Thermal shock is the name given to the cracking or explosion 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 strength, the glazing will fail and the outer panel will burst due to heat pressure buildup or, more commonly, the inner panel 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 Plate Glass Thermal Service” has published several reports blaming the glass industry for glass thermal shock incidents. We know that the temperature absorbed by glass fitted with silver reflective window film would increase the temperature by approximately 5°C, compared to an increase of around 3°C if curtains are used. A 5°C temperature rise would only produce a 500 psi resistance (10% of the resistance factor of a single glass) which is designed to withstand a resistance of 5000 psi. 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 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 there will be no thermal shock. 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 going 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 involved in thermal shock.
European glazing is more brittle than US glazing, 90% of glazing in the US is tempered which means 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 cracks in substandard or damaged windows, but these windows might be problem-free 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 a bad glass cut. The same can also be said when installing curtains.
It is not recommended to install a window film on double or triple glazed units due to the high heat absorption between the glass panes, although a low absorption film such as Reflective Silver 50 or Coolclear will do just 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 cause thermal shock. Most sheer films such as security film, UV clear or luxury 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 tolerance for expansion, as this also poses a risk of thermal shock.
- It is not recommended to put film on wired glass, since the iron wires absorb heat by conduction and the edges of the glass are cut with strips that leave chips that can become a source of cracks.
In short, your substandard glass, substandard edge cutting, mediocre glass installation, inappropriate window frames, external factors, and shading all combine to cause thermal cracking in the glass. If the glazing is of the correct standard, thermal cracking can never occur in any home or office window.
