Double glazed windows typically need replacing every 20 to 25 years, though some high-quality units can last up to 30 years with proper maintenance, according to the Energy Saving Trust (Energy Saving Trust, 2026). The exact lifespan depends on installation quality, frame material, and exposure to weather.
The main reason for replacement is seal failure, which allows condensation between panes and reduces thermal efficiency. A failed seal lets argon gas escape and moisture in, dropping the U-value from around 1.6 W/m²K to over 3.0 W/m²K. You do not need to replace windows on a fixed schedule, inspect them every few years for signs of fogging, draughts, or increased energy bills.
Seal failure is the most common reason for replacement
Double glazing units are sealed at the edges with a spacer bar and butyl or silicone. Over time, thermal cycling and UV exposure degrade this seal. Once broken, moisture enters and creates permanent condensation between the panes. The Glass and Glazing Federation states that a sealed unit with internal condensation has failed and should be replaced (BRE, 2026). Replacing just the sealed unit (the glass) is often cheaper than fitting a whole new frame, costing around £150 to £300 per window compared to £400 to £800 for a full replacement.
Frame material affects replacement intervals
uPVC frames typically last 20 to 25 years before becoming brittle or warped. Timber frames need repainting every 5 to 7 years but can last 30 to 40 years with regular maintenance, as noted by the Energy Saving Trust (Energy Saving Trust, 2026). Aluminium frames are durable for 30 to 45 years but are less energy-efficient without thermal breaks. If your frames are rotting, rusting, or draughty, replacement is needed regardless of the glass condition.
Energy efficiency standards have changed since older installations
Building regulations now require new windows to achieve a U-value of 1.4 W/m²K or lower for replacement installations in England (GOV.UK, 2026). Windows installed before 2002 often have U-values above 3.0 W/m²K. Replacing old double glazing with modern A-rated units can cut heat loss by up to 40% and save around £100 to £150 per year on heating bills for a typical semi-detached home. If your current windows are over 20 years old and not meeting current standards, replacement can pay for itself within 10 to 15 years through energy savings.
A worked example
A typical 1930s semi-detached house in Manchester with 12 double glazed windows would cost roughly £4,800 to replace all sealed units after 22 years, based on the average £400 per window for a full frame and glass replacement. The original windows, installed in 2002, now show condensation between panes on four units and noticeable draughts around three frames. Replacing all 12 windows with modern A-rated double glazing (U-value 1.2 W/m²K) cuts heating bills by approximately £210 per year, according to the Energy Saving Trust. 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). With a £4,800 upfront cost and £210 annual savings, the payback period is roughly 23 years, but the real benefit is eliminating draughty rooms and condensation mould. Over a 25-year lifespan, total savings reach £5,250, exceeding the replacement cost.
| Item | Figure |
|---|---|
| Upfront cost after grants | £4,800 |
| Yearly savings | £210 |
| Payback period | 23 years |
| 25-year lifetime savings | £5,250 |
What homeowners often get wrong
The most common mistake is waiting for visible condensation before replacing double glazing, when seal failure often starts years earlier without obvious signs. Here are three frequent errors that cost homeowners money and comfort.
- Ignoring draughts and higher energy bills Many homeowners assume draughts come from the frame, not the failed seal. A seal leak lets warm air escape and cold air enter, raising heating costs by up to 15% per year. Replacing just the sealed unit early (£150–£300) prevents this waste.
- Replacing the entire window when only the glass unit has failed Full frame replacement costs £400–£800 per window, but a failed seal only needs the sealed unit replaced for £150–£300. Unnecessary full replacements waste £250–£500 per window and void any remaining frame warranty.
- Believing that all double glazing lasts the same 20 years Hardwood or uPVC frames in sheltered locations can last 30 years, while aluminium frames in coastal areas may fail after 15 years due to salt corrosion. Using the Energy Saving Trust’s window lifespan tool helps you plan replacements based on your specific home and location.
Quick reference
- Double glazed windows typically last 20 to 25 years before seal failure requires replacement, according to the Energy Saving Trust.
- Seal failure drops the U-value from around 1.6 W/m²K to over 3.0 W/m²K, increasing heat loss by nearly 50%.
- Replacing just the sealed unit costs £150 to £300 per window, compared to £400 to £800 for a full frame and glass 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).
- Inspecting windows every two years for fogging, draughts, or rising energy bills catches seal failure early and avoids wasted heat.
Frequently Asked Questions
Look for condensation between panes, draughts, or rising energy bills. The Glass and Glazing Federation says internal condensation means the seal has failed and replacement is needed.
Yes, high-quality units with good maintenance can last up to 30 years according to the Energy Saving Trust. Timber frames can last 30 to 40 years with regular repainting every 5 to 7 years.
Yes, replacing only the sealed unit costs £150 to £300 per window, while a full frame replacement runs £400 to £800. Fitting new glass is often sufficient if the frame is still sound.
