Window Condensation in a Flat

The water is already in the room before it reaches the window.

Condensation is not the window's fault. Warm air holds moisture; when it touches a surface cold enough, that moisture comes out as liquid. The glass is simply the coldest thing in the room, which makes it the first place you see a problem that exists everywhere in the air.

So there are only three things that actually change the outcome: how much moisture you put into the air, how warm the surfaces are, and how much of it leaves. Wiping is none of those.

A note on this guide: this rests on how humidity and surface temperature interact, not on products tested here. Persistent damp, black mould, or damp plaster is a building problem and belongs with a landlord or a professional rather than a squeegee. See how we test.

The problem, as it appears

Every winter morning, water down the inside of the glass, a wet sill, and eventually black spots in the corners of the frame. It is worst in the bedroom and the kitchen, and worst of all on the coldest mornings.

Why the obvious assumption disappoints

Wiping treats the symptom. It stops water pooling on the frame, which genuinely matters — standing water is what rots a sill and feeds mould. It changes nothing about tomorrow.

And the window is a messenger. Replacing single glazing with double does raise the glass temperature and moves the condensation elsewhere, which can be worse: the next-coldest surface is often an outside wall behind furniture, where you will not see it until it is mould.

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What actually matters

1. Knowing the number

Relative humidity is the variable everything else acts on, and it is invisible. Broadly 40–60% suits most homes; consistently above that makes condensation and mould much more likely.

A cheap hygrometer turns "it feels damp" into something you can actually act on, and tells you which room is the source.

2. Making less moisture

The everyday sources are larger than people expect: drying laundry indoors, cooking without a lid, long showers, and simply breathing in a closed bedroom overnight.

Lids on pans, the bathroom door shut with the fan on, and above all not drying washing on a radiator in a closed room are free and do more than any device.

3. Getting it out of the flat

Short, sharp ventilation beats a permanently open window. Two windows open for a few minutes exchanges the air without significantly cooling the walls and furniture, because air holds very little heat compared with solid things.

Where ventilation is genuinely not available — an internal bathroom, a flat you cannot alter — a dehumidifier addresses the same variable mechanically.

4. Managing the water that still appears

Until the underlying balance changes, get the water off the glass before it reaches the frame. A window vacuum does it in seconds and, unlike a cloth, removes the water rather than moving it to the sill.

5. The enclosed spaces nobody ventilates

Wardrobes and cupboards on an outside wall get no air movement at all, and they are where damp turns into ruined clothing.

Where the free option is genuinely enough

Short bursts of cross-ventilation, twice a day. The single most effective free measure.

Lids on pans, and the extractor actually switched on while cooking rather than after.

Moving furniture a few centimetres off outside walls, so air can reach the surface that would otherwise go mouldy unseen.

Drying laundry anywhere but a closed room — the full argument, including what a heated airer changes, is in laundry with nowhere to dry it.

The mistake to avoid

Treating the window as the problem. It is the coldest surface, so it shows first. Warm the glass without reducing the moisture and the water simply condenses somewhere you cannot see — behind a wardrobe, in a corner, on an outside wall — which is worse, because you find it as mould.

The second mistake is managing it indefinitely with a cloth. Persistent condensation in a room someone sleeps in, black mould, or damp plaster has stopped being a housekeeping matter, and it is worth raising with a landlord or a professional rather than absorbing as a chore.