Modern windows are fitted with energy-efficient glass units that significantly reduce heat loss. Most such glass units use selective glass with a Low-E coating that reflects heat back into the room. However, these solutions have one feature — in certain conditions the glass can crack without an impact, due to so-called thermal shock.
What is thermal shock of glass
Thermal shock is the breaking of glass due to sharp temperature differences in different zones of it. When one part of the glass heats up more than another, internal stresses build up in the material. If the temperature difference becomes critical, the glass cracks.
Such a crack is usually straight or zigzag (like lightning), starts from the edge of the glass and has no point of impact.
Why it happens in new windows
In energy-saving glass units the inner glass surface retains heat better — which means the glass heats up faster and more strongly, while heat dissipates worse. This makes the glass more sensitive to local overheating.
- the inner glass surface retains heat better (Low-E coating)
- the glass heats up faster and more intensely than usual
- heat dissipates worse across the glass area
The main cause — a dark object near the glass
One of the most common causes of thermal shock is a dark object placed right against the glass from inside. Sunlight falls on the dark object, it heats up strongly and transfers heat to the glass in the contact zone, while the rest of the glass stays cooler.
Examples — common causes include:
- cushions or bedding left by the window
- boxes and packaging placed on the windowsill
- furniture pushed up against the glass
- heavy dark curtains or blinds opened only partially
- a dark windowsill surface material
How the crack forms — step by step
- Sunlight falls on a dark object near the glass
- The object heats up strongly, accumulating heat
- Heat is transferred to the glass in the contact zone
- The rest of the glass stays cooler
- A temperature difference arises — the warm zone expands, the cool one restrains it
- The glass undergoes thermal stress and cracks — usually from the edge
What cracks are typical
A thermal-shock crack visually differs from damage caused by a mechanical impact:
- it is not linked to mechanical impact — there is no point of impact or dust
- it starts from the edge of the glass and runs inward
- it often looks like a broken line or lightning
- it can appear suddenly, even without obvious causes
- it can develop gradually in the warmer seasons
Is it a defect?
- thermal shock is not a glass unit defect
- it is not related to the quality of installation
- it arises from the conditions of use
- the warranty usually does not apply in such cases
If the glass cracks by itself and the crack starts from the edge with no point of impact, it is very likely a thermal-shock crack and not a manufacturing or installation error.
How to avoid thermal shock
To prevent the glass from cracking due to overheating, follow these simple principles:
- Do not place dark objects right against the glass from inside
- Leave at least 10–15 cm between the object and the glass
- Do not cover the glass with dense objects or fabrics
- Avoid local heating (for example, a heater pointed directly at the glass)
- Use light-coloured or reflective surfaces near the window
- Keep blinds or curtains either fully open or fully closed
Conclusion. Thermal shock is not rare — it is a natural physical phenomenon, especially relevant for modern energy-efficient windows with Low-E glass. The main rule is simple: do not create a local source of overheating on the glass — and the window will last a long time without cracks or problems.
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