Window condensation is the single most common problem we're asked about as glaziers in London, and most of the advice online either sells you a dehumidifier or sells you new windows. The truth: where the water sits tells you everything, and only one of the three types needs a glazier at all.
Quick answer
Condensation forms when warm, moist indoor air meets glass that is colder than the air's dew point. To stop condensation on windows: keep indoor humidity at 40-60%, ventilate kitchens and bathrooms at the source, keep rooms above ~16°C, and open trickle vents. Condensation between the panes means the sealed unit has failed and the glass needs replacing, not the frame.
What causes condensation on windows?
Condensation is caused by moist air touching a cold surface, and glass is usually the coldest surface in the room. Air can only hold so much water vapour, and the amount falls sharply as air cools. When room air touches a cold pane, it cools below its dew point and sheds the excess moisture as droplets on the glass.
An average household of four puts 10-15 litres of water into the air every day: cooking, showers, kettles, breathing, and above all drying laundry indoors (a single load releases around 2 litres). Modern life makes it worse: draught-proofed, well-sealed London homes trap that moisture rather than leaking it away as Victorian houses once did. The moisture has to land somewhere, and the window is the first cold surface it finds.
The 3 types of window condensation, and what each means
There are only three places condensation can form on a window: the room side, the outside, or between the panes. The location is the diagnosis.
1. Condensation on the inside surface (room side)
Cause: too much moisture in the room, too little ventilation, or glass that gets too cold, usually all three. It's worst overnight in bedrooms (breathing adds moisture for eight hours while the heating is off) and in kitchens and bathrooms.
Fix: this is a household-habits problem, not a window problem. Cut moisture at the source (lids on pans, extractor fans run for 15 minutes after cooking and showering, no laundry on radiators), keep background ventilation going via trickle vents, and hold rooms above roughly 16°C so the glass never gets bitterly cold. If your windows are single glazed, the glass will always run cold, secondary glazing or double glazing raises the inner surface temperature dramatically.
2. Condensation on the outside surface
Cause: counter-intuitively, this is a sign your glazing is good. Modern low-E units keep so much heat indoors that the outer pane stays cold overnight. On clear, still mornings, typically spring and autumn, dew forms on it exactly as it does on a car windscreen.
Fix: none needed. It burns off by mid-morning and does no harm. If anything, treat it as free confirmation that your units are performing.
3. Condensation between the panes
Cause: the sealed unit has failed. Every double glazed unit is a factory-sealed sandwich with a desiccant-filled spacer bar keeping the cavity bone dry. When the edge seal perishes, moist air is pumped in and out with daily temperature swings until the desiccant is saturated, then mist, droplets and eventually milky staining appear inside the unit where no cloth can reach.
Fix: replace the sealed unit. It cannot be permanently dried out, and, crucially, the frame almost never needs to go. We cover this in detail in our guide to misted double glazing, or go straight to our misted double glazing repair service in London.
Wipe-away water is a ventilation problem. Water you can't wipe away is a glazing problem.
Dew point, in plain English
Dew point is the temperature at which the air in your room becomes too cold to hold its moisture. Think of air as a sponge whose size shrinks as it cools. At 20°C and 60% relative humidity, the dew point is about 12°C, so any surface colder than 12°C will collect water. Single glazing in winter can sit at 5-8°C on the room side; a modern double glazed unit sits closer to 15-17°C. That gap is the entire story of why old windows stream and new ones don't.
It also explains the two levers you control: lower the moisture (which lowers the dew point) or warm the glass (better glazing, steadier heating). Do either and condensation retreats; do both and it usually disappears.
What humidity should your home be? (40-60% RH)
Aim for 40-60% relative humidity, measured with a cheap digital hygrometer. Below 40% the air feels dry and scratchy; above 60% condensation and mould risk climb fast. Here's how the numbers play out room by room in a typical London home:
| Room | Target RH | Main moisture source | Most effective fix |
|---|---|---|---|
| Bedroom | 40-55% | Breathing overnight (~0.5 L per person) | Trickle vents open, door ajar, ≥16°C overnight |
| Bathroom | Spikes to 90%+, clear within 30 min | Showers & baths (~1.5 L each) | Extractor fan run 15 min after use, door closed while showering |
| Kitchen | 50-60% | Cooking & kettle (~3 L/day) | Lids on pans, extractor hood vented outside |
| Living room | 40-60% | Occupants, plants, aquariums | Background ventilation, steady heating |
| Anywhere laundry dries | Keep under 60% | Wet washing (~2 L per load) | Dry outdoors/vented dryer, or dehumidifier in a closed room |
Key takeaway
You cannot ventilate your way out of drying three loads of washing on the radiators. Deal with the biggest moisture sources first (laundry, showers, cooking) and the windows largely look after themselves.
When between-pane condensation means a failed unit
Any mist, fog or droplets sealed inside the glass, visible but untouchable, means the unit has failed and needs replacing. Early signs are subtle: a faint haze in cold morning light, a rainbow sheen, or a wet crescent along the bottom edge that comes and goes. Left alone it worsens; trapped moisture reacts with the glass over months and etches a permanent milky bloom that stays even on dry days.
The good news is the repair is far smaller than most owners expect. The failed unit lifts out of the existing frame and a new one, made to the same thickness, with fresh desiccant and, if you choose, upgraded low-E glass, goes in the same visit. Typical replacement takes 30-60 minutes per window once the unit is manufactured (usually 3-7 days after we measure). Our misted double glazing repair service covers all of Central and North London, and quoting is as simple as WhatsApping us a photo.
How to stop condensation on windows: prevention checklist
Work through these in order, the cheap habits first, the hardware last:
- Measure it. Put a £10 hygrometer in the worst room. Target 40-60% RH.
- Stop drying laundry on radiators. Use outdoor lines, a vented dryer, or a dehumidifier in one closed room.
- Run extractor fans properly: during, and for 15 minutes after, every shower and cooking session.
- Cook with lids on and keep the kitchen door closed so steam doesn't migrate to cold bedrooms.
- Open trickle vents and leave them open: they cost almost nothing in heat and run constant background air exchange.
- Keep rooms above ~16°C in winter; steady low heat beats short blasts for keeping glass above dew point.
- Wipe and dry sills each morning in a cold snap so water never sits long enough to feed mould.
- Upgrade cold glass. Single glazing? Consider secondary glazing (ideal for period and listed homes) or new double glazing via our window installation team.
- Replace failed units promptly: a blown unit has lost its insulation as well as its looks, so the glass runs colder and the problem compounds.
Window condensation FAQs
How do I stop condensation on my windows overnight?
Is condensation on the outside of windows a problem?
What does condensation between the panes of double glazing mean?
What humidity should my home be to avoid condensation?
Does condensation on windows cause mould?
Do dehumidifiers stop window condensation?
Misted glass that won't wipe clean?
That's a failed unit, and it's a quick fix: new glass in your existing frames. Free survey and fixed quote across Central & North London.