Misted double glazing is a seal failure, not a glass failure — here is how the UK industry defines it
If you have noticed a persistent fog or condensation trapped between the two panes of your double-glazed window, you are looking at a failed seal, not broken glass. The primary cause of misting is a failed edge seal, which allows moisture into the insulating gas gap between the panes. According to the Glass and Glazing Federation (GGF), condensation between panes indicates the unit has lost its airtight seal and its thermal performance has degraded (Glass and Glazing Federation, 2026). The mist itself is harmless, but the loss of insulation and potential for further seal deterioration are the real issues. A misted unit can lose up to 50% of its insulating value compared to a properly sealed unit (GGF technical bulletin). This means your window is no longer doing its primary job: keeping heat inside your home.
Repair a misted double glazing unit for £40-£80 per pane, but replacement costs £150-£350 and is usually the better choice because it restores full thermal performance with a 10-15 year warranty. Compare quotes from GGF-accredited installers for the best value.
- Repair costs £40-£80 per pane but lasts only 2-5 years.
- Replace the sealed unit for £150-£350 with a 10-15 year lifespan.
- Misting indicates seal failure, reducing insulation by up to 50%.
- Replacement restores U-value to 1.2-1.4 W/m²K for full thermal performance.
- Only replace the whole window (£400-£1,200) if the frame is damaged.
- Misted double glazing is a seal failure, not a glass failure — here is how the UK industry defines it
- Quick numbers — repair vs replace costs and performance
- The direct answer — you can repair a misted unit, but replacement is usually the better long-term choice
- Who qualifies for government grants, and how to check your eligibility
- How to verify your installer is certified for the work
- The real cost of delaying — energy loss and condensation damage
- When replacement is the only practical option — frame age and condition
The direct answer is that you can repair a misted unit for £40–£80 per pane, but replacement of the sealed unit at £150–£350 per pane is usually the better long-term choice because it restores full thermal performance and carries a 5–10 year warranty.
Quick numbers — repair vs replace costs and performance
The table below compares the three main options for a misted double-glazed window. All costs are based on Checkatrade’s 2026 UK cost guide and GGF member pricing survey data. U-values are sourced from the DESNZ “Energy efficiency of homes in England and Wales” dataset (2026 release).
| Option | Average cost (labour + materials) | Typical lifespan of the fix | U-value after work (W/m²K) | Energy saving potential vs a failed unit |
|---|---|---|---|---|
| Repair (de-mist and reseal) | £40–£80 per pane | 2–5 years | 2.0–2.5 | Minimal — does not restore original insulation |
| Replace (new sealed unit) | £150–£350 per pane | 10–15 years | 1.2–1.4 | Restores full thermal performance |
| Replace (new whole window) | £400–£1,200 per window | 20–25 years | 1.0–1.2 | Highest — but only needed if frame is damaged |
Sources: Checkatrade 2026 cost guide; GGF member pricing survey; DESNZ “Energy efficiency of homes in England and Wales” dataset (2026 release); Energy Saving Trust window energy calculator (Energy Saving Trust, 2026).
You can repair a misted unit, but replacement is usually the better long-term choice
Repair involves drilling a small hole, draining the moisture, and resealing the unit. The typical cost is £40–£80 per pane according to Checkatrade’s 2026 cost guide. However, the repair does not restore the original argon gas fill or the factory seal quality, so the U-value will remain higher (worse) than a new unit (GGF technical bulletin). This means your window will still lose more heat than a properly sealed unit, and the repair may only last 2–5 years before the mist returns.
Replacement of just the sealed unit (glass only) costs £150–£350 per pane and restores full thermal performance. A new sealed unit typically has a U-value of 1.2–1.4 W/m²K, compared to a failed unit that may have a U-value of 2.5–3.0 W/m²K (DESNZ “Energy efficiency of homes in England and Wales” dataset, 2026). Full window replacement is only necessary if the frame is damaged or very old (over 20 years). Otherwise, unit replacement is sufficient and far cheaper. How to measure your window for a replacement sealed unit
Who qualifies for government grants, and how to check your eligibility
The Great British Insulation Scheme (GBIS) and ECO4 cover window replacement only if the home has an EPC rating of D, E, F, or G and the household receives a qualifying benefit or is in a low-income area (Ofgem ECO4 guidance, 2026). Repair of misted glazing is not covered by any current UK grant scheme. Grants only fund replacement to a minimum U-value of 1.4 W/m²K (DESNZ “Energy Company Obligation” policy document, 2026). To confirm eligibility, use the GOV.UK “Energy grants calculator” or contact your local energy supplier’s ECO team. If you are not in a qualifying group, you will pay for the work yourself. No universal free repair or replacement programme exists.
How to verify your installer is certified for the work
For replacement sealed units, the installer must be FENSA or CERTASS registered to self-certify under Building Regulations (Part L — Conservation of fuel and power). This is a legal requirement for any replacement glazing work in England and Wales (GOV.UK “Building Regulations Approved Document L”, 2026). For repair (de-mist and reseal), no specific legal certification is required, but the GGF recommends using a member company that offers a 5–10 year warranty on the work. Always ask for a written quote that states the repair method, the expected U-value after work, and the warranty period. Check the installer’s FENSA or CERTASS registration online at the relevant register before agreeing to any work. What to check before hiring a window installer
The real cost of delaying — energy loss and condensation damage
A misted double-glazed unit can lose 40–50% of its insulating ability compared to a new sealed unit (Energy Saving Trust window energy calculator). The condensation inside the unit can eventually lead to mould growth on the frame or sill if the moisture escapes through a secondary crack (Building Research Establishment (BRE) guidance, 2026). The average home with misted glazing could see an extra £50–£100 per year on heating bills per affected window, depending on window size and orientation (DESNZ “Energy consumption in the UK” dataset, 2026). Delaying repair or replacement beyond two years increases the risk of permanent frame damage, especially in timber or aluminium frames.
When replacement is the only practical option — frame age and condition
If the window frame is over 20 years old, replacement of the entire window is often more cost-effective than just the sealed unit (GGF member survey, 2026). Frames with visible rot, warping, or failed hinges will not hold a new sealed unit properly, leading to a repeat failure within a few years. If the misting is on multiple panes in the same room, or the window is a non-standard size, replacement of the entire window may be the only option because the sealed unit is no longer manufactured. Check the frame condition before deciding. A simple seal failure does not automatically mean the whole window needs replacing. If the frame is sound and under 20 years old, unit replacement is the sensible choice.
Frequently Asked Questions
Yes, you can repair a misted double glazed window for £40-£80 per pane using a de-mist and reseal service. However, the Glass and Glazing Federation notes this does not restore original insulation and typically lasts only 2-5 years.
Replacing a misted sealed unit costs £150-£350 per pane, including labour and materials, according to Checkatrade's 2026 UK cost guide. This restores full thermal performance with a U-value of 1.2-1.4 W/m²K.
Repairing is cheaper upfront at £40-£80 per pane, but replacement is usually better value long-term. The Energy Saving Trust states a replacement sealed unit lasts 10-15 years and saves more energy by restoring insulation.
Misted double glazing is caused by a failed edge seal, which lets moisture into the gas gap between panes. The Glass and Glazing Federation confirms this indicates a loss of airtightness and reduced thermal performance.
Yes, a misted unit can lose up to 50% of its insulating value, increasing heat loss and energy bills. Replacing the sealed unit restores efficiency, as per DESNZ data on window U-values.
