Nothing has changed except the temperature
In September the house was fine. By the second week of October it smells stale, the windows are misted in the morning and there is a dark mark in the bedroom corner. The usual reaction is to assume water has got in somewhere.
Almost always it has not. The amount of water vapour in the air has not increased: what has dropped is the temperature of the walls. Warm air holds far more vapour than cold air, and as soon as a surface cools it releases what it can no longer hold.
Why it is always the same spots
Condensation does not spread evenly. It finds the coldest point in each room: the corner where two external walls meet, the edge of a window, the wall behind a wardrobe that stops air moving.
That is why the mark shows up in the same corner year after year. It is not that the wall is sick there, it is that it is the coldest part of the house.
How to tell it apart from a leak
A leak has a point of entry and a defined outline with a darker centre, and it gets worse right after rain. Condensation appears spread out, on the surface, in the cold spots, and gets worse in the morning after a night with everything shut.
The difference matters because the treatment is nothing alike. Breaking into a wall looking for a pipe that does not exist costs money and fixes nothing.
What generates it indoors
A hot shower with no extractor, cooking without a hood, laundry drying in the living room, and people breathing. A family of four releases several litres of water a day into the air doing nothing special.
In a coastal property, with ambient humidity already high and windows that seal well, that margin runs out quickly.
What to do, and in what order
First air the place fully for five or ten minutes a day, even in October. You change the air without cooling the walls, which are what hold the heat: it costs far less than people fear.
Then pull furniture a few centimetres off external walls, check the bathroom extractor actually moves air rather than just making noise, and stop drying laundry indoors without ventilation. If the mark still comes back every winter after that, then the insulation at that cold spot does need looking at, and that is a job.
