Four causes that look identical on a wall
Damp presents as a small number of symptoms, and each of them points at a different mechanism. That is what makes this branch the right place to start: you can see a black patch, a wet window or a musty smell without knowing anything about dew points, and each of those observations narrows the cause considerably before you measure anything.
The diagnostic questions are always the same four. Where is it? — high corners and glass mean condensation; a band low on the wall means ground moisture; a defined patch on an exposed elevation means water coming through. When does it appear? — mornings mean condensation, after rain means penetration, all year regardless of weather means a leak. Is the surface cold? — an infrared thermometer settles this in seconds. Is anything in the room making water? — laundry, an unvented dryer, cooking, a fish tank, a person sleeping.
Getting the answer wrong is expensive in a specific way: the repairs for the four causes have almost nothing in common. A dehumidifier does not fix penetrating damp, a damp-proof course does not fix condensation, and replastering over either one buys you a fresh surface to ruin.
Which one you are looking at
Condensation
Even, widespread, worst on the coldest surfaces, worst in the morning, worst in winter. Glass first, then exterior corners, then the wall behind furniture. This is the cause behind the large majority of household damp complaints and the only one you can fix by changing behavior.
Penetrating damp
A defined patch, usually on a weather-facing wall, that darkens after rain and dries between. Blocked gutters, cracked stucco, failed pointing, a slipped tile. The fix is outside the house, and no amount of ventilation touches it.
Rising damp
A horizontal tide line up to roughly three feet, often with salt deposits at the top edge, present all year. Much rarer than the internet and the damp-proofing trade suggest, and frequently a misdiagnosed bridge — soil piled above the damp-proof course, a rendered plinth, a raised patio.
A leak
Follows a pipe or an appliance rather than the weather or the temperature, and does not care what season it is. Under a bath, behind a washing machine, a slow weep at a compression joint. The one cause where speed matters most.
Before anything else: the four causes of household damp are condensation, penetrating damp, rising damp and leaks. Condensation is by far the most common and costs nothing to fix. The others involve water entering the structure, and no appliance will help. Each page below starts by telling the two apart.
Find your symptom
Condensation on windows
The most common damp symptom, and the most useful one — it tells you your indoor air has more moisture than your coldest surface can tolerate.
Black mold on walls
Cleaning it is the easy part. The reason it returns to the same spot is a surface temperature problem, and that is what most advice misses.
Musty smell
A smell is mold you have not found yet. It is the one symptom that appears before anything becomes visible.
Damp patch on a wall
Four causes, four completely different repairs, ranging from free to five figures. Getting the diagnosis right is the whole job.
High humidity in summer
The season where the usual advice is wrong. Opening windows on a muggy day imports water and your AC pays for it.
Dry air in winter
Entirely seasonal, easy to fix, and easy to overdo — over-humidifying puts water straight onto your windows.
Sweating pipes and tanks
Not a leak. Cold plumbing sits below the dew point of humid summer air, so water forms on the outside of it.
Peeling or blistering paint
Paint fails where water is moving through the wall. Where it blisters tells you which of the four causes you have.
Gaps and cupping in wood floors
A wood floor is a very large hygrometer. Gaps mean too dry, cupping means too wet, and both are readable.
Allergies and dust mites
Mites drink from the air. Below 50% they cannot, which makes humidity control more effective than vacuuming.
Ice on the inside of windows
Condensation that got cold enough to freeze — and a signal that moisture is reaching places you cannot inspect.
Mold on clothes and leather
Natural fibers hold moisture and sit in still air. They grow mildew while the room reads a comfortable 55%.
Common questions
What is the fastest way to tell condensation from a leak?
Watch the timing for a week. Condensation tracks the weather and the time of day — worst on cold mornings, better in mild weather, and it appears across whole cold surfaces rather than in one spot. A leak is indifferent from all of that: it is the same on a warm day as a cold one, and it usually has a wetter center and drier edges. If you can, dry the area completely and put a taped sheet of plastic over it overnight — moisture on the room side is condensation, moisture underneath it is coming from the wall.
Should I fix the symptom or wait until I have found the cause?
Both, in that order of urgency but not of effort. Wipe the water and treat visible mold now, because standing moisture does damage every day it sits there. But do not repaint, replaster or seal anything until you know the cause, because those repairs will fail in the same place and you will have paid twice.
My damp problem disappeared in spring. Is it fixed?
Almost certainly not — that is exactly the signature of condensation. Warmer weather raises every surface temperature in the house above the dew point, so the water stops forming without anything having changed. It will return with the first cold snap unless you have altered the humidity, the ventilation or the insulation in the meantime. Use the summer to fix it properly.