How do I tell room condensation from a failed double glazed unit?

Wipe it. Room condensation is on the surface facing you and comes off on the cloth. If the haze or water stays put, it is sealed inside the unit and the edge seal has failed.

Is condensation a sign my new windows were fitted badly?

Not on its own. New windows are far more airtight than what they replaced, so the moisture a house always produced no longer leaks out through gaps. That is exactly why the ventilation rules require a window replacement not to leave the ventilation worse than before — if your replacement arrived without trickle vents, the ventilation provision is the thing to look at.

Should I leave trickle vents open in winter?

Yes. They are background ventilation and are meant to stay open. The alternative people reach for — locking a window slightly ajar on the night-latch — is not accepted as background ventilation under the ventilation guidance, because a window on a night-latch is not secure.

Will a dehumidifier solve it?

It will lower the moisture in the air and reduce the symptom while it runs. It does not warm the cold surface or restore ventilation, so it is a way of managing the room rather than fixing the cause.

Low risk

Updated 30 August 2026

Condensation On The Inside Of Your Windows

Water runs down the room side of the glass most mornings, and wipes away with a cloth.

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Window and door fitting, illustrative image

What's happening?

This is room air meeting a cold surface. Warm air holds more water vapour than cold air, so when moist indoor air touches glass that has cooled overnight, the air at the glass drops below its dew point and gives up water. Because it is on the room side, you can wipe it — which is the quickest way to tell it apart from a misted sealed unit, where the water is trapped between panes and no cloth reaches it. Small amounts on the coldest mornings are physics, not a fault. Water pooling on the sill every day is a signal: either the glass is much colder than the rest of the room, or the room is generating more moisture than its ventilation can carry away.

What usually causes it?

Single glazing or an early double glazed unit with a metal spacer and no low-emissivity coating, so the glass runs cold
A sealed unit whose edge seal has failed, so the insulating cavity has gone and the inner pane is now the coldest surface in the room
Background ventilation missing or shut — trickle vents painted over, taped up, or never fitted at a window replacement
Bedrooms with the door closed overnight, where two sleeping adults release a litre or more of moisture into a sealed room
Drying laundry indoors, or a tumble dryer venting into the room rather than outside
An extract fan in the kitchen or bathroom that has failed, is disconnected, or discharges into the loft instead of outdoors
Heavy lined curtains or a blind closed tight to the glass, which stops room air washing the pane and lets it run colder still

Is it dangerous?

The water itself is harmless, but persistent surface condensation is the start of a mould problem. Where it runs off the glass onto timber sills, reveals and plaster it keeps those surfaces damp enough for mould to establish. Government guidance on damp and mould in the home is explicit that damp and mould are a health risk and should not be dismissed as a lifestyle matter — the underlying cause, whether cold surfaces or missing ventilation, is what needs fixing.

Can I fix it myself?

Yes, quite a lot of it. Open trickle vents and leave them open — they are designed to run permanently, not to be opened when you notice a problem. Run the extract fan while cooking and bathing and for a while afterwards, keep the bathroom door shut while it runs, dry washing outside or in a vented tumble dryer, and leave a gap between blinds and glass so air can reach the pane. Wipe standing water off timber sills rather than letting it sit. What you cannot fix yourself is glass that is simply too cold, or a window with no background ventilation provision at all.

When should you call a window fitter?

Call a window fitter when the ventilation and habits are already right and the glass is still streaming, when the window has no trickle vents and no other background ventilation, or when the condensation is between the panes rather than on the room face. Also call if water is standing in the frame drainage channels and not escaping, or if the sash no longer pulls up tight onto its seal, because a leaking closure lets warm moist air straight onto the coldest part of the unit.

What will the window fitter actually do?

Confirm which side of the glass the water is on, to separate room condensation from a failed sealed unit
Check the window for background ventilation and whether the provision meets the equivalent area expected for the room
Retrofit trickle ventilators into the frame or sash, or confirm an existing through-wall background ventilator is adequate
Clear and re-form the frame drainage slots so run-off leaves the frame instead of pooling on the sill
Renew perimeter gaskets and adjust hinges and keeps so the sash compresses evenly onto its seal
Quote a higher-specification unit — warm-edge spacer, low-emissivity coating — where the existing glass is the coldest surface in the room

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Misted Double GlazingLow risk No Trickle Vents After New WindowsCaution Black Mould On WallsHigh risk

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