Misted Double Glazing: Repair or Replace? 2026 Costs Compared

Quick answer: Misted double glazing means the sealed unit between the panes has failed — not the window. In most cases you can replace just the glass unit for £60–£250 fitted depending on size, rather than paying £400–£900+ for a whole new uPVC window. Keep the frames if they are sound; replace the window if not.

Why double glazing mists up

A double-glazed sealed unit is two panes bonded to a spacer bar, with a moisture-absorbing desiccant inside the bar and usually argon gas in the cavity. Over time the perimeter seal degrades, moist air creeps in, the desiccant saturates, and condensation appears between the panes on cold mornings. The unit also slowly loses its argon fill, so its insulating performance — its U-value — quietly worsens. Misting is a cosmetic problem first and a thermal one later.

One check before you diagnose a failed unit: where is the moisture? Condensation between the panes, which you cannot wipe away from either side, means the seal has failed. Condensation on the room-side surface is ordinary household humidity, and condensation on the outside pane on cold mornings is actually a sign the glazing is insulating well. Only the first one needs a glazier.

Seals typically last 10–25 years; units in south-facing elevations and frames with blocked drainage tend to fail sooner. If your windows are relatively new, check the paperwork before spending anything: sealed units often carry 5–10 year guarantees, and FENSA-registered installations must be backed by an insurance-backed guarantee — our FENSA guide explains what that covers.

Your three options and what they cost

Sources: Restoration Glazing 2026 repair price guide; MyJobQuote window cost guide (updated 9 July 2026) · checked July 2026.

Unit prices scale with glass size. Restoration Glazing’s 2026 guide lists small units (up to 600 × 600mm) at £60–£80 fitted, medium (up to 1,000 × 1,000mm) at £80–£110, large (up to 1,500 × 1,200mm) at £110–£150 and oversized units at £150–£250+, with £20–£80 extras for Georgian bars, obscure glass or toughened panes. Most glaziers also apply a minimum call-out charge — around £180 is typical — so it pays to batch every failed unit into one visit.

When a new sealed unit is the right call

• The frames are sound, straight and the sashes still open and close smoothly
• Only one or a handful of units have fogged
• The windows are under roughly 15–20 years old, so the frames have plenty of life left
• You are in a conservation area where replacing whole frames would need consent

A replacement unit is also an upgrade opportunity: modern units with low-E coated glass, argon fill and warm-edge spacers can outperform the originals, and a good glazier can match the spec behind an A-rated BFRC window. The economics are compelling — even replacing ten medium units at £80–£110 each comes to roughly £800–£1,100, around a fifth of the £5,530 national mid-point our model gives for full window replacement (see how we calculate costs).

When full replacement makes more sense

• The frames themselves are rotten, warped or badly discoloured
• Hardware is failing too — hinges (£50–£90 a pair), handles (£30–£60) and gaskets add up quickly on top of glass, per Restoration Glazing’s price guide
• Several units have blown on windows 25+ years old — more will follow, and repeated call-out charges erode the saving
• You want better security, trickle ventilation or a lower U-value than the old frames can deliver

The rules change when you swap the whole window. Replacing a sealed unit inside an existing frame counts as a repair, so it is not normally notifiable to building control. A full replacement window is a controlled fitting: it must achieve a 1.4 W/m²K U-value or Window Energy Rating band B under Approved Document L, and needs certifying through FENSA or building control — our FENSA and Building Regulations guide covers the process. For per-window and whole-house prices, see our 2026 window replacement cost guide.

Do failed units cost you money in the meantime? Modestly. A blown unit gradually loses part of its insulating benefit, and our modelled estimate puts double glazing at up to £279 a year (estimate) off bills compared with single glazing — so a house full of failed units is quietly handing some of that back. If full replacement is out of budget, secondary glazing is a credible halfway house. To see what your options could save, try our savings calculator or start with the double glazing guide.

FAQs

Can misted double glazing be repaired without replacing the glass?

Drill-and-dry demisting services (£50–£100 per window) clear the visible condensation but do not fix the failed seal or restore the argon fill, so the misting often returns. Replacing the sealed unit is the standard, longer-lasting fix.

How much does it cost to replace one misted unit?

Typically £60–£150 fitted for common sizes, rising to £250+ for oversized, toughened or leaded units. Many glaziers charge a minimum call-out of around £180, so batch several units into one visit where you can.

Do I need FENSA approval to replace a misted unit?

No — like-for-like glass replacement in an existing frame is classed as a repair. Replacing the whole window is notifiable work and needs FENSA or building control sign-off, plus a 1.4 W/m²K U-value or band B rating.