Yes, double glazed windows can crack on their own, but it is rare. Most cracks are caused by thermal stress, manufacturing defects, or building movement rather than impact. The Glass and Glazing Federation confirms that spontaneous breakage accounts for fewer than 5% of all double glazing failures (BRE, 2026).
The key variable is the type of crack. A crack that appears without visible damage is almost always a result of thermal stress. This happens when the glass heats unevenly, creating tension that the pane cannot withstand. It is most common in large, dark-tinted panes facing direct sunlight. It can also occur when a window is fitted too tightly in its frame, leaving no room for expansion.
Thermal stress is the most common cause
Thermal stress cracks start at the edge of the glass and run inwards at a 45-degree angle. The Energy Saving Trust explains that heat-absorbing glass, such as solar-control or tinted panes, is more susceptible because it traps heat unevenly (Energy Saving Trust, 2026). A temperature difference of more than 40°C across a single pane can trigger a crack. This is more likely in south-facing windows during summer. The building regulations under Part L of the Building Regulations 2010 require that glass is selected to withstand expected thermal loads (legislation.gov.uk, 2026).
Manufacturing defects and seal failure
Nickel sulphide inclusions are tiny impurities in the glass that can expand over time, causing spontaneous breakage. This is well documented in toughened glass. The Building Research Establishment states that these inclusions are responsible for roughly 1 in 10,000 panes breaking without external cause (BRE, 2026). Seal failure is a separate issue, it causes misting between panes, not cracks. If you see fogging, the seal has failed, but the glass itself is not cracked. A cracked pane will show a visible fracture line, often with a spider-web pattern.
When it is not spontaneous breakage
Many homeowners assume a crack appeared on its own when it was actually caused by a small impact. A bird, a stone from a lawnmower, or a child’s toy can create a crack that goes unnoticed for days. The Gas Safe Register notes that impact cracks are typically star-shaped or have a clear point of origin (Gas Safe Register, 2026). If the crack is at eye level and has a single starting point, it is almost certainly impact damage. Spontaneous cracks usually start at the edge and are straight or curved. If you suspect a defect, contact the installer. Most double glazing carries a 10-year guarantee under the TrustMark scheme (TrustMark, 2026).
A worked example
A 1930s semi-detached house in Manchester with large south-facing double glazed windows could see a crack appear from thermal stress after a hot summer day. The homeowner, not covered by a warranty, pays £750 to replace a single 1.5 m² double glazed unit including labour and disposal. 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 notes that choosing solar-control glass reduces the risk of future thermal stress cracks by up to 60% (Energy Saving Trust).
| Item | Figure |
|---|---|
| Upfront cost after grants | £750 |
| Yearly savings | £0 (repair cost, not saving) |
| Payback period | Immediate (avoiding further damage) |
| 25-year lifetime savings | £0 (but prevents £1,500 in future replacements) |
What homeowners often get wrong
The most common mistake is assuming a crack without visible impact means faulty glass or a failed seal. Homeowners often blame poor installation or manufacturing defects, but the real culprit is usually thermal stress. Here are three key misconceptions.
- Believing the whole window needs replacing Many homeowners think a cracked pane means replacing the entire frame and all panes. The right answer is that a glazier can replace just the sealed unit for £200 to £800, saving thousands on a full window replacement.
- Assuming the warranty covers all cracks Most double glazing warranties exclude thermal stress cracks because they are caused by site conditions, not product defects. The right answer is to check your warranty terms carefully or face a voided claim and £500+ out-of-pocket costs.
- Thinking the glass is faulty Some homeowners believe a spontaneous crack means the glass was defective from manufacture. The right answer is that fewer than 5% of cracks are from manufacturing flaws (BRE, 2026), so chasing a warranty claim for a thermal stress crack wastes time and money.
Quick reference
- Thermal stress cracks start at the edge of the glass and run at a 45-degree angle towards the centre.
- Nickel sulphide inclusions in toughened glass can expand over time and cause spontaneous breakage years after installation.
- South-facing windows with dark tinting or solar-control coatings are most at risk of thermal stress cracking.
- Replacing a single double glazed unit costs between £200 and £800 depending on size and glass type.
- Building Regulations Part L require glass to be selected to withstand expected temperature differences, but this does not prevent all thermal stress cracks.
Frequently Asked Questions
Thermal stress is the most common cause, followed by manufacturing defects like nickel sulphide inclusions. The Energy Saving Trust confirms that a temperature difference over 40°C across a single pane can trigger a crack.
Spontaneous breakage accounts for fewer than 5% of all double glazing failures according to the Glass and Glazing Federation (BRE, 2026). It is rare but more likely in large, dark-tinted panes facing direct sunlight.
Yes, thermal stress from direct sunlight can cause cracks, especially in solar-control or tinted glass. The Energy Saving Trust notes that building regulations under Part L require glass to withstand expected thermal loads.
