If a double glazing company has just quoted you four thousand pounds to replace a single window because the glass has misted up, this article will save you most of that money. Repair is almost always the right answer - but not always. Here's how to tell the difference.
The "must replace" myth
The double glazing industry has spent forty years convincing UK homeowners that a misty pane means a knackered window. It doesn't. The frame, the hinges, the locks, the trickle vents and the cill are almost always perfectly fine. The bit that fails - the sealed unit between the two panes of glass - is a serviceable component that any competent glazier can swap out in under an hour, leaving the rest of the window completely untouched.
A full window replacement costs between £600 and £1,800 per window depending on size and frame type. A sealed unit replacement on the same window typically costs between £90 and £220. The work involved is roughly 5% of the disruption of a full replacement. So before any "free quote" salesman rips the entire frame out of your wall, ask: can this be repaired instead?
Almost always worth repairing
Repair is the right call in nearly all of these scenarios:
- Misty, foggy or condensated panes - replace the sealed unit, keep the frame. Saves £500-£1,500 per window.
- Cracked or chipped glass with the frame intact - sealed unit swap, sometimes single-pane swap on stained glass.
- Stiff, dropped or noisy handles - hardware swap, £30-£80 fitted.
- Failed window locks - new lock mechanism in the existing frame, £60-£150.
- Hinges that have rusted or sagged - new friction stays, £40-£90 per window.
- Draughts around the glass - re-gasket the unit or fit new glazing tape, usually under £60.
- Whistling or rattling in wind - usually a worn gasket or hinge wedge, cheap fix.
The borderline cases
These need an honest assessment - sometimes repair, sometimes replace:
- Frame discolouration (yellowed or chalky PVCu). If structurally sound the frame can be cleaned with PVC restorer or, in stubborn cases, sprayed by a professional. Replacement isn't required for cosmetic reasons.
- One bad window in an otherwise fine set. If the frame is warped, the corners have split, or it's been jemmied during a break-in, replacing that one window is reasonable. Don't let anyone talk you into "doing them all while we're here".
- Old timber frames with rot. Small patches of rot are repairable with epoxy filler and re-painting. Large structural rot at the cill or jamb usually means replacement.
- Older 4-12-4 units in a draughty house. The sealed unit might still be intact, but a modern 4-16-4 argon-filled Low-E unit will perform far better. Either is a valid choice.
When replacement genuinely is the answer
- The frame is warped, twisted or no longer closing flush. Once a PVCu frame distorts it cannot be straightened - the welded corners will fail.
- Visible cracks running through the welded corners. Structural failure of the frame itself.
- Water penetration through the frame (not the seal) - usually means the cill or fixings have failed and there's wider damage behind.
- Single glazing in a habitable room - repair is rarely worth it, the thermal upgrade alone pays back in heating savings.
- Frames over about 30 years old in poor condition - rubber gaskets perished, drainage holes blocked, hinges seized, beading brittle. At this point you're chasing failures from window to window.
Cost comparison (typical 2026 Essex prices)
For a standard 600mm x 900mm casement window:
- Sealed unit replacement (4mm-16-4mm argon Low-E): £110-£160 supply and fit
- New handle: £30-£55 fitted
- New friction stays (hinges): £45-£75 per side fitted
- New full window (white PVCu): £650-£950 supply and fit
- New full window (anthracite grey or woodgrain PVCu): £800-£1,200
- New full window (aluminium): £1,100-£1,800
In rough terms: you can re-glaze every window in an average 3-bed semi for about the cost of replacing two of them.
Decision in 60 seconds
Ask yourself three questions:
- Does the frame open and close cleanly with no scraping or sticking?
- Are the welded corners solid (no visible cracks or splits)?
- Is the issue limited to the glass, gasket, handle, lock or hinge?
Three yeses means repair. Two yeses is worth getting an honest opinion. Less than that and you're probably looking at replacement - but get a second quote before you sign anything.