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Thermal Cracking: What Is it & How Do You Avoid It?


In the world of architectural glazing and windows, a fascinating and sometimes troublesome occurrence called Thermal Cracking has piqued the interest of homeowners and industry experts alike.

It often begins with the disheartening sight of a crack stretching horizontally from the edge of a nearby window.

This discovery can disrupt one's peace of mind, prompting an investigation into its origins. Was it a bird collision, a stray object, or a manufacturing flaw? As we delve into the intricacies of Thermal Cracking, we'll explore its causes and the preventive measures to protect your windows and doors.

What Is Thermal Cracking?

Thermal Cracking is a process in which glass is damaged after being exposed to uneven or concentrated heating or cooling. The warm glass expands but the cooler areas don’t, and if the glass isn’t strong enough to resist the expanding warm section – it will crack.

While not overly common, it is nonetheless a well-understood phenomenon – especially with thermally treated glass like Low-e, reflective or tinted glass, as these glass types tend to retain heat more readily.

In a household setting, thermal cracking is typically seen in the following scenarios:

  • When heaters or air conditioners are placed too close to windows and blow unevenly onto them.
  • Where glazing may be subjected to uneven shading (like behind a tree, building or other obstruction).
  • On sunny, but cold days.
  • When blinds or shutters with highly reflective backings are used.
  • On days of sudden and dramatic temperature changes.
  • Thermal cracking is also more often seen on larger windows.

It is important to note that thermal cracking is not the result of a manufacturing defect, but instead is a symptom of the environment in which the glass is placed. This means that practically all glass manufacturers will not cover thermal cracking under warranty.

How Do I Protect Myself From Thermal Cracking 


There are some simple ways that you can protect yourself from the risk of thermal cracking.

1.     Use Heat-Strengthened Glass 

Heat tempered toughened glass is considerably less likely to crack from thermal stress, simply because it is 4-5 times stronger than standard annealed float glass. This makes it more resistant to the pressure of an expanding heated area of glass.


2.     Ensure Heaters & Air Conditioners Don’t Vent Directly Onto Glass Surfaces. 

Always plan and place your heating and air conditioning so that it never blows directly onto glass surfaces or the gaps between glass surfaces and blinds. Doing so greatly reduces the thermal difference in your glass surfaces and reduce your risk of thermal breakage.


3.     Ensure Adequate Space Between Curtains & Blinds and Glazed Areas. 

Blinds that fit closely to your windows and glazed doors can help you control inside temperatures – but unfortunately, they can also contribute to thermal cracking. The link between how much your blinds put your glazed surfaces at risk of thermal cracking is a complicated one, and dependent on the colour, shape and material of your blinds or curtains.

The Australian Glass & Window Association recommends a space of at least 50mm between your windows and your blinds and glass, and that this gap is ventilated.     

Still Have Questions About Thermal Cracking?


If you’d like to know more about preventing thermal cracks (or if you need to replace a thermally cracked window), get in touch with our team. We’d be more than happy to talk you through your ways of preventing thermal cracks or help you find ways to minimise the chance of it happening again.

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