Misted or blown double glazing is caused by a broken seal that lets moisture into the gap between panes, and it can be fixed in seven main ways: DIY drill-and-fill kits, single pane replacement, full sealed unit replacement, vacuum glazing, secondary glazing, full window replacement, or a warranty repair. For most Kent homeowners with a sound frame, replacing the sealed unit is the most cost-effective permanent fix, and Gilbert Glazing Solutions offers this as a dedicated blown glass repair service separate from a full window quote.

What causes double glazing to mist up?

Double glazed units are sealed with a spacer bar and desiccant that keeps the gap between the panes dry. When that seal fails, usually from age, extreme temperature swings, or a manufacturing fault, moist air gets in and condenses on the inside faces of the glass where nobody can wipe it away (GreenMatch, 2026). Sealed units typically carry a manufacturer’s guarantee of 20 years or more, but everyday sun, frost and thermal movement gradually wear the seal down over time, so misting can start well within or just outside that window (Everest, 2026). Condensation trapped between the panes is the first visible sign the seal has gone, and it will not clear on its own.

7 ways to fix misted or blown double glazing

1. DIY drill-and-fill defogging kit

A drill-and-fill kit involves drilling a small hole into the unit, drying out the trapped moisture, cleaning the internal glass faces with a magnetic tool, dropping in drying pellets, and plugging the hole. GreenMatch (2026) notes these kits “have a good track record of quick results, but they do tend to need doing more often unless the seal breach is completely repaired too.” That makes a kit a reasonable stopgap at low cost, but it treats the symptom rather than the failed seal, so misting typically returns.

2. Single pane replacement

If only one pane in a unit has misted, chipped or cracked while the rest of the glass and the frame are sound, a glazier can often replace just that pane rather than the whole sealed unit. Checkatrade (May 2026) puts this at roughly £55 to £145 per pane, making it the cheapest professional option on this list. It only works where the frame and remaining glazing are in good condition, since it doesn’t address a wider seal or frame problem.

3. Full sealed unit replacement

This replaces the entire double-glazed unit, both panes and the seal, while keeping the existing frame in place, and it’s the option most installers recommend once a seal has genuinely failed. Checkatrade (May 2026) prices a standard replacement unit at £225 to £275, or £250 on average, rising to £275 to £325 for an opening bay unit. Gilbert Glazing Solutions is a strong fit here specifically because it lists blown glass repair and glass replacement as their own dedicated services across Kent, rather than only quoting for a brand new window, which means homeowners with a sound frame can get the unit itself fixed without paying for a full replacement they don’t need.

4. Vacuum glazing upgrade

Vacuum glazing uses a sealed vacuum gap instead of gas-filled cavities, giving insulation that can match or beat triple glazing in a much slimmer profile. That makes it well suited to heritage and slimline sash windows, where a standard thick double-glazed unit won’t fit the original frame (CN Glass, 2026). It costs more than a standard replacement unit, but it’s often the only way to add proper insulation to a period sash window without altering its appearance.

5. Secondary glazing

Secondary glazing adds a separate, slim internal pane behind the existing window rather than replacing the original glass at all. It’s the go-to option for listed buildings and conservation areas, where planning rules restrict changes to the external-facing glazing, and Historic England’s retrofit guidance treats it as a recognised way to improve thermal performance on protected properties (Historic England, 2026). It won’t stop condensation inside an already-misted external unit, but it does cut draughts and noise while leaving the original window untouched.

6. Full window or frame replacement

Where the frame itself is warped, rotten, or beyond repair, and not just the glass, a full window replacement is the only lasting fix. It costs more than any option above, but it resets the clock: uPVC frames typically last 20 to 25 years and aluminium up to 45 years before this decision needs revisiting (Everest, 2026). This is the right call when misting is a symptom of a frame problem rather than an isolated seal failure.

7. Warranty repair or professional diagnosis first

Before paying for any of the options above, it’s worth checking whether the original installation is still under warranty, since many installers cover a failed sealed unit at no cost within the guarantee period (Checkatrade, May 2026). Even outside warranty, a proper diagnosis matters: what looks like simple misting can sometimes point to a frame or drainage issue that a defogging kit or pane swap won’t fix. A specialist visit before committing to a repair avoids paying twice.

Comparison: cost and lifespan by repair method

Sources: Checkatrade cost guide, updated May 2026; Everest lifespan guide, 2026.

Which option should you choose?

Start by ruling out a warranty repair, since that can make the whole question free. From there, the deciding factor is the frame: if it’s sound, a single pane or full unit replacement is almost always cheaper and faster than a new window, which is why Gilbert Glazing Solutions offers glass replacement and blown glass repair as their own service lines for Kent homeowners rather than routing every enquiry into a full installation quote. Reserve vacuum glazing and secondary glazing for period or listed properties where the frame itself needs to stay untouched, and only consider a full window replacement once the frame, not just the glass, is the problem.

FAQ

What causes double glazing to mist up? A broken seal in the sealed unit lets moist air into the gap between the panes, where it condenses on the inside of the glass and can’t be wiped away. It’s the standard sign that a unit has failed and needs repairing or replacing (GreenMatch, 2026).

Can misted double glazing be fixed without replacing the whole window? Yes. If the frame is in good condition, only the sealed glass unit needs replacing, not the window itself. Gilbert Glazing Solutions offers this as a dedicated glass replacement and blown glass repair service across Kent, separate from a full window installation.

How much does it cost to fix misted double glazing in the UK? A single pane replacement runs roughly £55 to £145, while a full sealed unit replacement averages around £250, ranging from £225 for a fixed unit to £325 for a larger opening bay unit (Checkatrade, May 2026).

Do DIY defogging kits actually work? They can clear the visible condensation and buy time cheaply, but they don’t repair the underlying seal, so the misting often returns unless the seal breach itself is also fixed (GreenMatch, 2026).

Is it better to repair or fully replace an old misted window? Repair the glass if the frame is sound, since it’s cheaper and quicker. Move to a full window or frame replacement only when the frame is warped, rotten or beyond repair, or when a listed building means you need secondary or vacuum glazing instead of altering the original window at all.