The short answer
If only the sealed unit has misted or “blown” and the frame is sound, you can usually replace just the glass for around £60–£200 per unit — far cheaper than a new window. Misting between the panes means the seal has failed and the gas or air gap is now letting in moisture; the glass cannot be cleared, but the unit can be swapped. If the frame itself is rotten, warped or the hinges and locks are failing, replacing the whole window makes more sense. See can you replace just the glass for the detail.
Misting inside a double glazed unit is one of the most common window problems, and the good news is that it is often one of the cheapest to fix. This guide explains what causes a sealed unit to fail, what a glass-only repair involves and costs, and how to judge whether a repair is enough or the whole window has reached the end of its life.
Misted units at a glance
- Replace one sealed unit £60–£200
- Replace whole window £400–£900 (uPVC)
- Cause Failed perimeter seal
- Can the mist be cleaned off? No — it is between the panes
- Repair possible if Frame and hardware are sound
- Typical sealed-unit life 10–20 years
What “misted” or “blown” double glazing means
A double glazed unit is two panes of glass sealed around the edge with a spacer bar, with a gas-filled or dry-air gap between them. Over time the perimeter seal can fail, letting humid air into the gap. When the temperature drops, that moisture condenses on the inside of the glass — the misting or fogging you see between the panes. This is what people mean by a “blown” unit. The mist is on a surface you cannot reach, so it cannot be cleaned off, and the unit no longer insulates as well because the gas fill has been compromised. The only fix is to replace the sealed unit.
When you can repair (replace the glass only)
If the frame is solid — no rot, no warping, hinges and locks working — an installer can remove the failed sealed unit and fit a fresh one into the existing frame. This typically costs £60–£200 per unit depending on size and glass specification, far less than a whole new window. It is the right call for an otherwise good window that has simply suffered a seal failure, which is common after 10–20 years. The new unit can usually match or improve the original’s energy rating. See can you replace just the glass for what to check first.
| Situation | Best option | Typical cost |
|---|---|---|
| Misted unit, sound frame | Replace sealed unit only | £60–£200 |
| Misted unit, tired frame | Consider whole window | £400–£900 |
| Several misted units, old windows | Plan a phased replacement | Per-window pricing |
| Rot, draughts and failing locks | Replace the window | £400–£900 |
When to replace the whole window instead
Replacing the glass only makes sense when the frame is good. If the frame is rotten (timber), discoloured and brittle (older uPVC), warped, or the hinges, handles and locks are failing, you are putting a new unit into a window that is itself near the end of its life — better to replace the whole thing. The same is true if many units across the house have misted: that often signals windows of a similar age all reaching seal failure together, in which case a phased whole-house replacement may be more cost-effective than repeated single repairs. See signs you need new windows for the wider checklist.
How a sealed-unit replacement is done
An installer measures the failed unit, orders a matching replacement to the same dimensions and glass specification, then removes the beading, lifts out the old unit and fits the new one with fresh seals before re-beading. A single unit usually takes under an hour once the glass arrives. Because you are not changing the frame, a sealed-unit swap is not generally a notifiable replacement in the way a whole new window is — but using a competent, ideally registered installer still protects you. This is general information; whether to repair or replace depends on the condition of your specific windows.
Compare double glazing quotes
Whether you need a single sealed unit or new windows, comparing quotes helps you get a fair price. Use our service to reach FENSA or CERTASS registered installers in your area.
Frequently asked questions
Can misted double glazing be cleaned?
No. The misting is condensation between the two panes, on a surface you cannot reach. The seal has failed and the only effective fix is to replace the sealed unit. Defogging treatments rarely give a lasting result.
How much does it cost to replace a blown double glazed unit?
Replacing a single misted or blown sealed unit typically costs £60–£200, depending on its size and glass specification, provided the frame is sound. That is far cheaper than replacing the whole window.
Should I repair or replace misted windows?
If the frame is sound, replace just the glass — it is much cheaper. If the frame is rotten, warped or the hardware is failing, or many units have misted across old windows, replacing the whole window or planning a phased replacement usually makes more sense.
Why has my double glazing misted up?
The perimeter seal around the sealed unit has failed, letting humid air into the gap between the panes. When it cools, that moisture condenses inside the glass. Seals commonly fail after 10–20 years.
Sources & further reading
- Glass and Glazing Federation (GGF) — sealed unit failure, condensation and consumer guidance
- FENSA — guarantees, registered installers and replacement glazing
- Energy Saving Trust — double glazing performance and maintenance
- GOV.UK / Building Regulations Approved Document L — thermal standards for replacement glazing
This is general information, not advice for your specific property or installation. Costs and outcomes vary with your windows and chosen installer. Replacement windows should be fitted by a FENSA or CERTASS registered window installer. We are an independent information and introduction service, not an installer.