How long should a double glazed unit last?

A well-installed double glazed unit should last between 20 and 25 years before the seal fails and condensation appears between the panes, according to the Energy Saving Trust (Energy Saving Trust, 2026). This lifespan assumes the unit is made with a warm-edge spacer bar and a low-emissivity coating, both standard in modern sealed units.

The actual lifespan depends heavily on the quality of the original installation and the local climate. A unit exposed to strong direct sunlight on a south-facing elevation may fail sooner due to thermal stress. Units in sheltered, north-facing locations often last longer. The guarantee period is a useful guide, most reputable installers offer a 10-year guarantee on the sealed unit itself, but the frame and hardware may have a separate warranty of 5 to 10 years.

Seal failure is the main reason for replacement

The most common cause of double glazing failure is a broken seal, which allows moisture-laden air to enter the cavity. Once the seal fails, condensation forms between the panes and cannot be wiped away. The unit has then lost its insulating gas, typically argon or krypton, which reduces its thermal performance. The Building Regulations Approved Document L requires replacement windows to have a centre-pane U-value of 1.2 W/m²K or lower (GOV.UK, 2026). A failed unit cannot meet this standard and should be replaced entirely, not repaired, because resealing is rarely cost-effective.

Visible damage shortens lifespan significantly

Physical damage such as cracks, chips or scratches on the glass surface can weaken the unit and allow moisture ingress. Even a small chip can cause the seal to fail within months. The Glass and Glazing Federation advises that any cracked or chipped pane should be replaced as soon as possible to prevent further damage to the frame and surrounding wall (GGF, 2026). If the damage is limited to one pane, a replacement sealed unit can be fitted without replacing the entire window frame, which typically costs £200 to £400 per window depending on size and glazing type.

Regular maintenance extends the life of the unit

Simple care can help a double glazed unit reach or exceed its expected 20-year lifespan. Keep the drainage slots at the bottom of the frame clear of debris so condensation can drain away. Wipe the glass and frame seals with a soft cloth and mild detergent every few months to prevent dirt from degrading the rubber gaskets. The Fenestration Self-Assessment Scheme (FENSA) recommends checking the perimeter seals annually for signs of cracking or shrinkage (FENSA, 2026). If you notice draughts or a sudden increase in condensation on the inside of the glass, the unit may be approaching the end of its life and should be inspected by a certified installer.

A worked example

A 1930s semi-detached house in Manchester with twelve double glazed units across its windows will need to budget roughly £2,400 to replace all failed units after 22 years of use. The original units were installed in 2002 and by 2024 five units showed seal failure with condensation between the panes. Each replacement unit costs around £200 including fitting and a 10-year warranty from the installer. The homeowner can reclaim the 0% VAT on energy-saving materials until March 2027 which saves £480 on the total bill. The new units use a warm-edge spacer bar and low-emissivity coating which the Energy Saving Trust says can cut heat loss through windows by up to 40% compared to the old failing units. The annual heating saving from replacing those five leaky units is roughly £85 per year according to Energy Saving Trust estimates for a semi-detached property.

What homeowners often get wrong

The most common mistake is mistaking condensation on the outside of the glass for a failed sealed unit which leads to unnecessary replacement costs of hundreds of pounds. Here are three errors that waste money and void warranties

1. Confusing external condensation with seal failure External condensation on the glass surface in the morning is normal and actually indicates the unit is working well by keeping heat inside. Replacing a perfectly good unit for this reason costs £200 per window with zero benefit.
2. Thinking you can reseal a failed unit cheaply Some companies offer seal repair services for £50 to £80 per unit but these fixes rarely last more than two years. The unit has already lost its insulating argon gas so a proper replacement at £200 per unit is the only way to restore the original U-value of 1.2 W/m²K.
3. Ignoring trickle vents when replacing units Building Regulations require replacement windows to have trickle vents for ventilation and failing to include them can void your home insurance. A single replacement unit without a vent costs £180 but adding one costs £210 and keeps your policy valid.

Quick reference

• A double glazed unit with a warm-edge spacer bar lasts 20 to 25 years before seal failure according to the Energy Saving Trust.
• Seal failure causes condensation between panes which cannot be wiped away and signals the loss of insulating argon or krypton gas.
• The BUS grant offers £7,500 towards heat pump installation but does not cover window replacement costs.
• Replacing a single failed unit costs roughly £200 including fitting and a 10-year warranty from a MCS-certified installer.
• External condensation on the glass surface in the morning is normal and does not indicate a failed sealed unit.