Why Is My uPVC Double Glazing Misted or Foggy Between the Panes?
For a double-glazed uPVC window where mist, fogging, droplets or a cloudy film are trapped between the two panes rather than forming on the room-side surface.
What this usually means
Condensation trapped between the panes usually means the sealed insulating glass unit has failed and moisture has entered the cavity. The Glass and Glazing Federation distinguishes this from ordinary room-side condensation and explains that a failed sealed unit can normally be replaced without replacing the whole window frame. Check that the moisture really is between the panes and not on an accessible surface. If it is internal to the glass unit, cleaning will not remove it; a glazing specialist should measure and replace the sealed unit.
What may be causing this
The edge seal of the insulating glass unit has failed
Clues: Fogging is clearly trapped between panes and cannot be wiped from either accessible face.
What to do: Have the sealed glass unit measured and replaced.
The glass unit has been damaged or its edge has deteriorated
Clues: The misting follows impact, frame movement or long-term water exposure around the glazing system.
What to do: Have the glass and glazing pocket inspected for the cause as well as replacing the unit.
Ordinary surface condensation is being mistaken for failed double glazing
Clues: The moisture wipes away from the room-side or exterior surface.
What to do: Treat the indoor-humidity or exterior-condensation cause rather than replacing the glass.
How to check what is happening
The sealed unit itself has likely failed.
Moisture is trapped inside the insulating glass cavity.
The cause should be corrected when the sealed unit is replaced.
Can you fix this yourself?
Confirm the mist is genuinely between the panes. Replacing an insulating glass unit requires correct measurement, glazing practice and safe handling of glass.
- Soft cloth
- Phone camera
- Window specification if available
What you can safely try
- 1
Confirm where the condensation is
Wipe the inside and outside surfaces to make sure the moisture is trapped within the sealed unit.
- 2
Record the affected pane
Photograph the misting and any visible frame or seal damage for the glazier.
- 3
Arrange sealed-unit replacement
A glazier can usually replace the glass unit while retaining a sound uPVC frame.
Call a glazier if…
- The glass is cracked, chipped or unstable.
- The pane is at height or unusually large and cannot be inspected safely.
- Removing glazing beads or the glass would be required.
Get the right glazier.
We already know this problem maps primarily to a glazier. Choose the closest type of work, add your postcode and continue in the form.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- Condensation in double glazingGlass and Glazing Federation
- When to repair or replace your windowsGlass and Glazing Federation
Last reviewed: 6 September 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.