Condensation forms when moist air meets glass colder than the air's dew point. On the inside it usually means humidity is high and ventilation is low. Between the panes of double glazing it means the unit's seal has failed and the sealed unit needs replacing. On the outside, it is normally a sign that the glass is insulating well.
Why do windows get condensation?
Air holds water vapour, and warm air holds more than cold air. When warm, moist air touches a surface that is cold enough, it cools below its dew point and the vapour turns back into water on that surface. Glass is often the coldest surface in a room, so it is where condensation shows first, followed by window frames, reveals and the corners of external walls.
Government guidance on damp and mould describes condensation as moisture inside the home cooling and condensing onto colder parts of the building, such as window frames and wall corners. Everyday activities add the moisture: showering, cooking and drying laundry indoors all put water vapour into the air, and it builds up if it is not removed.
Where condensation appears tells you what is going on. The inside face of the glass, the gap between the panes of a double-glazed unit and the outside face of the glass each point to a different cause and a different fix, so it is worth checking which surface the water is actually on before deciding what to do.
What does condensation on the inside of the glass mean?
Condensation on the room side of the glass means the air indoors is humid and the glass is cold enough to drop below the dew point. It is common on single glazing, older double glazing without low-E glass, and at the edges of units with metal spacer bars, where the glass is coldest. It is usually worst on winter mornings and in bedrooms, kitchens and bathrooms.
A little misting that clears during the morning is normal. Heavy condensation every day, water running onto the cill, black mould on seals and reveals, or damp patches on nearby walls are signs that moisture is not being removed. The Energy Saving Trust notes that condensation on the inside of a window or wall can be a sign of a wider damp problem.
Government guidance says poorly insulated or poorly heated homes are more prone to condensation on the inside of external walls and windows, and that heating a home less, or irregularly, can increase it. Warmer glass helps: double glazing with low-E coatings and warm edge spacers keeps the room-side surface warmer, so it is less likely to reach the dew point.
How can I reduce condensation inside?
Reducing condensation comes down to producing less moisture, removing it where it is made and keeping surfaces warm. Government guidance on damp and mould highlights extractor fans in kitchens and bathrooms, and lists damaged, blocked, absent or switched-off mechanical ventilation as a risk. It also suggests opening a window wide for a period while cooking and after bathing.
Trickle vents in window frames provide background ventilation and should be kept clear and open, especially in bedrooms. A simple hygrometer shows whether relative humidity stays high, which the guidance treats as a key internal check. If condensation keeps coming back after these steps, look for other sources of moisture, such as leaks or rising damp, or ask a damp specialist.
Some rooms need more attention than others. Bedrooms gather moisture overnight from breathing, so a slightly open trickle vent and a door left ajar help. Kitchens and bathrooms produce bursts of steam, so an extractor fan that is powerful enough and actually used makes the biggest difference. Rooms that are rarely heated, such as spare bedrooms, often show condensation first because their walls and glass stay cold.
- Use extractor fans when cooking and bathing, and leave them running afterwards
- Put lids on pans and close kitchen and bathroom doors
- Dry washing outside or in a ventilated room
- Keep trickle vents open and clear of dust and paint
- Heat rooms steadily rather than in short bursts
- Leave a gap between furniture and cold external walls
- Wipe glass and cills dry in the morning
| Where it forms | What it usually means | What to do |
|---|---|---|
| Inside face of the glass | Humid air and cold glass | Improve ventilation, reduce moisture, keep rooms warm |
| Between the panes | The sealed unit has failed | Replace the sealed unit |
| Outside face of the glass | The glass is insulating well | Nothing; it clears as the glass warms |
What does condensation between the panes mean?
Condensation between the two panes of a double-glazed unit means the seal around the edge has failed. A sealed unit is two or more panes held apart by a spacer bar, with desiccant in the spacer to absorb any moisture trapped during manufacture. When the seal breaks down, moist air gets in, the desiccant becomes saturated and water condenses inside the unit where you cannot wipe it.
The Energy Saving Trust says condensation between the panes is likely to mean the double glazing is letting air in. The usual repair is to replace the sealed unit, not the window. The glazier measures the existing unit, orders a new one to size and fits it into the existing frame. If the frame is sound, the frame, hinges and locks stay as they are.
Misting between the panes may come and go with the weather at first, and white marks or staining may appear on the inner faces. The unit also loses some of its insulating value once the seal has failed. Check whether the window is still within its guarantee before paying for a replacement.
Why is there condensation on the outside of my windows?
Condensation on the outside face of the glass is usually a sign that the window is insulating well. Modern double and triple glazing with low-E coatings keeps heat inside, so the outer pane stays cold. On clear, still nights the outer pane can cool below the dew point of the outside air, and droplets form, much like dew on a car windscreen.
The Glass and Glazing Federation describes external condensation as a positive indication of a thermally efficient window, and the Energy Saving Trust says condensation on the outside is not a problem. It is most often seen on spring and autumn mornings and usually clears as the sun warms the glass. It does not mean the unit is faulty and needs no repair.
External condensation is more likely on windows that face open sky and are not sheltered by trees, nearby buildings or deep eaves, because the outer pane loses more heat to a clear night sky. Units with a higher insulation level show it more often than older ones on the same house, which can look odd when only some windows have been replaced. It is not a reason to choose glass that insulates less.
When should I replace a misted sealed unit?
Replace a sealed unit once misting between the panes is persistent, staining has appeared inside the unit, water is pooling at the bottom of the gap, or the glass is cracked. A failed unit will not recover on its own. Replacing it restores the view and the insulation, and the existing frame can usually stay in place.
A replacement unit is a chance to upgrade the glass. New units can have a low-E coating, argon gas fill and warm edge spacer bars, which keep the room-side glass warmer and reduce the chance of condensation forming on the inside. Where the unit is in a door or low down in a window, the replacement must be safety glass if it is in a critical location.
We can quote for replacement sealed units across the UK. A glazier measures the existing unit on a survey, and the written quote fixes the price for the specification shown. Manufacturer guarantees apply to new units, and the terms are confirmed in writing.
Can new windows make condensation worse?
They can if ventilation is lost. Older windows often let air leak through gaps around the sashes and frames, which removed moisture without anyone noticing. New windows are much more airtight, so the moisture produced in the home has fewer ways out unless trickle vents, extractor fans or opened windows take over that job.
Building regulations recognise this. In England, ventilation rules apply when windows are replaced, and government guidance on damp and mould says replacement windows must have trickle vents and that occupants should be able to control them. When replacing windows, ask how the new windows will provide background ventilation, and keep the vents open in daily use.
The detail of the ventilation rules differs between England, Wales, Scotland and Northern Ireland, so ask the installer to confirm what applies to your home and how the new windows meet it. If you rent and condensation keeps causing mould, report it to your landlord in writing, with photos, so the cause can be looked at rather than the mould simply being cleaned off.
Frequently asked questions
Can misted double glazing be fixed without replacing the unit?
The standard repair for misting between the panes is a new sealed unit. Once the edge seal has failed, the desiccant inside the spacer is saturated and moist air can keep getting in, so the problem returns. A replacement unit is made to the measured size and fitted into the existing frame, which usually stays in place. It can also be specified with better glass than the original.
Is condensation on the inside of windows normal?
A small amount on cold mornings, especially in bedrooms, kitchens and bathrooms, is common and clears as the room warms. Heavy condensation every day, water running onto cills, or black mould on seals and reveals means moisture is not being removed. Check extractor fans, open trickle vents, dry washing outside or in a ventilated room, and heat rooms steadily. Persistent damp may need a specialist.
Does triple glazing stop condensation?
Triple glazing keeps the room-side glass warmer than most double glazing, so condensation on the inside is less likely. It does not remove the moisture in the air, so ventilation still matters. Triple glazing is also more likely to show condensation on the outside on cold, clear mornings, because so little heat reaches the outer pane. That is a sign of good insulation, not a fault.
Why is only one of my windows misting between the panes?
Each sealed unit has its own edge seal, so they fail one at a time. A unit that faces the sun or the weather, sits in a frame where water collects, or was damaged during fitting may fail sooner than others in the same house. Only the affected unit needs replacing. A glazier can check the rest while on site and point out any showing early signs.
Do trickle vents really help with condensation?
Yes. Trickle vents are small openings in the window frame that let a steady trickle of fresh air in and moist air out, without opening the window. Government guidance on damp and mould says they should be kept clear and that occupants should be able to control them. They work most effectively alongside extractor fans in kitchens and bathrooms, which remove moisture where it is produced.
Sources
- Understanding and addressing the health risks of damp and mould in the home (GOV.UK)
- Energy efficient windows and doors (Energy Saving Trust)
- Low-E (low emissivity) glass and thermal efficiency (Glass and Glazing Federation)
- Energy efficient glazing: a useful guide (Glass and Glazing Federation)
- When to repair or replace your windows (Glass and Glazing Federation)
- Condensation consumer leaflet (Glass and Glazing Federation)
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