Search results for this problem are a mix of DIY videos, local glaziers and one franchise that promises to fix your windows without replacing them. None of them explain what has actually broken, which is why the same question keeps coming up on the trade forums: if it is the seal that has gone, why does the glass have to be replaced? The answer is in how a sealed unit is built, and once you have it, the rest of the decisions make themselves.
What blown means, and how to be sure that is what you have
A double glazed window is two things: a frame, and a sealed glass unit sitting in it. The unit is two panes bonded around a hollow spacer bar, with the cavity between them filled with argon and the whole perimeter sealed. Blown, misted, steamed up, cloudy and foggy all describe the same event: that perimeter seal has let go, outside air and moisture have got in, and the moisture is condensing on the inside faces of the glass where you cannot reach it.
The test takes a second. Wipe the glass on the room side, then on the outside. If either wipe clears it, the unit is fine and you have one of the two harmless kinds of condensation, which our guide to condensation on double glazing covers. If nothing you can touch clears it, the fog is inside the unit. In the early stages it comes and goes with the weather, cloudy on cold mornings and clear by afternoon. Left long enough it leaves permanent white streaks and tide marks on the inner glass faces that no amount of drying will remove.
Why a blown unit cannot be dried out
The spacer bar between the panes is not solid. It is a hollow tube filled with thousands of beads of desiccant, a molecular sieve whose sole job is to adsorb the trace moisture sealed in at manufacture and any that creeps through the seal over the years. The unit is sealed twice: a thin primary seal between the spacer and each pane that resists water vapour and gas movement, and a thicker secondary seal of polyurethane, polysulfide, hotmelt or silicone across the back of the spacer that bonds the whole edge together. Over 90% of European units are built this way, and the design is why a good unit stays clear for 15 to 20 years. Our explanation of how a sealed unit works goes through each layer.
Once the seal fails, moisture arrives faster than the desiccant can take it. When the desiccant is saturated it is finished: it cannot dry out and start again, and there is no way to replace it without taking the unit apart. The argon has already gone. Any soft-coat Low-E layer on the inner glass face is now sitting in water, and those coatings degrade within weeks of getting wet. So a blown unit is not a unit with some water in it. It is a unit whose three working parts, the seal, the desiccant and the gas fill, have all stopped working at once.
Can you fix blown double glazing without replacing it?
There are services that say yes. The method is to drill two small holes in a corner of the unit, pump dry air in through one and out through the other until the cavity clears, then plug the holes with small valves, sometimes with an anti-fogging agent added. It costs around £50 to £70 a unit, and DIY kits exist.
Read what the method does against what has failed. The seal is still broken, so moisture comes back in by the same route. The desiccant is still saturated, so nothing is absorbing it. The trade knowledge bases that describe the technique put the improvement at 6 to 24 months. It cannot be done on toughened glass, which rules out doors and any low-level pane, it comes with no guarantee from anyone we could find, and the firms offering it are candid that it is unlikely to remove the white staining once that has set in. Against that, a replacement unit costs from £55 fitted and comes with a guarantee of 10 years or more.
The drill-and-dry offer is priced to look like the cheap option. On a small pane it is not even that: the replacement unit is £55 to £74 fitted and it comes with a guarantee. The de-misting service is a cheaper way to buy the same problem again in a year or two.
Double Glazing Cost Calculator Team
What a replacement sealed unit costs
The published trade figures price the job by the size of the pane. A word on the label first: several cost guides call these "single pane" prices, which is misleading. A sealed unit is one component, and you cannot swap one leaf of it. Every figure below is for a complete new double glazed unit, fitted into your existing frame.
| Pane size | Typical window | Fitted price |
|---|---|---|
| 200 × 400 mm | Small top-light or door panel | £55 |
| 500 × 700 mm | Small casement opener | £74 |
| 800 × 1,000 mm | Standard casement | £117 |
| 900 × 1,200 mm | Large casement or fixed pane | £145 |
Source: Checkatrade, Blown window repair cost breakdown, June 2026. Ballpark averages for a standard clear A-rated unit.
| Job | Fitted cost | Notes |
|---|---|---|
| Replace a misted/blown sealed unit (single pane) | £55–£145 | Depends on pane size; the most common repair. |
| Replace the unit in a whole window | £100–£850 | Small £100–£200, standard £200–£325, large or bay £350–£850. |
| Reseal double glazing (per window) | £50–£100 | Specialist resealing where the gasket or seal has failed. |
| Replace a uPVC frame | £250–£750 | When the frame itself, not just the glass, has failed. |
Source: DGCC 2026 repair dataset (Checkatrade, MyBuilder). MyBuilder's own band is wider, £150 to £400 for a small unit and £400 to £800 for a large or bay. Price a whole house in the repair cost calculator.
Now separate the glass from the labour, because that is where the quotes diverge. A standard clear, A-rated, argon-filled unit costs around £100 per square metre supply-only from the online glass merchants, with a minimum charge at 0.3 square metres. Worked through, that is roughly £35 of glass in a 500 by 700 mm unit and roughly £110 in a 900 by 1,200 mm one. Fitters charge £25 to £35 an hour or £200 to £280 a day, and the fit takes one to three hours. So on a small pane the glass is the minor part of the bill, and the call-out is what you are paying for. That is also why doing several units in one visit is materially cheaper per unit, and why a glazier who quotes per unit without asking how many you have is not the one to use. To total up a house with several faults, the repair cost calculator prices each one.
Specification changes the glass price, not the labour. Toughened glass adds around £24 per square metre per pane, a higher-spec Low-E coating such as Planitherm 1.0 about £23, obscure glass about £27, and laminated glass roughly doubles the glass cost. A black warm-edge spacer adds about £5. None of these change what the fitter charges to put it in.
How to fix a blown double glazed window: the job step by step
Whether you do it yourself or watch a glazier do it, the job has the same four stages, and knowing them is the best protection against being sold a whole new window for a glass fault.
1. Remove the glazing beads
The beads are the strips that hold the unit in the frame. On uPVC made since the 1990s they are on the room side; that is a security design, because the 1980s externally beaded frames could be de-glazed from the garden in seconds. If yours are outside, modern versions have a wedge gasket that must be pulled from inside first. Start on the longest bead, mid-way along, with a wide stiff scraper held at right angles to the glass. Tap the handle until the blade opens the joint, lean it to about 45 degrees so the bead bows out of its groove, and work along until it is free. Mark each bead as it comes off, top, bottom, left, right, and keep every plastic packer that falls out. They go back in.
2. Measure and specify the new unit
With the unit out, measure it edge to edge, width first, then height, then its overall thickness through the spacer. Most units in uPVC frames are 24 or 28 mm thick and the two are not interchangeable, so if you are measuring with the glass still in, lay a straight edge across the frame and measure down to the glass on both sides to derive it. Order the new unit about 10 mm smaller than the tight opening in a uPVC frame, which is the common trade deduction, because glass is made to a tolerance of a couple of millimetres and a unit cut tight will not go in. The spacer bar is usually printed with the maker, the date and the build, for example 4 mm glass, 16 mm spacer, 4 mm glass, and the fitter or merchant will want that line.
Two specification points matter more than the rest. Match the old unit's coating and gas fill, or the window's rating drops. And if the pane is in a door, within 300 mm of a door edge, or any part of it is within 800 mm of the floor, the Building Regulations require safety glass, toughened or laminated, marked BS 6206 or its European equivalent. Toughened glass cannot be cut down after it is made, so it has to be right first time. The same rule is why a cat flap in double glazing means a new unit with the hole cut before the glass is toughened.
3. Fit and pack the unit
The unit sits on plastic packers, two along the bottom and one or two up each side, never directly on the frame. The packers do two jobs: they centre the glass so there is a drainage gap of about 5 mm all the way round, and on an opening window they are placed toe and heel, diagonally from the hinge side at the bottom to the lock side at the top, so the weight of the glass squares the sash rather than dragging it down. Bedding a unit solid in sealant, with no drainage, is the single commonest reason a replacement fails early. A large unit needs two people; a 900 by 1,200 mm unit is heavy and it is going in at arm's length.
4. Refit the beads
Short beads first, top and bottom, tapped home with a nylon or rubber mallet from one end. Then the long sides, bowed into position and tapped in. A little soapy water on the gasket helps. If the beads went back in the wrong places the corners will not meet, which is what the marks were for.
Fitting is one to three hours per unit once the glass has arrived. The unit is made to measure, so allow three to five working days from ordering for standard glass and seven to ten for toughened. If that sounds like a job you would rather not do yourself, the honest split from the installers is that a push-fit gasket is a realistic DIY job and a sealed unit swap is not, mainly because of the measuring and the weight, and anything on an upper floor should go to a glazier. Phone local glaziers rather than window companies; window companies would rather sell you the window.
Do you need FENSA or Building Regulations sign-off?
No, and the top-ranking cost guide for this search gets it wrong. It tells you to "always use a FENSA-registered professional" and that you will need the certificate when you sell. FENSA's own Guide to Compliance says the opposite: the scheme applies "only to full replacement work as repair work (for example, sash or glass only replacement) does not fall within the scope of the FENSA competent person scheme". Local authority building control puts it more bluntly: since April 2002 replacement windows fall under the Building Regulations, and "just replacing the glass is exempt". No certificate is issued for a unit swap, and none is missing when you sell. Our FENSA guide covers what the scheme does and does not certify.
What does still apply is the safety glazing rule above, which is part of the Building Regulations whether or not anyone certifies the work, and common sense on thermal spec: put a plain air-filled unit into a frame that had an argon Low-E one and the window is worse than it was.
Guarantees and insurance: check these before you pay anyone
A sealed unit should last 15 to 20 years. One that mists well inside that is a guarantee claim, not a repair bill, so find the paperwork first. The national installers typically give 10 years on parts and materials including fogging between the panes, and some go to 15 years on the sealed unit itself for casement and tilt and turn windows. Be aware that smaller suppliers often give only 5 years on the glass even when the frame carries 10 or 25, so read the split. Our aftercare guide explains what an insurance-backed guarantee adds if the installer has since closed.
Replacement units come with their own cover: 10 years is standard from glaziers and merchants for supply and fit, 5 years is common on supply-only glass, and the franchise repairers advertise up to 25 years on the glass they fit. Home insurance is the wrong place to look. Standard buildings policies treat seal failure as wear and tear and exclude it unless you have accidental damage cover, and a claim for a £74 unit is rarely worth the excess in any case.
How urgent is it?
Not an emergency, but not something to leave for years either. Act within three to six months if bills are rising or several windows are affected, and within six to twelve months for a single window in a low-use room. Three things get worse with time. The staining on the inner glass becomes permanent, which matters if you were hoping the drill-and-dry route might do. Water sitting in the cavity through a winter freezes and expands, which finishes off the seal and can crack the pane. And the heat penalty is real, even if the trade overstates it: the argon is gone, which costs roughly a third of the cavity's insulating benefit, and any soft-coat Low-E layer is degrading in the wet. Nobody publishes a single figure for the total, and honestly it ranges from a few percent on an old air-filled unit to more than double the original loss on a coated, gas-filled one full of water.
When to replace the whole window instead
The frame decides it. A uPVC frame lasts 20 to 25 years and a sealed unit 15 to 20, so on a window under about 15 years old a blown unit is a glass job and nothing more. Past 20 years the question changes: if the hinges and handles are also tired, the gaskets have hardened and several units have gone at once, you would be putting new glass into a frame with a few years left. A new uPVC frame alone is £250 to £750, and a complete uPVC casement window is £400 to £1,000 fitted, which the uPVC window calculator prices for your own house. The decision rule, with the numbers on both sides, is set out in repair or replace, and how long double glazing lasts gives the life expectancy by frame material. If you are not sure what has actually failed, the window fault checker sorts the symptoms first.
Why units blow, and how to make the next one last
Age and thermal cycling get every unit in the end, and south-facing windows go first because the unit expands and contracts hardest in direct sun. But premature failures, the eight-year-old unit that should have lasted twenty, are nearly always about water in the frame. The warranty inspectors' finding is specific: water gets trapped in the rebate under the glass, freezes over winter, breaks the seal, and the misting appears the following spring. The cures are all in the fitting. A drainage gap of about 5 mm around the unit, packers that keep it off the frame, a sill rebate that slopes to a clear drainage slot, and no bedding the unit solid in sealant. Frames whose rebates drain and ventilate, which is most uPVC, outlast undrained ones.
Two things you can specify. Ask for a warm-edge spacer rather than aluminium, because it transfers less thermal stress to the sealant as the unit cycles. And ask what the secondary sealant is; cheaper rubber edge seals deteriorate faster than silicone. Then keep the drainage slots along the outside bottom of the frame clear of moss and grit, which is the one piece of maintenance that actually reaches the seal.
If it is the whole window after all
One blown unit in a sound frame is a repair, and the figures above are the whole story. Where the frames are past twenty years, several units have gone together and the hardware is on its way, the repairs start to add up to a real share of what new windows cost. If you want that comparison priced on your own windows, tell us what you have and Leads 2 Trade will arrange quotes from installers covering your postcode.
Frequently asked questions
Blown, misted, steamed up and cloudy all describe the same fault: the edge seal of the sealed glass unit has failed, moisture has got into the cavity, and the desiccant in the spacer bar can no longer absorb it. The fog sits between the two panes, so you cannot wipe it from either side. The frame is usually unaffected.
Not permanently. The drill-and-dry services pump dry air through two holes drilled in the unit and charge around £50 to £70. The seal is still broken, the desiccant is still saturated, so the improvement typically lasts 6 to 24 months, it carries no guarantee, it cannot be done on toughened glass and it rarely removes the white staining. The lasting fix is a new sealed unit in the existing frame.
Fitted, £55 for a 200 by 400 mm pane rising to £145 for a 900 by 1,200 mm pane, with the whole-window figure at £100 to £850 depending on size. The glass itself is only part of that: a standard A-rated clear unit costs around £100 per square metre supply-only, so on a small pane most of the fitted price is labour and the call-out. Several units done in one visit are cheaper per unit.
Fitting takes one to three hours per unit once the glass has arrived. The unit is made to measure, so allow three to five working days from ordering for standard glass and seven to ten for toughened.
No. FENSA's own compliance guide says repair work, including glass-only replacement, does not fall within the scope of the scheme, and local authority building control describes replacing just the glass as exempt. Two things still apply: the new unit should match the thermal spec of the old one, and glass in a critical location such as a door or within 800 mm of the floor must be safety glass.
Usually not. Seal failure is treated as wear and tear, which standard buildings policies exclude unless you have accidental damage cover. Check the installation guarantee before the insurance: sealed units commonly carry 10 years, and some installers offer 15 on the unit itself.
Yes, but by an amount that depends on the unit. Argon conducts heat roughly a third less readily than air, so losing the gas costs about a third of the cavity's insulating benefit, and a soft-coat Low-E coating with water on it stops working within weeks. No trade body publishes a single figure; estimates run from a few percent on an old air-filled unit to more than double on a coated, gas-filled one with standing water.
Sealed units should last 15 to 20 years. Failure well inside that is almost always a site problem rather than a glass one: water trapped in the frame rebate that froze over winter, a unit bedded solid on sealant with no drainage path, or missing packers. A unit inside its guarantee is a claim, not a repair bill, so find the paperwork first.