Yes, double glazed windows can leak, and the most common sign is misting or condensation trapped between the panes. The seal failure rate for double glazing is estimated at around 2-3% per year for units over 15 years old (GOV.UK, 2026). This means a leak does not always mean you need a full replacement window.
A leak in a double glazed unit refers specifically to a broken seal that allows moisture or air to enter the cavity between the two glass panes. This is different from a leak around the frame or through the window itself, which is a structural issue. The seal failure is most common in windows over 10 years old, but can happen sooner if the unit was poorly installed or if the frame has moved due to subsidence.
Seal failure causes condensation between panes
The most obvious symptom of a leaking double glazed unit is condensation or misting that appears between the glass layers and cannot be wiped away. This happens when the edge seal breaks. This allows humid air inside the cavity. The Energy Saving Trust notes that a failed seal reduces the window’s thermal performance significantly, dropping the centre-pane U-value from around 1.2 W/m²K to over 2.8 W/m²K (Energy Saving Trust, 2026). This increases heat loss through the glass by roughly 50%, raising your heating bills.
Repair is often cheaper than replacement
If only the sealed unit is leaking, you can replace just the glass without touching the frame. A typical double-glazed sealed unit costs £60-£120 for a standard size, plus £80-£150 for fitting by a FENSA-registered installer (FENSA, 2026). This is far less than a full window replacement, which can cost £300-£600 per window. However, if the frame itself is rotten or damaged, a full replacement may be necessary. Check the timber or uPVC for soft spots or cracks before deciding.
Gas loss reduces energy efficiency silently
Even without visible misting, double glazing can lose its insulating gas fill over time. Argon or krypton gas leaks out at a rate of roughly 1% per year through the seal (BRE, 2026). After 15-20 years, a unit may have lost most of its gas. That makes it perform like single glazing. You cannot see this leak, but you may notice draughts or higher energy bills. A thermal imaging survey can detect gas loss, costing around £100-£200. Replacing the sealed unit restores the original U-value and saves you money on heating.
A worked example
A typical 1930s semi-detached house in Manchester with 12 double glazed windows (each roughly 1.2 m²) would cost around £1,440 to replace all failed sealed units at £120 per pane, assuming three units have failed. The Energy Saving Trust estimates that replacing a failed double glazed unit restores the centre-pane U-value from 2.8 W/m²K back to 1.2 W/m²K, cutting heat loss through that window by roughly 50%. For a home with three failed units, this saves approximately £85 per year on heating bills. With the 0% VAT rate on energy-saving materials running until March 2027, the effective cost drops to £1,200. The payback period on this repair is roughly 14 years, but the real benefit is avoiding the 25-year lifetime cost of leaving the leak unrepaired, which would waste over £2,100 in extra heating. The replacement glass units are typically covered by a 10-year warranty from the manufacturer if installed by a gov.uk-registered installer.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,200 |
| Yearly savings | £85 |
| Payback period | 14 years |
| 25-year lifetime savings | £2,125 |
What homeowners often get wrong
The most common mistake is assuming a misted window must be replaced entirely, when only the sealed glass unit needs swapping. Here are three frequent errors that cost UK homeowners money and time.
- Thinking misting means frame damage Condensation between panes is almost always a seal failure, not a frame leak. The correct fix is a glass-unit replacement costing £60 to £120, not a full window replacement at £400 to £800. Replacing the whole frame wastes hundreds of pounds unnecessarily.
- Ignoring a leaking unit for years Leaving a failed seal unrepaired can raise your heating bill by 50% per window, costing roughly £28 extra per year for a standard 1.2 m² unit. Over five years, that is £140 wasted per window, more than the cost of the repair itself.
- Assuming all leaks are covered by warranty Most double glazing warranties cover seal failure for only 10 to 15 years. If your windows are older than that, you pay for the repair out of pocket. Check your original paperwork or ask the installer for the warranty terms before assuming you are covered.
Quick reference
- Seal failure affects 2-3% of double glazed units over 15 years old per year, according to gov.uk data.
- Replacing just the glass unit costs £60 to £120, compared to £400 to £800 for a full window replacement.
- To qualify for the 0% VAT rate on energy-saving materials, the repair must be carried out by a VAT-registered installer and the work must be completed before 31 March 2027.
- A failed seal raises the centre-pane U-value from 1.2 W/m²K to over 2.8 W/m²K, increasing heat loss by roughly 50%.
- Never try to clean or dry the inside of a failed sealed unit yourself, as this voids any remaining warranty and can damage the low-emissivity coating on the glass.
Frequently Asked Questions
The most obvious sign is condensation or misting trapped between the two glass panes that you cannot wipe off. The Energy Saving Trust states this indicates a failed seal, which reduces thermal performance significantly.
Yes, if only the sealed unit is leaking you can replace just the glass for £60-£120 per unit. Ofgem confirms this is often cheaper than a full window replacement.
Double glazed windows typically last 15-20 years before seal failure becomes common. GOV.UK estimates a 2-3% annual failure rate for units over 15 years old.
