Misting inside a double glazed window means the seal on the sealed glass unit has failed and moisture has got between the panes. The frame is usually fine. If it is, a glazier removes the failed unit and fits a fresh one into the same frame for £130 to £300 all in, against £350 or more for a whole new window.
Here is what causes it, how to know whether a repair will do, and what it costs.
Points to take away
- Misting between the panes is a failed unit seal, not a failed window.
- If the frame is sound, reglaze the unit. It costs about a third of a new window.
- A new sealed unit restores insulation and lasts another 15 to 20 years.
- Drilling and defogging clears the fog briefly but does not fix the unit.
- Check for a FENSA or CERTASS guarantee before you pay.
Reading time: about 5 minutes.
Why double glazing mists up
A sealed unit is two panes of glass with a spacer bar around the edge, holding them apart. The spacer contains a desiccant, a drying agent that absorbs the tiny amount of moisture sealed in during manufacture. The whole edge is sealed with a butyl and polysulphide bond, and the cavity is often filled with argon gas.
Over 15 to 25 years, that edge seal degrades. Water vapour starts getting in faster than the desiccant can cope with, and on a cold morning it condenses on the inside faces of the glass as fog or droplets you cannot wipe away. Heat and moisture speed it up, so south-facing windows and bathroom windows tend to go first.
Repair or replace: what decides it
Reglaze the unit if:
- The frame is solid: no rot in timber, no warping or cracked corners in uPVC, no corrosion in metal.
- The window opens, closes and locks properly.
- It is a standard sealed unit that can be measured and remade. Most casement and sash windows from the last 30 years qualify.
Replace the whole window if:
- The frame has rotted, warped or cracked, or has pulled away from the wall.
- The window is a very old design where the glass is not a removable sealed unit.
- Most of the windows in the house are failing and the frames are past 20 years, in which case doing the whole house at once is better value. See the cost guide.
Our guide to the signs you need new windows covers the frame checks in more detail.
What it costs
| Job | Typical UK cost |
|---|---|
| Replacement sealed unit, small (bathroom) | £55 to £110 supply |
| Replacement sealed unit, standard casement | £90 to £150 supply |
| Replacement sealed unit, large or with a coating | £130 to £180 supply |
| Fitted, per window (measure and glaze) | £130 to £300 all in |
| Every blown unit in a 3-bed semi | £1,400 to £2,500 |
| Whole new uPVC casement window | £350 to £800 fitted |
Prices rise for toughened, laminated (acoustic or safety), Georgian-bar or leaded units, and for anything above the ground floor that needs access equipment.
Avoid the defogging gimmick
Some firms offer to “defog” a misted unit by drilling a small hole, drying the cavity and fitting a vent. It clears the visible misting for a time. It does not restore the edge seal, the desiccant or the argon fill, so the unit keeps losing heat like a failed one, and the fog often comes back. For a lasting fix, the unit is replaced.
Check your guarantee first
If the windows were fitted by a FENSA or CERTASS registered installer, the work usually came with a 10-year insurance-backed guarantee that covers sealed unit failure. Dig out the certificate and the installer’s paperwork before you pay for a repair. Even outside the guarantee, the original installer can often remake a matching unit faster than a general glazier.
Get a free quote and we will match you with vetted local repair firms and installers who cover your area.