Waking up to windows streaming with water is miserable — and it’s not just an annoyance. Persistent condensation causes mould growth, peeling paint, rotting window frames, and can trigger health problems including respiratory issues and allergies. The good news is that condensation is solvable once you understand what causes it.
Why Condensation Forms
Condensation occurs when warm, moist air hits a cold surface. The cold surface cools the air below its dew point, and the moisture in the air turns from vapour into liquid water — just like a cold glass of water on a warm day ‘sweats.’
Windows are the coldest surfaces in most rooms (especially single-glazed windows), so that’s where moisture condenses first. But the moisture isn’t coming from the windows — it’s coming from you, your cooking, your showers, your laundry, and your breathing. A family of four produces about 10 litres of moisture per day through normal daily activities.
Types of Window Condensation
| Location | Cause | Severity | Solution |
|---|---|---|---|
| Inside surface of window | Excess indoor humidity + cold glass | Common, manageable | Ventilation, heating, insulation |
| Between double-glazing panes | Failed window seal | Replacement needed | Replace the sealed unit (not the whole window) |
| Outside surface of window | Cold glass + warm, humid outdoor air | Not a problem — sign of good insulation | No action needed |
Fixing Internal Condensation
1. Improve Ventilation
This is the single most effective solution. Moisture needs to escape, and ventilation is how it does so:
- Open trickle vents — those small vents at the top of your windows are there for a reason. Keep them open, even in winter. They’re designed to provide background ventilation without significant heat loss
- Open windows briefly — 10–15 minutes of wide-open windows twice a day creates a rapid air exchange. The walls and furniture retain their heat, so the room warms back up quickly
- Use extractor fans — always run the extractor fan while cooking and showering, and for 15–20 minutes afterwards. If your fan is weak or noisy, replace it — modern fans are quiet and effective
- Don’t dry clothes indoors without ventilation — drying one load of washing releases about 2 litres of water into the air. If you must dry indoors, do it in a room with a window open or a dehumidifier running
The Energy Saving Trust condensation guide covers ventilation strategies in detail.
2. Reduce Humidity at the Source
- Cook with lids on pans — drastically reduces steam
- Close kitchen and bathroom doors when cooking and showering — this contains the moisture in the rooms with extractor fans
- Vent your tumble dryer outside — condenser dryers release moisture inside unless properly vented
- Don’t use unflued gas heaters — they produce significant moisture as a byproduct of combustion
3. Use a Dehumidifier
A dehumidifier removes moisture from the air mechanically. It’s particularly effective in bedrooms (where breathing produces moisture all night) and in rooms where drying clothes is unavoidable. Modern dehumidifiers are energy-efficient (200–300W) and can remove 10–20 litres per day from the air. Look for one with a humidistat so it only runs when needed.
4. Improve Insulation
The warmer your window glass, the less likely condensation is to form. Improving insulation raises the glass temperature:
- Upgrade to double or triple glazing — single-glazed windows are the worst offenders. Modern double glazing with low-E coatings keeps the inner pane significantly warmer
- Secondary glazing — a cheaper alternative to full replacement. A clear plastic or glass panel fitted inside the existing frame creates an insulating air gap
- Thick curtains — counterintuitively, heavy curtains can make condensation worse because they trap cold air between the curtain and the window. Open curtains during the day to let warm room air reach the glass
Our draught-proofing guide covers how to seal gaps around windows — but remember that some ventilation is essential. Don’t seal a house completely airtight without providing controlled ventilation.
Condensation Between Double-Glazing Panes
If moisture appears between the two panes of glass (inside the sealed unit), the perimeter seal has failed. This is not a ventilation problem — it’s a product failure:
- The seal allows humid air to enter between the panes
- It condenses on the cooler inner surface
- The window looks perpetually misted or foggy
You can’t fix this with ventilation or dehumidifiers. The sealed unit needs replacing. The good news is that you usually only need to replace the glass unit, not the entire window frame — this costs £50–£150 per unit compared to £300–£600 for a full window replacement. A glazier can do this in about 30 minutes per window. Which? explains the replacement process.
Preventing Mould
Where there’s persistent condensation, mould follows. Black mould on window frames, reveals, and surrounding walls is both ugly and a health hazard:
- Clean existing mould with a mould-specific spray (HG Mould Spray works well) or a solution of 1:4 bleach to water
- After cleaning, apply an anti-mould paint to affected areas
- Address the root cause (ventilation and humidity) — cleaning mould without fixing the condensation means it comes back within weeks
Quick Daily Habits That Help
- Wipe windows and sills each morning to remove overnight condensation — this prevents the moisture from soaking into frames and causing rot
- Leave bedroom doors open during the day for air circulation (if security allows)
- Keep a consistent low background temperature rather than blasting the heating and then turning it off — stable warmth reduces condensation cycles
