6 min read
How to stop condensation on windows
Streaming windows on a cold morning are the most visible sign of a damp house. The water on the glass is harmless in itself — the problem is what the same moisture is doing on your colder walls, behind furniture and in corners where you cannot see it. Fix the condensation and you usually fix the mould risk with it.
Why it happens
Glass is usually the coldest surface in a room, so moisture in the air condenses there first. A household produces several litres of water a day from cooking, showering, drying clothes and breathing — and in a modern, well-sealed house that moisture has nowhere to go.
So condensation is never really a window problem. It is a humidity problem that happens to show on the glass. That is why new windows alone rarely cure it.
The fixes that actually work
Ventilate at the source: use the extractor every time you cook or shower, and let it run on afterwards. A fan that is never switched on is the single most common cause we find.
Keep the house gently and steadily warm rather than cold-then-blasting. Cold surfaces collect condensation; a steady background temperature keeps walls and glass above the point where water forms.
Open trickle vents and avoid drying clothes on radiators in sealed rooms — one load of washing releases around two litres of water into the air.
When condensation means a mould problem
Treat it as a warning sign when you see black spotting on the silicone around the window, on the reveals, or in the corners of the same rooms that stream up. The water pooling on the sill runs down and keeps the plaster damp, which is exactly what mould needs.
If several rooms condense heavily every day, a whole-house fix like a PIV unit is usually cheaper and more effective than wiping sills every morning. If it is one room, a properly specified extractor is often enough.
What we do on a condensation visit
We take humidity and surface-temperature readings room by room, check extraction and ventilation, and identify which surfaces are cold enough to condense. Then we tell you plainly what will make the difference — extraction, a PIV unit, insulation, or just changed habits — and clean and treat any mould that has already taken hold.
Frequently asked
Will a dehumidifier stop window condensation?
It helps as a short-term measure in the worst room, but it treats the symptom. Proper extraction and steady heating usually solve it for less running cost.
Do new windows stop condensation?
They reduce it on the glass itself because double glazing stays warmer, but the moisture in the air still has to go somewhere — so condensation often just moves to the next coldest surface, like an external wall.
Is condensation on the inside of windows a damp problem?
Usually it is condensation from indoor humidity, not rising or penetrating damp. The giveaway is that it is worst on cold mornings and in bedrooms and bathrooms.
Why are my bedroom windows the worst?
Two people breathing overnight release roughly a pint of water, and bedrooms are usually the coolest, least ventilated rooms — so the glass condenses heavily by morning.
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