New double glazing in the UK should last between 20 and 25 years before performance drops significantly, according to the Glass and Glazing Federation (GOV.UK, 2026). The sealed unit’s lifespan depends on installation quality and maintenance, not just the frame material.
The key variable is the integrity of the sealed gas-filled cavity between the two panes. Once the seal fails, the insulating argon gas escapes and condensation forms inside the unit. This reduces thermal efficiency, and the window effectively becomes single glazing. The frame itself, uPVC, timber, or aluminium, can last 30 to 40 years, but the sealed unit typically needs replacement after two decades.
Seal failure is the main reason for replacement
The average double-glazed unit fails when the edge seal degrades. This allows moisture and air ingress. The Energy Saving Trust notes that a failed seal reduces the U-value from around 1.2 W/m²K to over 3.0 W/m²K, roughly doubling heat loss through the window (Energy Saving Trust, 2026). You will notice misting between the panes or a cloudy appearance. This cannot be repaired, only the sealed unit can be replaced, which costs £150–£250 per window. If the frame is still sound, a unit replacement extends overall lifespan by another 20 years.
Installation quality sets the baseline lifespan
Windows fitted by a FENSA-registered or TrustMark-approved installer meet Building Regulations Part L, which sets minimum thermal performance. The UK government requires new glazing to achieve a U-value of 1.2 W/m²K or lower (FENSA, 2026). Poor installation, gaps, incorrect sealing, or inadequate drainage, can cause seal failure within five years. A correctly installed unit with a 20-year warranty from a MCSCertified manufacturer typically reaches 25 years. Avoid units with less than a 10-year insurance-backed guarantee.
Maintenance and location affect longevity
Coastal or high-exposure homes may see seal failure eight to ten years earlier than sheltered inland properties due to salt spray and wind-driven rain. The Building Research Establishment advises annual checks: clean drainage channels, lubricate hinges, and inspect gaskets for cracking (BRE, 2026). uPVC frames need repainting every 10 years; timber frames need treatment every 3–5 years. If you ignore maintenance, you can expect the sealed unit to fail within 15 years rather than 20. Replacing the sealed unit alone costs around £150–£200 and restores full performance.
A worked example
A typical 1930s semi-detached home in Manchester with 10 double-glazed windows will see the first sealed unit fail around year 18, with full replacement of all units needed by year 25. The homeowner replaces just the sealed units (not the frames) at a cost of £2,000 for all 10 windows, using a FENSA-registered installer. After replacement, the U-value drops from a failed 3.0 W/m²K back to 1.2 W/m²K, saving roughly £180 per year on heating bills according to the Energy Saving Trust. The payback period on the unit replacement is 11 years, and over the 25-year lifespan of the new units, total savings reach £4,500. The original uPVC frames, still in good condition, last the full 40 years without needing replacement.
| Item | Figure |
|---|---|
| Upfront cost after grants | £2,000 |
| Yearly savings | £180 |
| Payback period | 11 years |
| 25-year lifetime savings | £4,500 |
What homeowners often get wrong
The most common mistake is assuming misting between panes means the whole window must be replaced, including the frame. Here are the three biggest errors homeowners make with double glazing.
- Replacing the entire window when only the sealed unit fails Many homeowners spend £500–£800 per window on a full frame-and-glass replacement when the frame is still sound. The right answer is a sealed unit replacement costing £150–£250 per window, which saves £350–£550 per window and avoids unnecessary waste.
- Ignoring trickle vents and condensation management Homeowners often block trickle vents thinking they cause drafts, but this traps moisture and accelerates seal failure. The right answer is keeping vents open year-round to maintain airflow, which can extend the sealed unit lifespan by 3–5 years and prevent mould growth.
- Choosing the cheapest installer without checking certifications A non-certified installation voids the warranty and often causes seal failure within 5 years. The right answer is using a FENSA or CERTASS registered installer, which guarantees compliance with Building Regulations and provides a 10-year insurance-backed guarantee on the work.
Quick reference
- A double-glazed sealed unit typically lasts 20 to 25 years before the gas-filled cavity fails and misting appears.
- Replacing just the sealed unit costs £150 to £250 per window, compared to £500 to £800 for a full window replacement.
- Warranties from FENSA-registered installers must cover the sealed unit for at least 10 years under current Building Regulations.
- Failed seals double heat loss through the window, raising the U-value from 1.2 W/m²K to over 3.0 W/m²K.
- Misting between panes cannot be repaired and is the clear sign that the sealed unit needs replacing immediately to avoid further heat loss.
Frequently Asked Questions
20 to 25 years for the sealed unit, according to the Glass and Glazing Federation (GOV.UK, 2026). The frame can last 30–40 years if maintained.
Replace when you see condensation between panes or the U-value climbs above 3.0 W/m²K, as the Energy Saving Trust notes heat loss doubles.
Yes, replacing only the sealed unit costs £150–£250 per window and extends lifespan by 20 years, per FENSA guidelines.
