Condensation between double glazing panes: what it means and how to fix it

Where is the condensation?

Wipe the glass on the inside. If the mist clears, it is on the room side and your problem is indoor humidity; see how to stop condensation on windows. If it does not clear, wipe the outside. Outside condensation on a cold clear morning means the inner pane is keeping heat in, so the outer pane stays cold. If neither wipe clears it, it is between the panes.

Why sealed units fail

A double glazed sealed unit is two panes separated by a spacer bar around the edge. The spacer contains a desiccant that absorbs any moisture in the trapped air, and the edges are sealed. Over years, sunshine, temperature changes and movement can break down the seal. Once moist air gets in, the desiccant eventually becomes saturated and condensation forms on the inside surfaces of the gap, especially on cold mornings.

Age is the most common cause, but water sitting in the bottom of the frame because of blocked drainage slots can speed up seal failure.

Why wiping or a dehumidifier will not fix it

The gap is sealed from the room, so nothing you do to the room air reaches it. A dehumidifier, ventilation or heating will not clear misting between the panes. Some companies offer to drill the glass, dry the cavity and reseal it. Results vary, and the unit may not perform as well as a new sealed unit, so ask questions about guarantees before choosing that route.

Replacing the sealed unit

In most cases a glazier can replace just the glass unit, leaving the frame in place. They measure the existing unit, order a new one to size and fit it, usually in one visit once it arrives. This is normally far cheaper than a new window. While you are at it, you can ask about a modern unit with a warm edge spacer or low emissivity glass for better insulation.

If the window is a few years old, check the original guarantee. Many installers guarantee sealed units for a number of years. If the installer was registered with a competent person scheme such as FENSA, you may have paperwork showing the installation date.

1. 1

   Confirm the mist is between the panes

2. 2

   Check the installer's guarantee and paperwork

3. 3

   Ask a glazier to measure and quote for a replacement sealed unit

4. 4

   Clear the frame drainage slots so water cannot sit against the new seal

Does it matter if I leave it?

A failed unit insulates slightly less well and looks cloudy, and in time mineral deposits can etch the glass inside the gap so it never fully clears. It does not usually cause damp in the room on its own. That said, if your rooms also suffer from condensation on the inside of other windows and walls, that is a separate indoor humidity issue worth tackling.

See dehumidifiers for indoor condensation →

Keeping inside condensation down

Whether or not a sealed unit has failed, streaming windows on the room side mean indoor air is too humid for the glass temperature. Use extractor fans while cooking and showering, keep trickle vents open, avoid drying washing in closed rooms without a dehumidifier, and keep a steady background heat. Our dew point calculator shows how close your room is to condensing on cold glass.

Frequently asked questions

Can condensation between double glazing be fixed? Yes, usually by replacing the sealed glass unit. The frame can normally stay.

Will a dehumidifier clear condensation between panes? No. The gap is sealed from the room, so room humidity has no effect on it.

Is condensation on the outside of windows a problem? No. It usually means the glazing is keeping heat in well, so the outer pane stays cold overnight.