Quick answer: Condensation forms when warm, moist indoor air touches cold glass and drops below its dew point — so the fix is lowering indoor humidity and warming the glass, not blaming the windows. The four changes that work: ventilate while you generate steam (cooking, showers, drying clothes), keep background heating steady rather than blast-and-off, use trickle vents or a dehumidifier in problem rooms, and keep the glass itself clean — grime gives moisture and mould a foothold. Persistent misting between the two panes is different: that's a failed seal, and the unit needs replacing.
Every winter morning, same ritual: curtains open, and the garden's gone — hidden behind a wall of fog and a sill full of water. By February the sealant's growing black speckles and you're googling whether your windows are broken.
They're not. Condensation is your house exhaling. An average UK household puts 10–15 litres of water into its indoor air every day — cooking, showers, kettles, breathing, and the big one, drying laundry on radiators (a single load releases around 2 litres). In winter, that vapour meets glass that's the coldest surface in the room, cools below its dew point, and turns back into water. Physics, not faulty windows.
Which is good news: physics responds to cheap adjustments.
What actually causes window condensation?
Window condensation is warm, humid air meeting a surface cold enough to force the moisture out of it. Three variables decide it:
- How much moisture your air carries (relative humidity — UK homes in winter often sit at 65–80%, when 40–60% is the healthy target)
- How cold the glass is (single glazing runs coldest; even double glazing chills overnight)
- How much air movement crosses the glass (still air pockets behind curtains fog first)
Change any of the three and the fog retreats.
The fixes that work (in order of impact)
1. Stop making invisible steam. Lids on pans. Bathroom door closed and extractor on during showers — and for 15 minutes after. And if you dry laundry on radiators: that's the single biggest condensation source in most homes. A heated airer in one ventilated room, a vented/condenser dryer, or an outdoor line in dry spells removes litres a day from your air.
2. Ventilate briefly but properly. Ten minutes of "shock ventilation" — windows wide open, morning and evening — swaps humid air for dry cold air, which your heating then warms cheaply. Open trickle vents (the slots on modern frames) permanently in bedrooms; they exist for exactly this.
3. Keep heat low and steady, not blast-and-off. Glass that never fully chills never hits dew point. A constant 16–18°C background beats 22°C bursts followed by cold nights — and often costs no more.
4. Dehumidify the losers. One room always worst? A compressor dehumidifier (£100–£200, roughly 8–15p per hour to run) pulls litres per day and dries laundry as a bonus. Moisture-trap tubs (£3) help only in cupboards and cars — they're too small for a room.
The mould connection (why this isn't just cosmetic)
Condensation that sits on frames and sealant feeds black mould (Aspergillus and Cladosporium species), and mould spores are a documented respiratory trigger — the NHS links damp and mould to asthma and airway problems, particularly in children and older adults. The pattern is always the same: water pools on the bottom bead of sealant, dust and grime on the glass give spores nutrition and grip, colonies establish in the corners.
Two-part defence: wipe standing water off sills on bad mornings (a window vac makes this a 30-second job), and keep the glass and frames actually clean — mould can't anchor to clean, dry surfaces nearly as easily. Grimy glass also chills air unevenly and fogs faster, so cleanliness is condensation control, not just cosmetics.
That maintenance layer is where a robot earns its keep in winter: the CleanBot R1 keeps interior faces properly clean — controlled mist, immediate wipe, no soap film left behind (why residue matters) — on a monthly button-press instead of a weekend chore. Clean glass, cleaner sealant lines, fewer footholds for mould. £173, 24-month warranty.
Make winter glass care automatic: CleanBot R1 — free UK delivery over £75, code WELCOME10 for 10% off your first order.
When condensation means the window really is broken
*Fog or droplets between the two panes of a double-glazed unit means the perimeter seal has failed — the argon gap has taken on moist air and no amount of ventilation will clear it. The glass unit (not the whole frame) needs replacing: typically £60–£150 per unit in the UK. Condensation on the outside* face on cold clear mornings, meanwhile, is actually a compliment — it means your glazing is insulating so well the outer pane stays cold. It burns off by mid-morning.
| Where's the moisture? | Verdict |
|---|---|
| Inside face of the glass | Humidity problem — fix the air (this guide) |
| Between the panes | Failed sealed unit — replace the glass unit |
| Outside face, cold mornings | Sign of good insulation — ignore |
FAQ
Why do my windows have condensation every morning?
Overnight, heating drops, glass chills, and the moisture your household added to the air all evening (cooking, showers, breathing, drying clothes) condenses on the coldest surface in the room — the glass. Lower the humidity and keep background heat steady and it stops.
Do dehumidifiers stop window condensation?
Yes — a compressor dehumidifier removing litres per day typically clears persistent room condensation within a few days, at roughly 8–15p per hour to run. It treats the symptom; also reduce the moisture sources.
Is condensation on windows bad for health?
Indirectly — persistent condensation feeds black mould on frames and sealant, and the NHS links damp and mould to respiratory problems, especially in children. Wipe standing water and keep glass and frames clean.
Why is there condensation between my two panes of glass?
The sealed unit's perimeter seal has failed and moist air has entered the gap. Ventilation won't fix it; the glass unit needs replacing (typically £60–£150 per unit in the UK).
Cleaner glass is part of the fix: CleanBot R1 robot window cleaner — £173, 24-month warranty.