What causes condensation in a house?

Reviewed and updated 22 October 2026 · How we assess condensation

Condensation is water vapour from the air turning back into liquid when it touches something cold. In a home, three things decide whether that happens: how much moisture is in the air, how quickly it's removed, and how cold the coldest surfaces get.

Water vapour: the invisible part

The air in your home is a mix of gases, and one of them is water vapour. You can't see it — steam from a kettle is actually tiny droplets that have already condensed. How much vapour air can hold depends on temperature: air at 20 °C can hold roughly twice as much as air at 10 °C, and about four times as much as air at 0 °C.

That's what relative humidity (RH) measures: how full the air is, as a percentage of what it could hold at its current temperature. 60% RH means the air holds 60% of its maximum. Because the maximum changes with temperature, the same air reads a different RH as it warms or cools — which is the key to understanding condensation. Our guide to ideal humidity levels goes into this in more depth.

Cold surfaces: where it shows up

When room air touches a cold surface, the thin layer of air right against it cools down. Its relative humidity rises. If that layer cools all the way to the dew point, it reaches 100% and water condenses on the surface.

That's why condensation appears first on the coldest things in a room:

• Windows, especially single glazing and the edges of double glazing.
• Outside walls, particularly solid brick or stone walls without insulation.
• Corners and junctions where walls meet ceilings or floors — known as thermal bridges.
• Behind furniture on an outside wall, where the room's warmth can't reach.
• Cold water pipes, toilet cisterns and metal frames.

Where the moisture comes from

Everyday life puts a surprising amount of water into the air. Typical published estimates (they vary with household and habits) look like this:

None of this means people are doing something wrong. Homes should be able to cope with normal cooking, washing and drying clothes. When they can't, it usually points to too little ventilation, surfaces that are too cold, or both.

Ventilation: where the moisture goes

Moisture leaves the home when moist indoor air is swapped for outdoor air, through extractor fans, trickle vents, open windows and the gaps and cracks of an older building. Cold outdoor air holds very little water, even when the forecast says "90% humidity", so swapping it in almost always dries the house. Our open-window calculator shows exactly how much.

Modern improvements such as double glazing, draught-proofing and sealing chimneys cut heat loss, but they also close off accidental ventilation. Without deliberate ventilation to replace it, moisture builds up. That's one reason condensation often starts after new windows are fitted.

Putting it together

Condensation happens when the moisture level is high enough, for long enough, at a surface that is cold enough. Change any of the three and things improve:

1. Less moisture — lids on pans, laundry dried outside or in a vented dryer, extractor fans used.
2. More ventilation — trickle vents open, fans that actually work, short wide window openings.
3. Warmer surfaces — steadier heating, furniture away from outside walls, insulation done properly.

The fixes overlap, so two together rarely give double the benefit. Our full room assessment recalculates combinations properly for your room.

Condensation or something else?

Not every damp patch is condensation. Rain getting in through a defect, leaking pipes and ground moisture can look similar but need completely different repairs. If a patch gets worse after rain or sits near plumbing, read damp or condensation? before treating it.