Condensation on Windows Every Morning and What Causes It

You open the curtains on a cold morning and the glass is streaming, with a little puddle already forming on the sill and a grey line of mould creeping along the seal. Wipe it today and it is back tomorrow. Condensation on windows is a clue rather than the actual problem: it tells you how much moisture your home holds and where it is escaping, and with a bit of detective work you can find out which.

This is the method I use to track the cause down, with the numbers that matter and the fixes ranked by cost.

Why condensation on windows forms

Air always carries some water vapour. Warm air can hold more of it than cold air. When warm, moist indoor air touches a surface colder than its dew point, the vapour turns back into liquid. Windows are usually the coldest surface in a room, so they get it first.

Two numbers decide whether it happens:

• Relative humidity indoors. Healthy homes sit roughly between 40 and 60 percent. Above 60 percent in winter, condensation becomes very likely.
• Glass surface temperature. Single glazing on a 0 degree night can be just a few degrees above freezing on the inside. Modern double glazing usually stays well above 10 degrees.

As an example, air at 20 degrees and 65 percent humidity has a dew point of about 13 degrees. Any glass colder than that will fog. Drop the humidity to 45 percent and the dew point falls to about 8 degrees, and the same window often stays clear.

Where all that moisture comes from

A typical household of four puts somewhere around 10 to 15 litres of water into the air every day. Most people are surprised by how much each activity contributes.

Drying clothes on a radiator is often the single biggest source. A full load of washing can hold more water than the rest of the household produces overnight.

Detective work: which window and when

The pattern tells you the cause. Spend a week noting which windows fog, how badly, and at what time. A cheap digital hygrometer, around 10 to 15, makes this far easier because you can see the humidity in each room.

Only the bedroom windows, only in the morning

This is breath. Two adults in a closed bedroom raise humidity steadily all night, and the glass is coldest just before dawn. A trickle vent left open or a door ajar usually fixes it.

Kitchen and bathroom windows during use

Normal, and fine if it clears within an hour. If it lingers all day, the extractor fan is too weak or not running long enough. Hold a single sheet of toilet paper to the grille. If the fan cannot hold it in place, it is not pulling enough air.

One window much worse than others

Check the window itself. A failed seal, a missing trickle vent, or a cold spot in the wall around the frame can all make one window an outlier. A cold draft around the frame is easy to find with smoke, as we show in tracing a cold draft with an incense stick.

Fog between the two panes of glass

This is not a room humidity problem at all. The seal on the sealed glazing unit has failed and moist air has got inside. No amount of ventilation fixes it. The glass unit needs replacing, which usually costs far less than a whole new window.

Every window, all day, plus damp patches

Humidity in the whole house is too high. Look for a bigger source: indoor drying, a blocked vent, or water getting in somewhere. If you see damp patches away from windows, our guide to tracking down a hidden water leak walks through ruling out pipes and roofs.

Fixes ranked from free to expensive

1. Open trickle vents and leave them open. Free, and they exist for exactly this reason.
2. Put lids on pans and run the cooker hood for 10 minutes after cooking.
3. Run the bathroom fan for 20 minutes after every shower, and close the bathroom door.
4. Dry washing outside or in a tumble dryer that vents outdoors. If you must dry inside, do it in one room with the door shut and the window cracked.
5. Wipe windows each morning with a squeegee or absorbent cloth so water does not soak into seals and frames.
6. Use a dehumidifier. A 10 to 12 litre compressor model suits most homes and costs a few pennies an hour to run.
7. Upgrade glazing or add secondary glazing if you still have single panes.

The heating advice that is half wrong

A common piece of advice is "just turn the heating up". It is partly right: warmer glass reaches the dew point less easily, and warmer air holds moisture without dropping it.

But heat alone does not remove a single drop of water. It moves the problem. The moisture stays in the air until it finds the next cold surface, which is often the back of a wardrobe against an outside wall or the corner behind a sofa. People who turn up the heat and keep everything sealed often swap visible window condensation for hidden mould in places they do not look. The real fix is getting moist air out, then heating steadily rather than in short bursts.

When to worry about mould

Black spots on the seals and the sill are common and can be cleaned with a mould spray or diluted household bleach. Wear gloves and ventilate the room. Mould spreading onto walls, a musty smell that does not clear, or wet patches far from any window suggest a bigger damp issue. A persistent smell is worth chasing on its own, and our guide to tracking down a mystery smell at home helps narrow it down.

Frequently asked questions

Is condensation on windows a sign of damp?

Some condensation on cold mornings is normal, especially in bedrooms and kitchens. It becomes a sign of damp when it appears on every window, lasts all day or comes with mould on walls. In that case indoor humidity is too high and needs bringing down.

Should I open my windows to stop condensation?

Opening a window for 10 to 15 minutes in the morning swaps humid air for drier outdoor air. Leaving trickle vents open all the time works even better because the airflow is constant. You do not need to keep windows wide open in cold weather.

Does a dehumidifier stop window condensation?

Yes, in most homes a dehumidifier lowers humidity enough that windows stay clear. Place it in the worst room and set it to about 50 percent. It works best alongside reducing moisture sources such as indoor drying.