If you can see moisture on the glass but cannot wipe it off from inside or out, it is sitting in the cavity between the two panes. That is a failed sealed unit, and unlike ordinary condensation there is nothing about your heating or ventilation that will fix it.
The reassuring part is that in most houses the window itself is fine. This guide explains why units fail, what your options are and what it should cost.
Why sealed units fail
Age and thermal cycling
The perimeter seal expands and contracts every day as the glass heats and cools. After fifteen to twenty years that constant movement eventually breaks the bond, and once air can get in, the desiccant saturates and misting begins.
Blocked drainage
This is the biggest avoidable cause. Every frame has drainage slots that let water escape from the rebate. When they clog with dirt or moss, water pools against the edge seal of the unit and destroys it, often within eight to ten years.
Poor installation
Units bedded without correct glazing packers sit on their edge seal and carry the weight of the glass through it. Beads forced on with a mallet, or silicone smeared over the drainage, produce the same early failure.
South-facing heat build-up
Dark blinds or furniture pressed hard against south-facing glass trap heat between the blind and the pane and stress the seal. It is a surprisingly common cause of one window failing while its neighbours are fine.
Your options and what they cost
| Option | Cost | Lasts | Verdict |
|---|---|---|---|
| Do nothing | £0 | — | Misting worsens; insulation already compromised |
| Drill and dry ('defogging') | £40 – £90 | 6 months – 2 years | Cosmetic only, cavity stays unsealed |
| Replace the sealed unit | £95 – £260 | 15 – 20 years | The proper repair in most cases |
| Replace the whole window | £450 – £900 | 25 – 35 years | Only if the frame has also failed |
| Claim on guarantee | £0 – labour only | — | Worth checking first |
How the repair is done
The glazing beads are removed from inside, the failed unit lifted out, and the rebate cleaned and checked. The new unit is set on fresh glazing packers so its weight is carried at the correct points and not on the seal, the beads are refitted, and the drainage is cleared.
It takes around half an hour to forty-five minutes per window and is usually done as two short visits — one to measure, one to fit once the glass is made.
Preventing the next one
- Clear the drainage slots at the bottom of each outer frame once a year.
- Do not seal over the drainage with silicone when re-pointing around the frame.
- Keep dark blinds a few centimetres off south-facing glass.
- Deal with any window that holds standing water in the rebate after rain.
- Ask for warm-edge spacer bars when replacing units — they cope better with thermal movement.
Does misting affect your heating bills?
Yes, though usually less than people fear. Once the seal fails, the argon fill escapes and the cavity fills with ordinary damp air, so the unit's insulating performance drops back roughly towards that of a basic 1990s double glazed unit. You are not back to single glazing, but you have lost a meaningful part of the improvement you paid for.
The bigger effect for most households is comfort rather than cost. A failed unit runs colder on the inside face, which makes the area near the window feel draughty and encourages surface condensation and mould on the reveal, even where the room's overall humidity is reasonable.
Across a whole house of failed units the difference does become noticeable on a bill, which is one reason it is worth doing them all on a single visit rather than one at a time.
When it is not just the glass
Call it a window problem rather than a glass problem if the frame is rotten or bowed, if the sash cannot be adjusted to close square, if water is getting past the frame into the room, or if most of the units in the house have gone at once — that last one usually indicates the whole installation is at the end of its life.
If you are anywhere in Central Scotland we will look at it and tell you which it is. Most of the time it is the pane, not the frame.
Common questions
No. It is a comfort and appearance issue rather than a safety one, so it can wait until convenient — unless the glass is also cracked.
Want a straight answer on your own windows or doors?
DM Installations fits and repairs windows and doors across Central Scotland — Cumbernauld, Glasgow, Falkirk, Stirling and the surrounding towns. Free survey, written quote, and an honest answer on whether you need a repair or a replacement.
Related guides
Misted Double Glazing Repair Cost
Failed units are usually a glass problem, not a window problem. Here is what replacing the pane costs against replacing the whole thing.
How-toHow to Stop Window Condensation
Where the moisture comes from, the fixes that actually work, and how to tell harmless condensation from a failed unit.
How-toWindow and Door Maintenance
Thirty minutes a year that adds years to your windows and doors, with the products to use and the ones to avoid.