How to Stop Condensation on Windows Overnight UK

A closed bedroom door traps all your overnight moisture in one small room. You and your partner breathe out around a litre of water vapour between you between bedtime and morning — and with the door shut, every drop ends up on the coldest surface in the room, which is almost always the window glass.

Opening the door even slightly lets that moisture spread through the rest of the house, where it's diluted across a much bigger volume of air. People who try this one change alone often report a 30–40% drop in morning window condensation, with no purchases required.

When curtains are pulled tight against the windowsill, they trap a pocket of cold, still air between the curtain and the glass. The radiator under the window can't reach this trapped air, so the glass surface gets even colder — and the colder the glass, the more moisture condenses on it.

Leaving a 2–3cm gap at the bottom of your curtains lets warm room air circulate up the inside of the glass. Warmer glass means less condensation. Costs nothing and makes a noticeable difference within the first night.

If you've already got condensation forming overnight, you can stop the worst of the damage by catching the water before it pools on the windowsill. Self-adhesive absorber strips sit along the bottom edge of the glass and soak up runs of condensation as they appear.

They won't prevent the condensation forming, but they will protect your wooden sills, paint and seals from the daily soaking that eventually causes peeling, rot and black mould.

Anti-fog sprays leave an invisible coating on the glass that stops water droplets from forming a film — instead, the moisture sheets off or evaporates without pooling. It's the same technology used on car windscreens and bathroom mirrors.

One application typically lasts several weeks on a bedroom window, and a single bottle treats the whole house. Pair it with the curtain gap above for a near-clear pane in the morning.

Window insulation film creates an extra layer of trapped, insulating air across the inside of your window. That extra layer raises the temperature of the inner surface — and warmer glass simply doesn't condense as much moisture out of the air.

Reusable kits are renter-friendly: they peel off cleanly in spring and can be refitted next winter. Single-glazed windows benefit most, but it also helps tired double-glazing where the seals have failed.

This is the permanent fix. A dehumidifier with a built-in humidistat keeps the air in your bedroom at a set humidity — typically 50–55% — and switches itself on and off through the night to maintain it. At that humidity, the air simply doesn't have enough moisture in it for condensation to form on the glass.

It also works with the bedroom door closed, which matters if you have kids, pets, or you just like a quiet, dark room to sleep in. Running cost is around 4–7p per hour while it's actively cycling — far less than the cost of repainting mouldy frames.

Overnight, two adults produce about 1 litre of moisture just through breathing. That moisture has to go somewhere, and in a closed bedroom there's nowhere for it to escape to.

At the same time, the heating is usually off, so the room cools through the night. Cold air can hold less moisture than warm air — and the coldest surface in the room is almost always the window glass. The moisture condenses out of the air and onto that cold glass.

In the morning, the heating comes on and evaporates some of it — but by then, the sills, paint and seals have already been wet for hours. Do that every night for a winter and you've got peeling paint, swollen wood and black mould around the frames.