Yes, double glazing does lose its effectiveness over time, with the sealed unit typically lasting 20 to 25 years before noticeable performance drops occur, according to the Energy Saving Trust (Energy Saving Trust, 2026). The primary cause is the gradual failure of the gas seal between the panes, which reduces thermal insulation.
The rate of decline depends on the quality of the installation, the type of gas used (usually argon), and the exposure to weather. Most modern double glazing starts with a U-value around 1.2 to 1.6 W/m²K. When the seal fails, the U-value can rise to 2.8 W/m²K or higher, making the window perform little better than single glazing. You cannot reverse this loss, once the seal is broken, the unit needs replacing.
Seal failure is the main cause of lost effectiveness
The most common sign of seal failure is condensation forming between the glass panes. This misting or fogging indicates that the inert gas (such as argon) has leaked out and moisture has entered, as explained by the Glass and Glazing Federation (GGF, 2026). A failed seal reduces the window’s insulating ability by up to 50%, increasing heat loss and your heating bills. If you see persistent condensation inside the sealed unit, the entire pane assembly must be replaced. Replacing just the glass unit costs roughly £150 to £300 per window, depending on size and frame type.
Frame and hardware degrade independently of the glass
While the glass unit loses effectiveness mainly through seal failure, the frame and opening mechanisms also wear out. uPVC frames may become brittle or warp after 20 to 30 years. This allows draughts that bypass the glazing, as noted by the Fenestration Self-Assessment Scheme (FENSA, 2026). Timber frames require regular painting and can rot if neglected. Draughts around the frame can account for up to 15% of heat loss from a home, so replacing worn seals and hinges restores some performance even if the glass unit is still intact.
Annual checks can catch early loss of performance
Simple inspections help you spot declining effectiveness before it costs you money. Check for condensation between panes, feel for draughts around the frame, and look for cracked or damaged putty. The Energy Saving Trust recommends inspecting window seals each autumn (Energy Saving Trust, 2026). If your windows are over 20 years old, replacing them with modern double or triple glazing (U-value 0.8 to 1.2 W/m²K) can cut heat loss by up to 40%, saving around £165 per year on energy bills for a typical semi-detached house.
A worked example
A typical 1930s semi-detached house in Manchester with eight standard double-glazed windows will see its sealed units lose effectiveness after around 22 years, costing an extra £210 per year in heating bills. According to the Energy Saving Trust, the original argon gas fill degrades gradually, and once the seal fails the U-value can climb from 1.4 W/m²K to 2.8 W/m²K or higher.Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). If your household income is under £31,000 you may qualify for ECO4 funding to cover part of the cost. The table below shows the financial picture for this typical upgrade scenario.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,800 |
| Yearly savings | £210 |
| Payback period | 8.6 years |
| 25-year lifetime savings | £3,450 |
What homeowners often get wrong
The most common mistake is believing that misting between the panes is just cosmetic and can be ignored. Here are three critical errors homeowners make with ageing double glazing.
- Thinking misting is just a cleaning issue Condensation inside the sealed unit means the gas seal has failed and the insulating argon has escaped. Leaving it unrepaired can double your heat loss through that window, adding £30 to £50 per window per year to your heating bill.
- Believing you need to replace the whole window frame You only need to replace the sealed glass unit, which costs £150 to £300 per window rather than £500 to £1,000 for a full frame replacement. This keeps the job simple and avoids unnecessary waste going to landfill.
- Assuming new windows are always better than repairing old ones A 2023 Energy Saving Trust report found that replacing perfectly functional frames just because the glass has misted wastes thousands of pounds. Replacing only the sealed unit preserves the original frame and is often covered by a 10-year warranty from the installer.
Quick reference
- Double glazing loses its effectiveness after 20 to 25 years, with seal failure being the primary cause of performance drop.
- A failed seal can raise the U-value from 1.4 W/m²K to over 2.8 W/m²K, making the window perform little better than single glazing.
- Replacing just the sealed glass unit costs £150 to £300 per window, not the £500 to £1,000 for a full frame replacement.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- Condensation between the panes is not a cleaning issue it is a sign of seal failure that requires immediate replacement to avoid higher heating bills.
Frequently Asked Questions
Double glazing typically lasts 20 to 25 years before losing effectiveness, according to the Energy Saving Trust. After this, the gas seal often fails and insulation drops significantly.
The main cause is seal failure, which lets inert gas escape and moisture enter, as explained by the Glass and Glazing Federation. This reduces the window's insulating ability by up to 50%.
No, you cannot reverse the loss once the seal is broken. You must replace the sealed unit, which costs roughly £150 to £300 per window, according to the Energy Saving Trust.
