Misting between the panes is the single most common thing homeowners call us about, and it is also the job most often over-sold. A sealed unit that has failed does not automatically mean the window has failed. In most houses the frame, hinges and locks are perfectly serviceable and only the glass needs to come out.
That distinction matters because it is the difference between spending around £90 and spending around £600 on the same opening. This guide covers what a unit swap costs, when it is worth doing, and when the whole window genuinely does need to go.
What misting actually is
A double glazed unit is two panes of glass bonded around a spacer bar, with a desiccant inside that absorbs any residual moisture. The perimeter seal is what keeps the cavity dry. When that seal breaks down — through age, standing water in the frame drainage, or heat stress — damp air gets in, the desiccant saturates, and you get that grey haze or the visible tide mark that comes and goes with the weather.
Once it starts it does not recover. The various 'defogging' drill-and-dry treatments remove the visible symptom for a while, but the cavity is no longer sealed, so the insulating performance stays compromised.
Replacement unit costs
| Unit size | Standard A-rated | Toughened safety glass | Obscure / patterned |
|---|---|---|---|
| Small (up to 0.3m²) | £70 – £110 | £95 – £150 | £90 – £140 |
| Medium (0.3 – 0.8m²) | £95 – £170 | £130 – £220 | £120 – £200 |
| Large (0.8 – 1.5m²) | £150 – £260 | £200 – £330 | £190 – £300 |
| Very large / patio pane | £250 – £450 | £320 – £600 | n/a |
| Leaded or Georgian detail | £160 – £320 | £210 – £400 | n/a |
Pane replacement vs a whole new window
This is where 'replace the pane, not the frame' comes from. If the frame closes properly, seals cleanly and drains, there is very little reason to skip a unit swap.
| Replace the glass unit | Replace the whole window | |
|---|---|---|
| Typical cost | £95 – £260 | £450 – £900 |
| Time on site | 30 – 45 minutes | 2 – 4 hours |
| Mess and making good | Minimal | Internal and external finishing |
| Fixes misting | Yes | Yes |
| Fixes draughts and stiff hinges | No | Yes |
| Improves frame insulation | No | Yes |
| Sensible when | Frame and hardware are sound | Frame is rotten, warped or leaking |
How to check whether your frame is still sound
If everything passes, a unit swap is the right call. If two or three checks fail, spending on glass is money you will spend again.
- Open and close each sash — it should move without lifting, dropping or being shouldered shut.
- Look at the drainage slots along the bottom of the outer frame; they should be clear, not blocked with silicone or dirt.
- Press the gasket seals — soft and springy is fine, hard and shrunken is not.
- Check the corners of uPVC frames for welded joints opening up.
- On timber, push a key into the bottom rail; if it sinks in, that is rot and glass alone will not save it.
Is it worth claiming on a guarantee?
Sealed units are usually guaranteed for five to ten years, and the guarantee normally sits with the company that installed them. Dig out the paperwork before paying anyone — if the original installer is still trading and the unit is in date, you may only be liable for the labour.
Where the installer has gone under, an insurance-backed guarantee taken out at the time may still stand. If neither applies, a straightforward paid unit swap is rarely expensive enough to be worth a long fight.
What to expect on the day
A unit swap is a tidy job. The beading is taken off from inside, the failed unit is lifted out, the rebate is cleaned and re-packed with the correct glazing packers so the sash sits square, and the new unit goes back with the beads re-fitted. Half an hour to forty-five minutes per window is typical.
Measurements are usually taken on a first visit and the glass ordered to size, so most jobs are two short visits rather than one. If you are anywhere in Central Scotland — Cumbernauld, Glasgow, Falkirk, Stirling and around — we will tell you honestly on the survey whether it is a pane job or a window job. Call 07538 882579 if you would like it looked at.
Common questions
Drilling and drying clears the haze temporarily but leaves the cavity unsealed, so insulation performance stays poor and the misting usually returns. Replacing the unit is the proper repair.
Want a straight answer on your own windows or doors?
DM Installations fits and repairs windows and doors across Central Scotland — Cumbernauld, Glasgow, Falkirk, Stirling and the surrounding towns. Free survey, written quote, and an honest answer on whether you need a repair or a replacement.
Related guides
Condensation Between Glass Panes
The one type of window condensation you cannot fix with ventilation, and what to do about it.
Cost guidesHow Much Does Double Glazing Cost?
Typical supplied-and-fitted prices per window, whole-house costs by property size, and the things that quietly push a quote up or down.
When to call a proSigns You Need New Windows
A proper inspection checklist so you can tell a repair job from a replacement job before anyone quotes you.