How to Stop Condensation on Windows on Cold Mornings

You open the bedroom curtains on a cold morning and the glass is streaming, the sill is wet and there is a grey line of mould creeping along the rubber seal. Condensation on windows is one of the most common winter complaints in any home, and wiping it away each day only treats the symptom.

The good news is that it follows simple physics, so you can fix it with a few cheap habits and, in harder cases, one sensible purchase. Here is what causes it, what works and what is mostly a waste of money.

Why condensation on windows happens

Warm air holds more water vapour than cold air. When warm, damp air in a room touches a surface colder than its dew point, the vapour turns back into liquid. In winter, window glass is usually the coldest surface in the room, so that is where the water collects first.

Two things control how bad it gets: how much moisture is in the air and how cold the glass is. A household of four can release roughly 10 litres of water vapour a day from breathing, showering, cooking, boiling kettles and drying laundry. If that moisture has nowhere to go, it ends up on the coldest thing it can find.

The target for indoor relative humidity is between 40 and 60 percent. A digital hygrometer costs around 8 to 15 pounds and is the single most useful tool here, because it tells you whether your changes are working. Put one in the bedroom, since that is where most morning condensation forms.

Cut the moisture at the source

Most of the water in your air comes from a handful of daily activities. Tackle these first and you will often halve the problem without spending anything.

Dry laundry outside or in a tumble dryer. One load of washing drying on a rack can release about 2 litres of water into the room. If you must dry indoors, do it in a small room with the door shut and a window open a crack or a dehumidifier running.
Put lids on pans. Boiling pasta uncovered pumps steam into the kitchen for 10 minutes. A lid cuts that sharply and the water boils faster.
Run the extractor fan for 15 to 20 minutes after a shower, not just while it runs. Keep the bathroom door closed so steam does not drift into cooler bedrooms.
Move some houseplants. A cluster of large plants releases water through their leaves. Spread them out, or keep them in rooms with good airflow. Our piece on why houseplants keep dying in the same exact spot explains how cold glass affects them too.

Let the damp air out

Ventilation is what removes the moisture you cannot avoid making. It does not mean freezing the house.

Open trickle vents, the thin openings at the top of most modern window frames. They are designed to stay open all winter and let a small, steady stream of air through.
Air the bedroom for 10 minutes each morning. Open the window fully, close the door, then shut it again. A short burst exchanges the damp air without chilling the walls the way a window left ajar all day does.
Leave a gap behind furniture. Wardrobes pushed tight against outside walls trap cold, still air. A gap of 5 cm lets it circulate.
Keep internal doors open in the evening so warm air can reach the colder rooms.

Keep the glass and the room a little warmer

Because condensation forms on cold surfaces, a slightly warmer room and warmer glass both help. Keeping the bedroom at a steady 16 to 18 degrees overnight, rather than letting it drop to 12 and blasting it in the morning, often stops the worst of it.

Open curtains during the day so the glass warms in whatever sunlight there is. At night, heavy curtains trap cold air against the window and make condensation worse, so leave a small gap at the top or bottom.

For old single glazed windows, a secondary glazing film kit costs about 15 to 25 pounds per window and is fitted with a hairdryer in half an hour. It creates an insulating air gap and keeps the inner surface warmer.

Why moisture absorber tubs are not the answer

Supermarket shelves are full of small plastic tubs of crystals that promise to beat condensation. They contain calcium chloride, which does pull water from the air. The trouble is scale.

A typical tub collects a few hundred millilitres over several weeks. Your household may be producing 10 litres a day. Putting a tub on the sill is like bailing out a boat with a teaspoon. They are useful in a closed wardrobe or a caravan in storage, but not for a lived-in bedroom.

If cutting moisture and ventilating are not enough, a proper electric dehumidifier is the tool that matters. A compressor model rated at 10 to 12 litres a day costs around 120 to 200 pounds and uses roughly 150 to 250 watts. In unheated rooms below about 15 degrees, a desiccant dehumidifier works better, because compressor models lose efficiency in the cold. Run it with the door closed and empty the tank daily at first so you can see how much it is pulling out.

Deal with the water you already have

Each morning, take the water off the glass rather than letting it soak into the frame and sill. A small squeegee or a battery window vacuum clears a large pane in under a minute. Dry the sill and the rubber seals with a cloth.

If black spots have already appeared on the seals, clean them with a diluted bleach solution or a mould spray, the same approach as removing mould from shower caulk, then keep them dry. For more fixes like this one, browse our home how-to guides.

One warning: condensation that forms between the two panes of a double glazed unit is a different problem. It means the seal has failed. No amount of ventilation fixes it, and the sealed unit needs replacing, usually for 60 to 150 pounds per pane.

Frequently asked questions

Is condensation on windows a sign of a bigger problem?

A little condensation on the coldest mornings is normal, especially in bedrooms. Heavy daily condensation that runs onto the sill means the indoor air is too humid, which can lead to mould on walls and furniture. A hygrometer reading above 65 percent is a sign to act.

Does opening a window stop condensation?

Yes, a short burst of ventilation lets humid air out and replaces it with drier outdoor air. Ten minutes with the window wide open each morning works better than leaving it slightly ajar all day. Trickle vents give steady background ventilation without the heat loss.

Why do I only get condensation in the bedroom?

Two people sleeping release a lot of moisture through breathing over eight hours, and bedrooms are often kept cooler than living areas. Closed doors and curtains trap that damp air against cold glass. Airing the room in the morning and leaving the door ajar at night usually helps.