Double Glazing · Updated August 2026
Blown Window Repair Cost UK 2026: Misted Unit Prices
A blown window — condensation trapped between the panes — means the sealed unit has failed, not the window. Replacing just the glass unit costs £75–£250 fitted for most casements, rising to £200–£450 for toughened patio-door glass. The frames stay put, which is why a unit swap runs at a fraction of the £400–£1,800 a full new window costs.
Blown window repair cost by unit — 2026
Supply and fit, glass unit only — frames, hinges and handles unchanged. Most fitters carry a call-out minimum of £80–£150 that covers the first unit.
| Unit | Typical cost | Time on site | Notes |
|---|---|---|---|
| Small casement unit (to ~0.5m²) | £75–£150 | Under an hour | Top openers, cloakroom windows |
| Standard casement unit | £100–£200 | ~1 hour | The commonest replacement |
| Large fixed / picture unit | £150–£300 | 1–2 hours | Two fitters above ~1.8m² |
| Patio door glass (toughened) | £200–£450 | 1–2 hours | Safety glass is required by regs |
| Bay window (3–5 units) | £300–£850 | Half a day | Per-unit price falls with quantity |
| Whole-house refresh (8–12 units) | £700–£1,900 | A day | Worth batching for the discount |
| “Demisting” (drill and vent) | £60–£120 / unit | Under an hour | Temporary — the seal is still failed |
Indicative 2026 UK prices, compiled from published installer pricing (Checkatrade, MyJobQuote and MyBuilder misted-unit guides, August 2026). Sealed units are made to measure - treat these as a budgeting guide and confirm against written per-unit quotes.
What changes the cost
| Cost factor | Typical impact | Why it matters |
|---|---|---|
| Unit size and shape | the base variable | Sealed units are priced per m2 and made to measure - arched and shaped units cost noticeably more |
| Glass specification | +20–40% | Toughened is compulsory in doors and low-level glazing; low-E (K) glass should match the originals or one pane reads darker |
| Access | +£30–£100 | Upper-floor units needing ladders or towers take longer and sometimes a second visit |
| Quantity | −15–30% per unit | The call-out is spread across the batch - doing one misted unit at a time is the expensive way |
| Frame condition | the replacement signal | If gaskets are perished or frames warped, new glass mists again - that money belongs in full replacement |
Blown window repair cost by UK region — 2026
Modelled from regional labour rates rather than measured in this category - materials cost much the same everywhere, the labour does not.
| Region | Typical installed cost | vs UK average |
|---|---|---|
| London | £120–£270/unit | +25–30% |
| South East | £110–£250/unit | +15–20% |
| South West | £90–£220/unit | +5% |
| Midlands | £90–£210/unit | baseline |
| North England | £80–£190/unit | −5–10% |
| Scotland | £80–£190/unit | −10% |
| Wales | £80–£190/unit | −10% |
| Northern Ireland | £80–£180/unit | −15% |
Getting it priced properly
1. Count and photograph every misted unit
Fitters price per unit and discount by quantity. One phone photo per window is enough for a firm per-unit quote.
2. Get the glass spec in writing
Toughened where regulations require it, low-E to match the originals, and trickle-vent or Georgian-bar details noted - the quote should name them all.
3. Check the frames before you spend
Press the gaskets and check corners for warp. Sound frames make a unit swap the obvious win; failing frames mean new glass will mist again.
What blown window repairs cost - 2026
Every figure comes from the cost table above.
Why sealed units blow, and how to slow it
A sealed unit is two panes of glass bonded to a spacer bar, with a drying agent inside the bar that keeps the trapped air dry. The unit blows when that edge seal fails: moist air creeps in, the desiccant saturates, and the first cold morning shows it as misting or droplets between the panes that no amount of cleaning will shift.
Age is the main cause — most units are built to last around 15–25 years — but a few things bring the day forward. Units that sit in standing water because the drainage slots in the frame are blocked fail early, so keeping those weep holes clear genuinely helps. South- and west-facing windows take the most heat cycling and tend to go first, and units knocked during earlier work are on borrowed time. None of that makes a blown unit repairable once it has gone — the glass has to come out — but it does explain why one window failed and the one beside it hasn’t.
What a misted-unit price includes, and what it doesn’t
A unit swap is a small, quick job, so the per-unit prices on this page cover most of it. The extras below aren’t hidden costs so much as choices — glass spec and quantity, mostly — and they explain why two quotes for the same window can differ. Knowing which column your job falls into makes the quotes easy to read.
| In the per-unit price | Adds to it |
|---|---|
| A made-to-measure sealed unit fitted into your existing frame | Toughened or laminated safety glass where regulations require it — roughly 20–40% more |
| Removing the old beads and refitting the new unit | Matching soft-coat low-E glass so a new pane doesn’t read darker than its neighbours |
| A standard clear or low-E double-glazed unit | Georgian bars, obscure or patterned glass, or stepping up to a triple-glazed unit |
| A call-out that covers the first unit | Further units at the per-unit rate — doing several at once is what brings the price down |
| Taking away and disposing of the failed unit | New gaskets, hinges or handles if the frame hardware has also failed |
Expect two short visits: one to measure, then about an hour a unit to fit once the made-to-measure glass arrives, usually a week or two later. VAT may or may not be on the quote depending on the firm’s size, so check. If you want a figure for your own windows before you call anyone, the blown window repair cost calculator prices a unit swap in under a minute, and the double glazing cost guide shows where full replacement takes over.
Do you need FENSA or building regs to replace a blown unit?
This is the question that saves people money, and the answer turns on one distinction. Replacing just the failed sealed unit in an existing frame is a like-for-like repair. It is not controlled work, so you don’t need a FENSA or CERTASS certificate and you don’t need to tell building control — a straight unit swap leaves no paperwork behind.
Replacing the whole window, frame and all, is different. A new window is a controlled fitting under Building Regulations Part L, so it has to be either installed by a FENSA- or CERTASS-registered firm that self-certifies the work, or notified to building control who inspect it. The replacement also has to meet a minimum energy rating. So the moment a quote moves from new glass in your frames to new windows, ask the installer for their FENSA or CERTASS registration number and check it on the scheme’s register. Our guide on whether you need permission to replace windows and the FENSA register spell out where the line sits.
Choosing a glazier: what to check
Sealed units are made to measure, so the quote hinges on getting the count and the spec right up front. Three checks do most of the work.
Ask for a per-unit price
Count and photograph every misted pane, then ask for a firm per-unit quote rather than a day rate. It makes the batch discount visible and stops the price creeping as more units are found on the day.
Pin down the glass spec
Toughened where the regs require it, low-E to match the originals, and any bars or obscure patterns noted — all named on the quote. Mismatched coatings are the usual reason one new pane looks different from the rest.
Check what’s guaranteed
Sealed units generally carry a manufacturer guarantee against re-misting, commonly five to ten years, so ask for the exact term. Get the fitter’s own workmanship warranty in writing alongside it.
Finding installers worth quoting is where we come in. Tell us the windows and we pass it to vetted local glazing firms for up to three quotes to compare, free and with no obligation — you decide who, if anyone, to use. The double glazing hub has more on choosing between repair and replacement.
Blown window repair — frequently asked questions
A double-glazed sealed unit whose edge seal has failed. The desiccant in the spacer bar saturates, moisture creeps between the panes and shows as misting or droplets you cannot wipe off. The glass unit needs replacing; the window itself usually does not.
“Demisting” services drill the unit and vent or flush it for £60–£120. It clears the visible mist for a while, but the seal is still failed and the insulating gas is long gone — most units mist again. Treat it as a cosmetic stopgap, not a repair.
When the frames are sound, yes — a unit swap typically costs £75–£250 against £400–£1,800 for a full new window. When gaskets are perished or frames warped, the new unit will fail early and full replacement is the honest advice.
Two visits: one to measure, then about an hour per unit to fit once the made-to-measure units arrive — usually one to two weeks later. Beads are removed, the unit swapped and the window back in use the same hour.
Related glazing guides
When repair stops making sense: full window prices.
A realistic figure for full replacement in 30 seconds.
The national installers compared on price and guarantees.
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Reviewed by the BestBuilders editorial team · Last updated: 5 September 2026 · Next scheduled review: December 2026 · See our editorial standards.