Thermal shock is the name given to the cracking or exploding of glass due to rapid change in temperature. Glass is particularly vulnerable to this due to its low hardness, low thermal conductivity, and high thermal expansion. Thermal shock of glass occurs when thermal gradients cause different parts of the glass to expand by different amounts. When the stress exceeds the resistance, the glazing will fail and the outer pane will explode due to heat pressure build-up or, more commonly, the inner pane will crack. Thermal cracking can only occur in low strength glass, so toughened or tempered glass can never be affected by thermal shock.

There are a number of internal and external factors that contribute to thermal cracking, such as:

• Shading caused by stickers, walls, trees or curtains.
• Intense reflection of the snow or the sea.
• Sudden changes in temperature such as air conditioning or the rising sun.

The “Pittsburgh Glass Plate Thermal Service” published several reports blaming the glass industry for glass thermal shock incidents. We know that the temperature absorbed by glass fitted with reflective silver window film would increase the temperature by about 5°C, compared to about 3°C ​​if curtains are used. A 5°C rise in temperature would only produce a 500 psi strength (10% of the single pane strength factor) which is designed to withstand a 5000 psi strength. This without taking into account the reinforcement provided by the film itself. This means that thermal shock should not occur if the glazing has been cut to the correct standards.

The main factors attributed to the thermal shock of glass are the installation of curtains or window films. However, these are not the cause, they only contribute to it. Without defective glass, thermal shock will not occur. Many windows have slight cracks around the edge of the glass that are hidden within the frames, thermal cracking will occur from this point. Even shaving the head of a small screw that goes into the slot would be enough to initiate breakage. The age of the frame and the intensity of the differential radiation resulting from the shadows are also factors that intervene in the thermal shock.

European glazing is more brittle than US glazing, 90% of glazing in the US is tempered, meaning it will withstand changes of 150°C vs. 25°C for non-tempered glazing which it is often found in Europe. This means that Europeans should be much more aware of the effects of thermal cracking.

Thermal shock causes substandard or damaged windows to crack, but these windows might not have a problem for ten years, then window film is installed and the window cracks within a few months. This is not a coincidence, although the window film did not cause the thermal shock crack, it contributed to the increased heat absorption of the glass, which then results in the formation of a crack, the crack will start from a small hidden crack. inside the frame due to poor glass cutting. The same can also be said when installing curtains.

It is not recommended to install window film on double or triple glazed units due to high heat absorption between the glass panes, although a low absorption film such as Reflective Silver 50 or Coolclear will work fine. It is not recommended to install a film that has too high an absorption over reflective glazing as this will increase the absorption parameters and could lead to thermal shock. More transparent films, such as security film, clear or luxury UV window film, or any of the lighter tints, can also be installed perfectly; alternatively, any film can be installed externally.

Things to consider before installing window film:

• You should pay special attention to rounded and split trapezoidal windows or other modern architecture
• Holes or places where fans are used.
• Beware of old and large glazing systems that have a frame that no longer guarantees expansion tolerance, as this also poses a risk of thermal shock.
• Filming on wired glass is not recommended as the iron wires absorb heat by conduction and the edges of the glass are cut with strips leaving chips that can become a source of cracks.

In short, its poor quality glass, substandard edge cutting, mediocre glass installation, improper window framing, external factors, and shading all combine to cause a thermal crack in the glass. If the glazing is of the correct standard, thermal cracking can never occur in any home or office window.