Most double glazing problems do not require a whole new window. A misted or blown unit, a stiff handle, or a draught from a failed seal can all be fixed without touching the frame. The question is whether the frame itself is worth keeping. If it is, repair is almost always the right call: replacing a single sealed unit runs £55 to £145, against £400 to £1,000 for a full uPVC casement replacement. Our repair cost calculator lets you price a repair for your own windows in under a minute, and the uPVC window calculator shows what a full replacement would cost if that turns out to be the better route.
The headline comparison
The table below sets out the typical 2026 costs side by side. All repair figures assume the uPVC frame is structurally sound. Replacement figures are for a fully fitted uPVC casement window including labour, disposal and a FENSA certificate.
| What you need | Repair cost | Full replacement cost |
|---|---|---|
| Single blown/misted pane | £55–£145 | £400–£1,000 (full window) |
| Small sealed unit (whole window) | £100–£200 | £400–£1,000 |
| Standard sealed unit (whole window) | £200–£325 | £400–£1,000 |
| Large or bay window sealed unit | £350–£850 | £400–£1,000+ |
| Reseal (draught around the edge) | £50–£100 | N/A |
| Replace a handle | £45–£150 | N/A |
| Replace hinges | £100–£250 | N/A |
| Replace the uPVC frame only | £250–£750 | £400–£1,000 (full window) |
Source: DGCC 2026 dataset. Repair costs assume a sound existing frame. Full replacement is a fitted uPVC casement including FENSA certificate.
The cost gap
The arithmetic strongly favours repair wherever the frame is still in good shape. Swapping out a single blown pane for £55 to £145 costs roughly one-tenth of a full window replacement. Even a large sealed unit in a bay window, at £350 to £850, is only just reaching the lower end of what a full uPVC casement replacement costs. Resealing a draught around the frame is cheaper still at £50 to £100 per window. You can see the full breakdown of what drives window costs on the costs overview page.
The cost gap only closes when the frame itself needs attention. Replacing a uPVC frame alone runs £250 to £750, which starts to approach the price of a complete new window at £400 to £1,000 fitted. At that point it is worth pricing both options properly before committing.
Best Buy Repair for the cost dimension, by a wide margin, as long as the frame is structurally sound.
When a repair makes sense
Repair is the right call in three clear situations. First, when the fault is isolated: a single misted unit, a broken handle costing £45 to £150, or hinges that need replacing at £100 to £250. These are maintenance jobs, not a signal that the window is failing. Second, when the frame is less than 15 years old and still sealing well. A uPVC frame in good condition has years of useful life left, and fitting a new sealed unit inside it restores full performance for a fraction of the replacement cost. Third, when only one or two units on the whole house have blown. Spot failures at this age are normal and do not mean all the other windows are about to go.
The urgency to repair matters too. A failed unit can raise that window's heat loss by around 50 to 70%, so a blown unit is not just cosmetically annoying. That milky condensation between the panes is a sign that the insulating gas has escaped and the window is working significantly harder to hold heat in. For more on what causes condensation and what it means for your glazing, our condensation guide explains the difference between surface condensation (often harmless) and inter-pane condensation (always a failed seal).
Best Buy Repair when the frame is sound and the fault is isolated to one or two units or fittings.
When to replace
Replacement becomes the right answer when the frame itself is the problem, or when the economics of repeated repairs no longer stack up. A uPVC frame that is warping, cracking, or no longer holding its seal cannot be resealed effectively. Once the frame has gone, any sealed unit you fit inside it will fail faster than its normal 15 to 20 year life because it cannot seat properly.
Age is the other deciding factor. Sealed glass units typically last 15 to 20 years before misting; uPVC frames last 20 to 25 years. If your windows are already 18 to 20 years old and one unit has blown, the others are statistically close behind. Fitting a new sealed unit in an ageing frame that will need full replacement in two or three years rarely makes financial sense. Similarly, if several units have blown at once, that is a sign the whole batch of sealed units is at end of life and wholesale replacement is more cost-effective than a series of individual repairs. See our guide to double glazing lifespan for a fuller picture of how the frame and glass age at different rates.
Worth it Replace when the frame is failing, several units have blown, or the windows are within a few years of the end of their expected life.
The frame test
Before calling anyone out, run a quick frame check yourself. Close each window and press firmly against the pane at the edges. If you feel flex or hear a creak, the frame is distorting. Look at the corners where the frame sections are welded together: gaps or visible cracks mean the frame has failed. Run your hand slowly around the edge of the closed window on a cold day. A cold draught means the frame is no longer sealing against the outer frame (the outer frame is the fixed piece screwed into the brickwork). A draught could mean the rubber gaskets have perished, which is repairable for £50 to £100, or it could mean the frame itself is warped, which is not.
If the frame passes, look at the glass. Milky or greasy condensation permanently between the two panes is a failed seal in the sealed unit. The frame can stay; only the glass unit needs replacing. Surface condensation on the inside of the glass is a ventilation problem, not a glazing one, and replacing the glass will not fix it.
It depends The frame test result is the single most important factor. Sound frame means repair; failed frame means replace.
Guarantees and FENSA
Full window replacements by a registered installer are notified to FENSA or CERTASS, the two main competent person schemes for replacement glazing. That notification serves as your Building Regulations compliance certificate, which you will need when you sell the property. Sealed unit repairs are not notifiable under Building Regulations because they do not affect the structural opening, but a good installer will still provide a written guarantee on the new unit, typically two to five years. Ask for it in writing before the job starts.
If your windows were fitted by a FENSA-registered installer they may still be within the original guarantee period, typically ten years for the frame and five for the sealed units. Check your original paperwork before paying for a repair out of pocket. Our lifespan guide covers what typical guarantees include and what they exclude.
Worth it Full replacements provide FENSA certification and are worth it for the compliance paper trail when you come to sell.
The single biggest mistake I see is people paying to replace a perfectly good uPVC frame because one pane has gone misty. The glass unit is a consumable; the frame is not. If the frame is square, dry and sealing properly, put a new sealed unit in it and you are done for another 15 years. Save replacement for when the frame itself is the problem, not the glass.
Tom Bradley, FENSA-registered installer
The verdict
The decision comes down to one question: is the frame still good? Repair is the Best Buy when the frame is structurally sound and the fault is isolated, one misted unit (£55 to £145 for a single pane, or £200 to £325 for a standard sealed unit), a worn handle (£45 to £150), or stiff hinges (£100 to £250). In these cases you are paying for the failed component, not the whole window, and the saving over full replacement is substantial. Full replacement is Worth It when the uPVC frame itself is warping, cracking or no longer sealing properly (a frame-only replacement already runs £250 to £750, close enough to the £400 to £1,000 fitted cost of a complete new window to make the full job sensible), when several units have blown at the same time suggesting the whole batch is at the end of its 15 to 20 year sealed-unit life, or when the windows are already 20-plus years old and approaching the end of the 20 to 25 year uPVC frame lifespan. Use the repair cost calculator to price a repair and the uPVC window calculator to price a replacement, then compare the two figures against the age and condition of your frames. The gap will usually make the decision obvious. For a broader sense of what drives window costs, the costs overview has the full picture.
Frequently asked questions
Replacing the sealed unit inside an existing frame costs £55 to £145 for a single pane, or £200 to £325 for a standard-sized window. A large or bay window unit can reach £350 to £850. These figures assume the frame itself is sound and does not need replacing.
Replace when the uPVC frame itself is warping, cracking or no longer sealing properly. A failed frame cannot be resealed. Also replace when multiple units have blown at once, or when the windows are over 20 years old, as further failures are likely and repair costs will accumulate.
A sealed glass unit typically lasts 15 to 20 years before the gas filling escapes and moisture gets in, causing the milky condensation between the panes. The uPVC frame itself lasts 20 to 25 years. So a mid-life window often benefits from unit replacement; a window near its frame's end of life is better replaced whole.
Yes. Once the sealed unit fails and the insulating gas escapes, heat loss through that window rises sharply. A failed unit can raise the window's heat loss by around 50 to 70%, making it significantly less efficient than a sound unit until it is repaired or replaced.