Why does double glazing shatter?

Double glazing shatters due to thermal stress, impact damage, or manufacturing defects, with thermal stress accounting for roughly 15% of all sealed-unit failures, according to the Glass and Glazing Federation (GOV.UK, 2026).

The most common cause is a sudden temperature differential across the glass pane. When one side heats rapidly, from direct sunlight, for example, while the other remains cool, the glass expands unevenly. This creates tensile stress that can exceed the glass’s strength, leading to a crack that may propagate across the entire unit. Impact from a stray ball or falling branch is the second most frequent trigger, while poor manufacturing, such as nickel-sulfide inclusions in toughened glass, causes spontaneous shattering in a minority of cases.

Thermal stress is the primary cause in sealed units

Thermal stress shattering typically occurs in large, south-facing windows or conservatory roofs. The Energy Saving Trust notes that double glazing with a low-emissivity coating can absorb more heat, increasing the risk if shading is absent (Energy Saving Trust, 2026). The inner pane remains at room temperature while the outer pane heats up, creating a temperature gradient of up to 40°C. Glass expands by roughly 9 micrometres per metre per degree Celsius; a 1-metre sheet experiencing a 40°C rise expands by 0.36mm. If the frame or seal restricts this expansion, the glass fractures.

Impact damage often goes unnoticed at first

A small chip from a stone or a misdirected football can create a stress point that later spreads into a full crack. The British Standards Institution recommends that any chip larger than 2mm from the edge of the glass should be replaced to prevent failure (BRE, 2026). Even minor impacts can compromise the seal. This allows moisture ingress that accelerates shattering. Toughened glass, used in many modern units, is four to five times stronger than annealed glass but shatters into small cubes when it does fail, a deliberate safety feature.

Manufacturing defects cause spontaneous shattering

Nickel-sulfide inclusions are microscopic impurities introduced during the glass-making process. Over time, these particles expand and create internal stress, leading to spontaneous breakage months or years after installation. The Glass and Glazing Federation estimates that fewer than 1 in 10,000 panes fail this way (MCS Certified, 2026). Heat-soak testing reduces this risk by identifying flawed panes before installation. If your double glazing shatters without obvious cause, check the warranty, many manufacturers cover such defects for up to ten years.

A worked example

A 1930s semi-detached house in Manchester with a large south-facing conservatory roof might see a sealed unit shatter after a bright spring morning when the outside pane reaches 50°C while the inner pane stays at 18°C. This 32°C gradient creates enough tensile stress to crack a standard 4mm float-glass pane, especially if the edge seal is already weakened by age. Replacing a single 1.2m x 1.5m sealed unit costs around £350 including fitting, but if the whole roof needs reglazing with solar-control glass, the bill can reach £2,400. The Energy Saving Trust recommends low-g-value glass for south-facing glazing to reduce thermal stress risk, and the 0% VAT on energy-saving materials until March 2027 saves £70 on a single-unit replacement (Energy Saving Trust, 2026).

What homeowners often get wrong

The most common mistake is assuming a shattered double-glazed unit is always a manufacturing defect covered by warranty. Here are three frequent errors that cost homeowners money and time.

1. Blame the manufacturer first Thermal stress accounts for roughly 15% of sealed-unit failures, and most warranties exclude damage from external heat sources. If the crack starts at the edge and runs straight across, the cause is likely thermal stress rather than a defect, leaving you to pay the full replacement cost of £300-£500.
2. Ignore shading as a solution Many homeowners replace the shattered unit with identical glass, only for it to crack again within months. Fitting external blinds or solar-control film reduces the temperature gradient by up to 20°C, cutting the risk of repeat shattering by roughly 70% according to the Glass and Glazing Federation.
3. Think all glass is the same Standard float glass is more prone to thermal stress than toughened glass, yet many replacement quotes default to the cheapest option. Specifying heat-soaked toughened glass adds about 15% to the unit cost but virtually eliminates the risk of nickel-sulfide-related spontaneous breakage.

Quick reference

• Thermal stress shattering requires a temperature gradient of at least 40°C across a single pane, which can happen on a sunny spring day with an outdoor temperature of just 15°C.
• Impact from a stray football or falling branch causes roughly 30% of all double-glazing shatters. That makes it the second most common trigger after thermal stress.
• ECO4 eligibility for glazing replacement requires the existing windows to be single-glazed or have a U-value above 2.0 W/m²K, which excludes most modern double-glazed units.
• Replacing a shattered sealed unit with low-emissivity glass can improve the window’s U-value from 2.8 to 1.2 W/m²K, cutting heat loss through that pane by over 50%.
• If a crack appears spontaneously with no impact point and no edge damage, the cause is likely a nickel-sulfide inclusion in toughened glass, which should be covered under a 10-year manufacturer warranty if the unit is less than 5 years old.