Misting between the panes means your sealed unit has failed, here is what that costs and how to fix it
If you see mist or fog trapped between the two panes of glass in your double glazing, the airtight seal has failed. That seal is what holds the insulating gas (usually argon) in the gap, and once it breaks, moisture from the outside air seeps in and condenses inside the cavity. The result is a permanent mist that cannot be wiped away.
The failed seal means your window no longer insulates as it should. A standard double-glazed unit with a working seal has a U-value (a measure of heat loss) of around 1.2–1.4 W/m²K. A failed unit with air and moisture in the gap can climb to 2.7–3.0 W/m²K, according to DESNZ standard double-glazing U-value tables (DESNZ, 2026). That is roughly equivalent to single glazing, so your heating bills will rise and the room will feel colder.
The only effective fix is to replace the sealed glazing unit, the glass and spacer bar, or the whole window if the frame is damaged or the unit cannot be removed. The Glass and Glazing Federation (GGF) states that misting between panes is a clear sign of seal failure and cannot be repaired with sealants or DIY kits (GGF, 2026).
How to tell misting between panes from condensation on the surface
Misting between the panes sits inside the sealed cavity, so you cannot wipe it away from either the inside or the outside of the glass. Surface condensation, by contrast, forms on the room-side glass when warm, moist air hits a cold surface, and it clears with a cloth or a dehumidifier. External condensation on the outside pane in cold weather is normal and not a sign of failure, it happens when the glass is colder than the outside air and will clear as the day warms up.
To confirm the difference, inspect the window on a dry day. If the mist is still there and does not move when you wipe both sides, the seal is broken. The GGF technical bulletin on condensation types advises that internal cavity misting is a warranty issue, not a cleaning problem (GGF, 2026).
Quick numbers, typical costs, lifespan, and energy savings for replacement
| Item | Typical cost (GBP) | Typical U-value before failure | Typical U-value after replacement | Lifespan (years) | Source |
|---|---|---|---|---|---|
| Replacement sealed unit (glass only) | £150–£400 | 2.7–3.0 W/m²K | 1.2–1.4 W/m²K | 15–20 | Checkatrade 2026 |
| Whole new double-glazed window (frame + glass) | £400–£1,200 | 2.7–3.0 W/m²K | 1.2–1.4 W/m²K | 20–25 | Checkatrade 2026 |
| Average lifespan of a sealed unit (from new) | N/A | N/A | N/A | 15–20 | GGF, 2026 |
| Energy saving per window (annual) | N/A | N/A | Up to 20% reduction in heat loss per window | N/A | Energy Saving Trust, 2026 |
The direct answer to “double glazing misted between panes”, what you must do next
Inspect the window on a dry day. If the mist persists and cannot be wiped from either side, the seal is broken and you need a replacement. Contact a GGF-registered or FENSA-registered installer to measure the unit and give you a quote.
Replacing only the glass unit (the sealed unit) costs roughly £150–£400 per window in 2026, according to Checkatrade’s cost guide (Checkatrade, 2026). A full frame and glass replacement runs £400–£1,200 per window. The GGF consumer guide recommends getting at least two quotes and checking the installer’s registration before paying a deposit (GGF, 2026).
FENSA data for 2026 shows that unit-only replacements are the most common repair for failed seals, with an average installer charge of £250 per window including labour (FENSA, 2026).
How replacement costs break down, unit-only vs full window replacement
A unit-only replacement (new glass and spacer bar, kept in the existing frame) is cheaper because the frame stays in place. The unit cost itself ranges from £100 to £300 depending on size and glass specification. Labour adds £50–£150 per window, according to Checkatrade’s 2026 installer survey (Checkatrade, 2026).
Full window replacement is needed if the frame is damaged, rotting (common with old timber frames), or if the window is over 20 years old and the frame design is obsolete. FENSA notes that many older frames cannot accept modern sealed units because the rebate depth is too shallow (FENSA, 2026). Full replacement costs more but gives you a new frame, new seals, and a fresh warranty.
How to verify your installer is certified for the work
For double-glazing replacement, the installer must be registered with a competent person scheme such as FENSA, CERTASS, or BSI. This is a legal requirement under Building Regulations (Part L) for replacement windows and doors (GOV.UK, 2026). If the property is part of a renewable-heat scheme, the installer may also need MCS certification.
Check the installer’s FENSA number on the FENSA website or call the GGF consumer helpline. A FENSA certificate is required if you sell the house within two years of the repair, without it, the buyer’s solicitor may flag the windows as non-compliant (FENSA, 2026).
Will a warranty or insurance cover the repair?
Most new sealed units come with a 10-year manufacturer’s warranty against seal failure. Check your original paperwork, if the window is less than 10 years old and the installer is still trading, you may be able to claim a free replacement unit. The GGF consumer warranty guidance states that the warranty covers the sealed unit, not the frame, and excludes damage from impact or misuse (GGF, 2026).
Home insurance rarely covers seal failure unless it is caused by accidental damage, such as a stone hitting the glass. The Association of British Insurers (ABI) notes that gradual deterioration, including seal failure, is typically excluded from standard home insurance policies (ABI, 2026). If you have accidental damage cover, check your policy wording for exclusions on glass and glazing.
How long a replacement sealed unit should last and when to consider upgrading
A new sealed unit typically lasts 15–20 years before the seal degrades again, based on GGF technical data (GGF, 2026). If your existing windows have no Low-E coating (a microscopically thin metallic layer that reflects heat back into the room) and the frame is sound, consider upgrading to a Low-E argon-filled unit. These achieve a U-value of around 1.2 W/m²K, compared to 2.7–3.0 for an uncoated failed unit (DESNZ, 2026).
Upgrading to triple glazing for a single failed unit is rarely cost-effective. Triple glazing costs roughly twice as much as a standard double-glazed unit and saves only an additional 0.1–0.2 W/m²K in U-value, according to Energy Saving Trust guidance (Energy Saving Trust, 2026). It makes more financial sense if you are re-glazing the whole house at once.
