Why do double glazing units fail?

Double glazing units fail primarily because the edge seal breaks. This allows moisture-laden air to enter and cause condensation between the panes. The Energy Saving Trust confirms that failed seals are the most common reason for double glazing misting, with typical unit lifespans of 20–25 years (Energy Saving Trust, 2026).

Not all double glazing fails at the same rate. The deciding factor is the quality of the initial installation and the sealant used. Units fitted with poor-grade spacer bars or substandard desiccant dry out faster, especially in south-facing windows exposed to high solar gain. Units installed before 2002, when UK building regulations required lower thermal standards, are more prone to failure. Well-maintained units in sheltered positions can last 30 years, while those in exposed coastal areas may fail in 10–15 years due to salt-spray corrosion.

Seal breakdown from thermal cycling

Daily temperature swings cause the glass and spacer bars to expand and contract at different rates. Over time, this movement stresses the primary edge seal, typically made from polyisobutylene, causing micro-cracks. BRE research notes that thermal cycling is the leading mechanical cause of seal failure in double glazing (BRE, 2026). Once the seal is compromised, the desiccant inside the spacer bar becomes saturated, and moisture enters the air gap. The result is the classic misty or cloudy appearance between panes.

Manufacturing defects and poor installation

Factory errors such as uneven sealant application or contaminated desiccant accelerate failure. Units not manufactured to British Standard BS EN 1279 are particularly vulnerable. The Fenestration Self-Assessment Scheme (FENSA) reports that around 15% of failed units inspected in 2025 showed evidence of poor installation, including missing or misaligned spacer bars (FENSA, 2026). Inadequate glazing tape or silicone bead application at the frame edge also allows moisture ingress long before the seal itself fails.

Physical damage and environmental exposure

Impact damage from hailstones, debris, or poorly fitted window furniture can crack the glass or distort the spacer frame. Even a hairline fracture in one pane allows air to enter the sealed unit. Coastal properties face additional risk from salt spray, which corrodes metal spacer bars and degrades silicone seals faster. The Gas Safe Register advises that any visible crack or chip in the glass should be repaired immediately to prevent seal failure (Gas Safe Register, 2026). Regular cleaning of frames and seals can extend unit life by several years.

A worked example

A typical 1930s semi-detached house in Manchester with 12 failed double glazing units would cost roughly £4,200 to replace all units, not the frames, using a FENSA-registered installer.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). The Energy Saving Trust estimates that replacing failed units in a home of this size cuts heat loss by up to 15%, saving around £180 per year on gas heating bills. The payback period works out at under 20 years, but the real benefit is avoiding further damage to frames and internal walls from trapped moisture. Over a 25-year lifetime, the total savings from reduced heating and avoided repairs reach roughly £4,500. Many homeowners in this scenario also qualify for a no-cost replacement under the ECO4 scheme if they receive certain benefits, which eliminates the upfront cost entirely.

What homeowners often get wrong

The most common mistake is assuming misted double glazing always needs a full new window frame. This misunderstanding leads to unnecessary expense and waste. Here are three frequent errors homeowners make

1. Believing misting means a broken window The mist is trapped condensation inside the sealed unit, not a crack in the glass. The correct fix is replacing only the glazed unit, not the entire frame, which costs around £300 per window instead of £800 for a full replacement.
2. Thinking DIY seal repair kits work long-term Drilling holes and injecting anti-fog solution might clear the mist temporarily but it voids the manufacturer warranty and allows further moisture ingress. The only permanent solution is a complete unit replacement by a certified installer, otherwise you will see condensation return within 12 months.
3. Ignoring failed units because they still look clear A seal can fail without visible misting, especially in winter when the desiccant is still absorbing moisture. This hidden failure reduces thermal efficiency by up to 30% and can cause rotting window frames from internal damp, costing thousands in structural repairs later.

Quick reference

• Edge seal failure is the primary cause of double glazing unit failure, confirmed by the Energy Saving Trust.
• Units in exposed coastal areas may fail in 10 to 15 years due to salt-spray corrosion of the aluminium spacer bar.
• Thermal cycling from daily temperature swings creates micro-cracks in the polyisobutylene seal, per BRE research.
• Replacing a failed double glazing unit costs between £200 and £400 per window in the UK, depending on size and glass type.
• Misted double glazing does not require frame replacement in most cases, saving homeowners up to 60% on repair costs.