Damp, mould and structural
Condensation: why it happens and how to stop it
Condensation forms when warm, moist indoor air meets a cold surface such as a window or external wall and the moisture turns back to water. It is the most common cause of household damp and mould in the UK, and it is largely controllable through ventilation, even heating and cutting indoor moisture.
- Also known as
- surface condensation, interstitial condensation (within the structure)
- Easily confused with
- penetrating damp, rising damp, a plumbing leak
- How serious
- Low to moderate: usually fixable without building work, but feeds mould
- Typical cost
- Usually no cost for DIY control
How to identify condensation
The clearest sign is water droplets on cold surfaces: window glass and frames in the morning, cold external walls, and cold pipes or cisterns. Black mould then grows in the coldest, least ventilated spots.
Condensation is worst in winter and in moisture-heavy rooms (kitchens, bathrooms) and where air does not move, such as behind furniture against an external wall. Unlike rising damp it appears upstairs as well as down, and has no ground-level tide mark.
How serious is it?
Low to moderate: usually fixable without building work, but feeds mould
Condensation itself is harmless water, but the mould it feeds is the main everyday damp health issue in UK homes, and the NHS links damp and mould to respiratory problems, especially for vulnerable people.
It is rarely a structural problem and is usually the cheapest damp to address, because the fix is about how the home is ventilated, heated and used rather than building work.
How to fix it yourself
- Ventilate moisture at source: use extractor fans (or open windows) when cooking and bathing, and keep lids on pans.
- Dry washing outdoors where possible. Drying clothes on radiators indoors releases litres of water into the air; if you must, do it in a ventilated room.
- Heat the home more evenly and steadily. Brief bursts of heat in one room leave cold walls elsewhere where moisture condenses.
- Improve background ventilation: open trickle vents, and avoid blocking air bricks.
- Keep furniture a little off external walls so air can circulate, and wipe down window condensation each morning until habits change.
When to call a professional
- Condensation and mould persist despite better ventilation and heating, which can point to poor insulation, cold bridging or inadequate extraction.
- You are considering positive input ventilation (PIV), extractor upgrades or insulation and want it specified correctly.
Who to call
- A ventilation specialist or a Property Care Association member for diagnosis and a PIV or extraction solution.
- A registered heating engineer or insulation installer if cold surfaces and uneven heating are the root cause.
Frequently asked questions
How do I stop condensation on my windows?
Reduce the moisture in the air and warm the glass. Ventilate when cooking and bathing, dry washing outdoors, open trickle vents, and heat rooms more evenly. Wiping the glass each morning helps in the short term, but ventilation and steady heating are what stop it returning.
Is condensation the same as damp?
Condensation is one type of damp, and in UK homes it is the most common. It comes from moist indoor air meeting cold surfaces, rather than water entering from outside or below. Because the source is internal, the fix is usually ventilation and heating, not building repairs.
Why is my house full of condensation in winter?
Cold weather makes surfaces colder while closed windows trap moisture from cooking, washing and breathing. Warm, damp air then condenses on cold glass and walls. The remedy is more ventilation to remove the moisture and steadier heating to keep surfaces above the temperature where water forms.
Sources
Editor, HomesAndHedge
Oliver leads HomesAndHedge's editorial coverage of home and garden problems. He researches and writes the plain-English explainers on pests, invasive plants, damp and mould, drainage and wildlife, drawing on guidance from bodies such as the Property Care Association, the RHS and the NHS, and is clear about when a job needs a qualified professional.
Last reviewed: 8 June 2026