Condensation on windows: inside, between the panes or outside?

Reviewed and updated 22 October 2026 · How we assess condensation

Before you buy a dehumidifier or call a glazier, check exactly where the water is. Touch the glass: if your finger comes away wet from the room side, it's indoor condensation. If it stays dry but the glass still looks misty, the moisture is between the panes or outside.

1. Condensation on the inside of windows

This is by far the most common type. Glass is usually the coldest surface in a room, so it's the first place indoor moisture shows up. It's typically worst first thing in the morning in bedrooms (two sleeping adults breathe out roughly half a litre of water overnight), and after showers or cooking.

Why it gathers at the bottom of the glass

The edges of a double-glazed unit are colder than the centre, because the spacer bar between the panes conducts heat. Cold air also sinks down the glass, so the bottom edge is coldest of all. Modern "warm edge" spacers reduce this but don't remove it.

Is a little condensation normal?

A thin strip on cold mornings that clears within an hour or two is common in UK homes, especially with older double glazing. It becomes a problem when water runs onto the sill, the seals and reveals go black, or it lasts most of the day. Wipe it off rather than leaving it to soak into timber or plaster.

What it tells you about your walls

If the glass is wet, the air near it is at 100% humidity. Corners, reveals and the wall behind furniture may be well above 80% at the same time — enough for mould without any visible water. Check those spots too.

2. Condensation between the panes of double glazing

A sealed unit is two (or three) panes bonded to a spacer filled with a desiccant that keeps the gap dry. When the seal fails, humid air gets in, the desiccant saturates and the inside of the unit mists over — often in a pattern that comes and goes with the weather.

• It's not caused by your room humidity, and a dehumidifier or extra ventilation won't clear it.
• The fix is a replacement sealed unit. A glazier can usually fit a new unit into the existing frame, which costs far less than a new window.
• Check any guarantee. Units are often guaranteed for 10 years; FENSA or CERTASS paperwork from the installation may help.
• "Demisting" services that drill and dry the unit exist, but the insulating gas has gone and the result is usually less durable than a new unit.

3. Condensation on the outside of windows

Dew on the outer face forms on clear, calm nights when the glass radiates heat to the sky and cools below the outdoor dew point. It's more likely with modern low-emissivity double or triple glazing precisely because so little heat escapes from inside to warm the outer pane. It's harmless and clears as the sun comes up.

4. Secondary glazing

With secondary glazing you have a room-side panel and the original window. If moisture forms on the inside of the original window, room air is getting past the secondary panel. The general principle used in historic-building guidance is: make the inner (room-side) layer the tighter one, and let the outer layer breathe to the outside. Add or renew seals on the secondary panel; don't seal up the old window's gaps.

5. Fixes that work — and ones that don't

To see which of these matters most in your room, use the full assessment. It calculates the glass, edge and wall temperatures together and recalculates combinations of fixes as a package.