Three different problems produce a dark patch on a wall, and the fix for one is useless against the others. Before anyone quotes you for anything, work out which one you have. It takes two days and a roll of kitchen foil.
The three, and what each looks like
Condensation. Moisture from inside the house landing on a cold surface. Shows up in corners, behind furniture on external walls, on window reveals and around cold bridges. Often comes with black spot mould. Worse in winter, worse in bedrooms and bathrooms, worse when the heating is on intermittently.
Penetrating damp or a leak. Water arriving from outside at a specific point. Usually a defined patch that grows after rain, often traceable to a cracked render, a failed seal, a blocked gutter or a roof problem.
Rising damp. Groundwater drawn up through the structure. Rare compared to how often it is diagnosed. The giveaway is that it stops at a fairly consistent height, often with a visible tide line and salt deposits.
The foil test
This is the one everybody should know and almost nobody does.
- Cut a square of kitchen foil roughly the size of a sheet of paper.
- Tape all four edges to the damp patch, sealing it against the wall.
- Leave it for two days, with the room used normally.
- Peel it off and look at both sides.
Moisture on the room side of the foil means the water is coming out of the air in the room. That is condensation.
Moisture on the wall side, or the wall behind it darker than before means the water is coming through the wall. That is penetrating or rising damp, and it needs a different investigation.
Why condensation is so common
A household puts a surprising amount of water into the air every day. Cooking, showering, drying clothes indoors, and simply breathing. Warm air holds that moisture happily until it touches something cold, and then it lets go of it.
Two things have made this worse in modern houses. Buildings have been made more airtight without always adding ventilation to match, and heating has moved from constant background warmth to short intense bursts, which leaves surfaces cold for longer.
Fixing condensation, in the order that works
- Reduce the moisture going in. Lids on pans, the extractor fan actually running during and after a shower, and drying laundry outside or in a ventilated room rather than on a radiator.
- Ventilate deliberately. Trickle vents open, extractor fans that vent outside rather than into a loft, a short cross-ventilation burst each day.
- Warm the cold surfaces. Steadier heating rather than short bursts, and pulling furniture a few centimetres off external walls so air can move behind it.
- Insulate the cold bridges. Only after the first three, and only where the pattern points at a specific cold spot.
Note what is not on the list: painting over it. Anti-mould paint deals with the symptom on the surface while the water carries on arriving.
Rising damp is diagnosed far more often than it occurs, and the treatment is expensive and invasive. If a surveyor reaches that conclusion, ask what ruled out condensation and penetrating damp first, and do the foil test yourself.
When it is definitely not condensation
- The patch appears or grows within hours of heavy rain.
- It is on a chimney breast, under a window sill, or beside a soil pipe.
- It is high on a wall or on a ceiling, where warm moist air would not settle.
- It is present in summer as well as winter.
Any of those points outward rather than inward. Start with gutters, downpipes and external seals, which are the cheapest things to check and the most common culprits.
Questions people ask
Does a dehumidifier solve condensation?
It manages the symptom and it can be a reasonable stopgap in one problem room. It costs money to run continuously, so treat it as a bridge while you fix ventilation and heating.
Is black mould dangerous?
It is worth removing promptly, particularly for anyone with asthma or a respiratory condition. Clean it with an appropriate product, and then deal with the moisture so it does not return.
Should I keep windows open in winter?
Not all day. Short, deliberate bursts of cross ventilation clear moist air quickly without cooling the structure, which is far more efficient than one window on the latch all day.
Why is it worse in the bedroom?
Two people breathing overnight in a closed, unheated room puts a lot of moisture into cold air. Bedrooms are the classic condensation room for exactly that reason.
