Water on the inside of a bedroom window in the morning is the most visible symptom of a problem that is usually invisible the rest of the day. The glass is not leaking. Moisture that was already in the air has condensed on the coldest surface in the room, and if that surface stays wet for hours at a time, mould is not far behind.
Where the water on your windows comes from
Air holds water vapour, and warm air holds more of it than cold air. Every breath, every shower, every pan of boiling pasta and every load of drying laundry adds vapour to the indoor air. When that moist air meets a surface cold enough that it can no longer hold the vapour, the excess turns to liquid. That temperature is the dew point, and in winter a single-glazed window sits well below it.
This is why condensation appears on the glass first and on external walls second: those are the coldest surfaces. It is also why the problem gets worse in a well-sealed flat with the heating on — the air carries more moisture, the surfaces are colder relative to it, and there is nowhere for the vapour to go.
Ventilate when the steam appears, not an hour later
Timing is the whole trick. Open the window or turn on the extractor fan at the moment steam is produced — while the shower runs, while the pan boils, while the dryer spins — and the vapour leaves before it has a chance to settle on a cold surface. Waiting until the room feels stuffy means the moisture has already condensed and now has to be evaporated again, which takes far more energy.
Bathrooms and kitchens need mechanical extraction that vents outside, not a recirculating filter. Check that the fan actually moves air: hold a sheet of tissue against the grille and see whether it is pulled. A fan that hums without pulling is decorative, and it is one of the most common reasons a bathroom stays damp through the winter.
Keep surfaces above the dew point
The second half of the fix is temperature. Condensation needs a cold surface, so the cheapest prevention is keeping the room warm enough that the glass, walls and frames stay above the dew point. That does not mean heating an empty house — it means not letting bedrooms drop so low overnight that every morning brings a puddle on the sill.
Trickle vents, window vents and a slightly open window overnight are all doing the same job: exchanging moist indoor air for drier outdoor air while the heating is off. The energy cost of that exchange is far lower than the cost of repairing mouldy plaster, and lower than the heating needed to dry a damp wall out again.
Deal with a patch of mould properly
Small surface mould on paint, grout or sealant can be cleaned with a damp cloth and a household mould-and-mildew cleaner, or a diluted bleach solution on non-porous surfaces. Wear gloves, ventilate the room while you work, and do not dry-brush mould on a dry wall — that sends spores into the air. Dry the area afterwards and fix the moisture source, because cleaning without fixing the cause just resets the clock.
Two cases are not a cleaning job. Mould that covers more than roughly a square metre, and mould that keeps returning to the same patch after cleaning, both point at water getting in rather than air staying wet: a failed sealant line, a leaking gutter above, a bridged damp course, or a cold bridge inside the wall. Those need a professional assessment, and the health case for acting is real — the WHO guidance on household air quality sets out why damp and mould matter for respiratory health.
Room by room: where it starts
Bedroom — the worst offenders are making the bed immediately and closing the window. Pull the duvet back for twenty minutes and let the mattress dry. Bathroom — squeegee the shower screen, run the fan during and for ten minutes after, and keep the door closed while the fan runs so moist air leaves outside rather than spreading through the flat. Kitchen — a lid on the pan costs nothing and removes most of the steam. Utility or spare room — an unvented dryer dumps litres of water into the air per load; duct it outside or use a condensing model.
Furniture against an external wall is another quiet contributor. Leave a gap of a few centimetres so air can move behind it; a wardrobe pushed flat to a cold wall traps damp air and grows mould on its back panel where you will not see it for a year.
The two habits that prevent most of it
If you only change two things, make them these: ventilate at the moment steam is produced, and never let a room sit cold and sealed for days. Everything else — dehumidifiers, window vacs, anti-mould paint — is a support for those two habits rather than a replacement for them.
Where the moisture is damaging equipment as well as walls, the precautions in protecting electronics from humidity are worth applying in the same rooms, and if the damp is coming in through doors and frames, weatherproofing doors closes one route at the source. The physics of why vapour turns to water on a cold surface is explained at condensation.