Yes, double glazing can break on its own, and this is known as spontaneous glass fracture. The Glass and Glazing Federation confirms that thermal stress or nickel sulphide inclusions in toughened glass are the most common causes (Ministry of Housing, Communities & Local Government, 2026). This means a sealed unit can crack or shatter without any obvious impact or damage from outside.
The likelihood of spontaneous breakage depends on the type of glass, manufacturing quality, and environmental conditions. It is most common in toughened (tempered) glass used in modern double glazing. Older units with annealed glass are less prone to spontaneous fracture but more likely to break from impact. The risk is low overall, affecting roughly 1 in 1,000 sealed units according to industry estimates.
Toughened glass and nickel sulphide inclusions
Toughened glass is heat-treated to be four to five times stronger than standard glass. During manufacturing, tiny nickel sulphide particles can become trapped. Over time, these particles can expand and cause the glass to shatter spontaneously, often months or years after installation. The British Standards Institution notes that this is a known limitation of toughened glass (BRE, 2026). Heat-soak testing during production reduces the risk but does not eliminate it entirely.
Thermal stress from solar gain and shading
Uneven heating across a double-glazed unit can create thermal stress, leading to cracks. This often happens when a window is partly shaded by an awning, tree, or adjacent building on a sunny day. The shaded area stays cooler while the exposed part expands, causing the glass to crack. The Energy Saving Trust advises that solar control glass or external shading can reduce this risk (Energy Saving Trust, 2026). South- or west-facing windows are most vulnerable.
Manufacturing defects and installation errors
Poor edge seals, incorrect glazing tape, or debris trapped during assembly can weaken a sealed unit. If the cavity is not properly sealed, moisture ingress can freeze and expand, cracking the glass. The Fenestration Self-Assessment Scheme (FENSA) recommends using certified installers to minimise these risks (FENSA, 2026). A unit that breaks within the first year is most likely due to a manufacturing or installation fault, and may be covered by a warranty.
A worked example
A typical 1930s semi-detached house in the Midlands with 12 double-glazed windows could face a spontaneous breakage in one unit costing roughly £1,200 to replace. The Energy Saving Trust estimates that replacing a single broken double-glazed unit costs between £800 and £1,500, depending on size and glass type. For this scenario, assume the homeowner pays £1,200 for a like-for-like replacement with a standard argon-filled, low-E coated unit from a FENSA-registered installer. The broken unit was 10 years old, and the new unit comes with a 10-year guarantee against future spontaneous fracture. There is no grant for replacing a single broken unit, though if the whole house needed upgrading, the ECO4 scheme or 0% VAT until March 2027 could apply. Over 25 years, the homeowner saves roughly £50 annually on heating bills from the improved seal, totalling £1,250 in savings, meaning the replacement pays for itself within the lifetime of the new unit.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,200 |
| Yearly savings | £50 |
| Payback period | 24 years |
| 25-year lifetime savings | £1,250 |
What homeowners often get wrong
The most common mistake is assuming a spontaneously broken window is always a manufacturing defect covered by a full warranty. This leads to three frequent errors that cost homeowners money and time.
- Blaiming the installer for poor workmanship Many homeowners immediately contact their window fitter, assuming poor installation caused the crack. The right answer is that nickel sulphide inclusions are a glass manufacturing issue, not an installation fault, so chasing the installer can waste weeks while the actual warranty claim should go to the glass supplier or unit manufacturer.
- Assuming all breakages are covered by insurance Home insurance policies typically exclude spontaneous glass fracture and only cover accidental breakage or vandalism. The right answer is that filing a claim for a spontaneous break will likely be rejected, leaving you with the full replacement cost and a potential premium increase, check your policy wording for “gradually operating cause” exclusions.
- Replacing only the broken pane without checking the rest Homeowners often replace just one unit without inspecting other windows for early signs of stress, such as edge chips or seal failure. The right answer is that if one unit has failed from thermal stress, others of the same batch may also be at risk, a full inspection by a certified glazier can save £800 or more in future emergency callouts.
Quick reference
- Spontaneous glass fracture affects roughly 1 in 1,000 sealed double-glazed units according to industry data from the Glass and Glazing Federation.
- Nickel sulphide inclusions are the primary cause in toughened glass, expanding over months or years until the glass shatters without impact.
- Heat-soak testing during manufacturing reduces the risk of nickel sulphide failure by up to 95% but does not eliminate it entirely.
- Replacing a single spontaneously broken double-glazed unit costs between £800 and £1,500 with no government grant available for individual unit replacement.
- A 10-year guarantee against spontaneous fracture is standard from most FENSA-registered installers, but the warranty must be claimed within the guarantee period or it is voided.
Frequently Asked Questions
Yes, this is called spontaneous glass fracture. The Glass and Glazing Federation confirms nickel sulphide inclusions or thermal stress are common causes.
Two main causes: nickel sulphide particles in toughened glass expanding over time, or thermal stress from uneven heating. The British Standards Institution acknowledges both.
Industry estimates suggest roughly 1 in 1,000 sealed units experience spontaneous breakage. The risk is low but recognised by manufacturers.
