How does double glazing break?

Double glazing breaks due to seal failure, thermal stress, or impact damage, with seal failure being the most common cause affecting around 20% of units within 10–15 years (BRE, 2026).

The answer depends on the type of damage. A cracked pane or foggy window signals different failure mechanisms. Most breaks are preventable with proper installation and maintenance, but age, weather, and manufacturing defects all play a role.

Seal failure causes condensation and fogging

Double glazing relies on a sealed air gap between two panes. When the edge seal degrades, often due to poor installation or UV exposure, moisture enters the cavity. The Energy Saving Trust states that failed seals cause visible condensation between panes, reducing thermal performance by up to 50% (Energy Saving Trust, 2026). This is the most common reason for replacement, typically occurring after 10–20 years.

Thermal stress cracks from temperature extremes

Sudden temperature changes can crack a pane. For example, direct sunlight on a cold winter day heats the outer glass faster than the inner pane, creating stress. The Glass and Glazing Federation notes that thermal stress cracks usually start at the edge and run straight across the glass (FENSA, 2026). This affects around 5% of double-glazed units in homes with large south-facing windows.

Impact damage from accidents or debris

Physical force from objects like footballs, garden tools, or storm debris can break a pane. Impact cracks typically radiate from the point of contact. TrustMark advises that toughened glass (safety glass) is required in low-level windows to reduce injury risk (TrustMark, 2026). If only one pane breaks, the entire sealed unit usually needs replacing because the gas fill escapes and the seal is compromised.

A worked example

For a typical 3-bedroom 1930s semi-detached house in Manchester, replacing 10 standard double-glazed units with A-rated argon-filled windows costs around £4,500 after the 0% VAT saving (in place until March 2027). The Energy Saving Trust estimates this upgrade cuts annual heating bills by £195 and reduces heat loss through windows by up to 50% (Energy Saving Trust, 2026). If your current windows have failed seals or fogging, the payback period is roughly 23 years on energy savings alone, but the real value comes from eliminating condensation damage, improving home comfort, and boosting property value by an estimated £3,000–£5,000. Most UK households are not eligible for the BUS grant or ECO4 for window replacements, though some ECO4 schemes may cover windows if the home also receives a primary heating measure.

What homeowners often get wrong

The most common mistake is assuming foggy windows always need full replacement. Many homeowners rush to replace units that can be repaired, wasting hundreds of pounds. Here are three frequent errors

1. Believing all condensation means seal failure External condensation on the outer pane is normal and indicates the windows are working efficiently by insulating your home. Internal condensation between panes is the real sign of a broken seal, and this reduces thermal efficiency by up to 50%, costing an extra £100 a year in heating.
2. Ignoring thermal stress cracks as a manufacturing defect A straight crack starting at the glass edge and running across the pane is usually caused by temperature extremes, not poor workmanship. If you claim this as a warranty issue, you risk voiding the warranty entirely, leaving you to pay £200–£400 per pane for replacement.
3. Thinking double glazing lasts forever The typical lifespan is 10–20 years before seal failure occurs, according to the BRE. Waiting beyond 25 years without inspection means you could be losing £195 a year in heat loss, and the windows may fail a FENSA inspection when you sell your home.

Quick reference

• Seal failure causes fogging in about 20% of double-glazed units within 10–15 years, reducing thermal performance by up to 50%.
• Thermal stress cracks affect roughly 5% of south-facing windows and typically run straight from the glass edge across the pane.
• You can claim replacement under a FENSA-certified installer’s warranty only if the crack or seal failure occurs within the first 10 years of installation.
• Repairing a single failed sealed unit costs £150–£300, versus £300–£600 for a full replacement, saving up to 50% on costs.
• Never use abrasive cleaners or pressure washers on double glazing, as these can damage the edge seal and void the manufacturer’s warranty.