What is actually happening
Warm air holds more water vapour than cold air. When that air touches a cold surface it cools suddenly and releases the water it can no longer hold. A window pane is, almost always, the coldest surface in the room.
That is why it appears there first and not on a wall. The window is not making water: the room is holding more moisture than it can carry at that temperature.
Where all that moisture comes from
From very ordinary things. A hot shower with no extractor, cooking without an extractor hood, drying laundry indoors, and the people themselves, who breathe. A family of four generates several litres of vapour a day doing nothing special.
In a coastal property, closed for months and starting from high ambient humidity, the margin is narrower still.
When it is normal and when it is not
A little mist in the bottom corner of the pane on a cold morning after a night with the windows shut is normal. It clears as soon as you air the room and leaves no trace.
It stops being normal when water runs down to the frame, when it happens daily, or when black mould has appeared on the rubber seal or in the corner of the wall. At that point it is no longer passing condensation, it is a ventilation problem that is damaging something.
When it genuinely is the window's fault
There is one case where the window is responsible: when the mist is between the two panes of a double glazed unit. That means the cavity has lost its seal and no longer insulates. It cannot be cleaned or dried, the unit gets replaced.
The other case is condensation on a metal frame with no thermal break, typical of eighties and nineties aluminium joinery. There the frame is literally cold on the inside because it conducts the cold from outside.
What to do, in order of cost
First, air the place fully for five or ten minutes a day, even in winter. Changing the air costs far less energy than people think, because you replace the air without cooling the walls.
Then tackle the sources: an extractor in the bathroom that actually works, a hood in the kitchen, and no drying laundry indoors without ventilation. If it persists after that, it is time to look at the joinery, the ironmongery and the seals, which is our part.
