The stains look the same, but the cause changes everything. Treating condensation as a leak is money thrown at the wall.
Condensation
Shows up in ceiling corners, behind furniture pushed against walls and on north-facing walls. It gets worse in winter with the house shut up. The mould is superficial and wipes off.
A leak
Comes from outside and draws a stain with a clear outline, often with the paint lifting. It gets worse after rain and does not improve with the window open.
Rising damp
Starts at the skirting and climbs to about a metre, with crumbling plaster and white salt marks on the surface.
Three quick tests
Tape a square of plastic film over the stain for two days. If the water sits on the outside, it is condensation. If it appears under the film, it comes from the wall. A hygrometer above sixty-five per cent confirms the first case.
What to do in each case
Condensation is solved with ten minutes of cross ventilation a day, drying laundry outside and keeping furniture a couple of fingers off the wall. Leaks and rising damp need building work, and no anti-mould paint fixes the cause.
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