Where the water actually comes from
Every home produces moisture constantly: cooking, showers, drying laundry indoors, even breathing while asleep adds noticeable humidity to a bedroom overnight. Warm air holds that moisture as invisible vapour. Glass is the coldest surface in most rooms, particularly on a frosty night, so when the warm humid air touches it the vapour condenses back into visible water droplets.
This is why condensation is worst first thing in the morning after a closed-up night, worse in bedrooms and bathrooms, and worse the colder the outside temperature gets. It is a sign the room is holding more moisture than it can shed, not a sign the window itself is letting weather in or has stopped insulating.
Humidity and ventilation
Newer, well-sealed uPVC windows are actually part of why this has become more common than it used to be. Older draughty windows constantly leaked a little air, which carried moisture out with it. A modern well-fitted window is much more airtight, which is good for heat loss but means moisture has fewer ways to escape unless ventilation is deliberately provided.
- Run an extractor fan, or open a window briefly, during and after cooking and showering
- Dry laundry outside or in a ventilated room with the door shut, not on radiators in a closed living room
- Keep a low background heat on rather than letting rooms go cold and then heating in bursts
- Leave a small gap around furniture pushed against external walls so air can circulate
- Wipe window sills each morning if condensation has already formed, to stop it soaking into wood or sills
Not sure if it is condensation or a seal fault
Send us a photo or description of where the water is forming and we can tell you quickly whether it is a ventilation issue or something we need to look at.
Trickle vents
Most uPVC windows fitted in the last couple of decades have small trickle vents built into the top of the frame. These allow a constant trickle of background ventilation even with the window shut, which is exactly the kind of steady air exchange that keeps humidity from building up overnight. If yours are painted shut, blocked with dust, or were taped over by a previous owner trying to stop draughts, that is often enough on its own to tip a room into constant condensation. Opening or cleaning them is free and worth doing before anything else.
When it does point at a window fault
The exception is water or misting that appears between the two panes of glass rather than on the inner room-side surface. That cannot be wiped away and does not clear once the room airs out, because it is trapped inside the sealed unit itself and means the seal has failed. That is a genuinely different problem with its own fix, which we cover in full in our guide on misted double glazing.
Room-side condensation that keeps recurring despite good ventilation and heating, especially if it is pooling enough to damage sills or paintwork, is still worth a call so we can check the window is closing fully and sealing properly, since a window not shutting tight can make the surface run colder than it should.