.
- Stop drying washing on radiators. A single load puts a couple of litres of water into the air. If it has to be indoors, do it in one room with the door shut and a window open or a fan running.
- Cover pans and use the cooker hood. Cooking is the second largest source after washing and bathing.
- Leave trickle vents open. They are there for exactly this and are very often taped shut or painted over.
- Heat the house more evenly. Cold rooms are where condensation lands, so a rarely heated spare room with the door shut is usually the worst room in the house for mould.
- Move furniture off external walls. Fifty millimetres of air behind a wardrobe changes the surface temperature of the wall behind it.
If all of that is being done and the problem persists, the next step is usually improving ventilation properly rather than doing more of the same.
Dealing with the mould itself
Mould is the symptom. Removing it without changing the conditions means it returns, usually within weeks and usually in the same place.
When you do clean it, wipe rather than brush or vacuum, since disturbing it dry releases spores into the room. Use a product intended for the job, ventilate while you work and wear gloves and a mask. Soft furnishings that are heavily affected are often not worth saving.
Persistent damp and mould in a home is worth taking seriously rather than living with, since it can affect breathing, particularly for anyone with asthma or a respiratory condition and it damages plaster and timber over time. If it keeps coming back after you have addressed ventilation and heating, that is a sign the cause is structural rather than behavioural and it is time for the independent survey.
Write down what you found
Damp is slow, which makes memory a poor instrument. A photograph with a date on it is worth far more than a recollection of whether the patch was bigger last winter.
Keep the photographs, the date, what you changed and any survey or report you paid for. Three things depend on it: knowing whether the problem is actually improving, giving a surveyor something to work from rather than an anecdote and being able to answer honestly when a buyer asks, because damp is one of the questions on the standard property forms and "we had it looked at, here is the report and here is what was done" is a very different answer from a shrug.
That is the same record as the maintenance history and the same reason for keeping a digital property logbook.
Keep the evidence, not just the memory
homehogs keeps the photographs, the surveys and the work you had done against the property itself, so next winter you can see whether it is actually getting better.
Join the waiting list
