Stop condensation & mould (UK guide)
Streaming windows every morning isn’t a window problem — it’s a moisture problem. Here’s why it happens and the fixes in order of effectiveness.
The 30-second version
Your home makes 10–15 litres of water vapour a day (breathing, cooking, showers, drying clothes). Cold surfaces condense it. Fixes, in order: make less moisture (lids on pans, dry clothes with extraction), ventilate briefly (10 minutes of cross-flow beats a window cracked all day), and extract what remains — which is where a dehumidifier earns its keep at ~5p/hour.
Why your windows stream
Air holds less water when cold. Overnight, room air cools against the glass — the coldest surface in the room — and sheds its moisture there. That’s also why mould blooms in corners, behind wardrobes and around window reveals: they’re the cold, still-air spots. Black mould (and the musty smell before it) starts appearing when surfaces sit above roughly 70% local humidity for sustained periods.
The fixes, honestly ranked
1. Cut moisture at source (free). Lids on saucepans, bathroom door shut with the fan running during and 15 minutes after showers, and — the big one — stop drying laundry on radiators unless something is extracting that water. One wash load releases about 2 litres into your air.
2. Ventilate in bursts (free). Ten minutes of windows open on both sides of the home swaps humid air for dry outside air with minimal heat loss — far better than trickling heat out of a cracked window all day. Winter outdoor air is dry once warmed; this genuinely works.
3. Hold 50–60% humidity with a dehumidifier. The always-on fix for the moisture you can’t avoid making. A humidistat unit holds the target automatically — see our dehumidifier picks; the quiet Meaco ABC suits bedrooms, the Pro Breeze 20L handles laundry-drying households.
4. Treat existing mould once, then fix the cause. Clean with a proper mould remover or diluted bleach on hard surfaces (ventilate while doing it), repaint bad areas with anti-mould paint if needed — but know that mould always returns while the humidity that fed it remains. That’s why steps 1–3 come first.
When it isn’t condensation
Damp low on walls (rising), a spreading ceiling stain (leak), or a wall wet after rain (penetrating damp) are building problems, not humidity problems — a dehumidifier will mask them at best. Condensation’s signature is cold surfaces, mornings, and worse in winter. If your pattern differs, investigate the fabric of the building first.
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Frequently asked questions
Why do my windows have condensation every morning?
Overnight, warm humid indoor air cools against the glass — the room's coldest surface — and sheds its moisture there. It signals indoor humidity is too high, typically from cooking, showers, breathing and indoor laundry-drying.
Will a dehumidifier stop mould?
It removes the condition mould needs — sustained humidity above ~70% at a surface. Clean existing mould once, then hold 50–60% humidity and it won't re-establish. Mould from leaks or rising damp needs the building fault fixed instead.
Is it better to leave a window open or use a dehumidifier?
Short cross-ventilation bursts (10 minutes, both sides of the home) plus a dehumidifier holding 50–60% beats either alone. A permanently cracked window in winter mostly just exports heat.
What humidity should a UK house be in winter?
40–60% relative humidity. Above 65–70% sustained, condensation and mould risk climbs fast; below 40% feels dry and scratchy. A £10 hygrometer or a dehumidifier's display tells you where you are.