Condensation forms when warm, moist room air meets a cold surface — usually the inside of the glass or an uninsulated upstand on a cold night. Reducing indoor humidity, improving ventilation, insulating the reveal and upstand, and choosing better-insulated glass stop most of it.
Is it condensation or a leak?
Condensation appears on cold mornings on the inside face, often in the same pattern each time, and does not depend on rain. A leak shows up during or after rain and often runs down one corner of the reveal.
What causes it?
- Cooking, bathing and drying clothes without extraction
- An uninsulated upstand or reveal acting as a cold bridge
- Older single or poorly sealed double glazing
- Heating that does not reach the high ceiling
How do I reduce it?
- Run extractor fans and open a window or vent after showers and cooking
- Insulate the upstand and line the reveal
- Add an opening rooflight or trickle ventilation
- Replace failed or older glass with warm-edge double or triple glazing
Is condensation in a roof window normal?
Occasional light condensation on very cold nights can happen on any glazing. Persistent streaming water is a sign of high humidity or a cold bridge that needs fixing.
This guide gives general information, not legal or structural advice. Rules change and every property is different — check with your council or building control for your address. We review our guides regularly; the date above shows the last review.
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