Thermal shock is the name given to cracking or explosion of glass due to rapid temperature change. Glass is particularly vulnerable to this due to its low toughness, low thermal conductivity and high thermal expansion. Glass thermal shock occurs when thermal gradients cause different parts of the glass to expand by different amounts. When the stress overcomes the strength the glazing will fail and either the external pane will explode due to the heat pressure build up, or more commonly the internal pane will crack. Thermal cracking can only occur on 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 drapes.
- Intense reflection from snow or the sea.
- Abrupt temperature changes such as air conditioning or the rising sun.
“The Pittsburgh Plate Glass Thermal Service” published several reports blaming the glass industry for glass thermal shock incidents. We know that the temperature absorbed by glass fitted with a silver reflective window film would increase the temperature by approximately 5oC, versus around a 3oC increase if drapes are used. A 5oC increase in temperature would only produce strength of 500 psi (10% of single glass resistance factor) which is conceived to withstand resistance of 5000psi. This is without taking into account the reinforcement provided by the film itself. This means that thermal shock shouldn’t occur if the glazing has been cut in accordance with the correct standards.
The main factors that are blamed for glass thermal shock are the installation of window film or drapes. However these are not the cause, they only contribute to it. Without faulty glass thermal shock will not occur. Many windows have slight cracks around the edge of the glass which are hidden inside the frames, the thermal crack will occur from this spot. Even the shaving of a small screw head that gets into the groove would be enough to initiate the break. The age of the frame and intensity of the differential radiance resulting from shadows are also factors involved in thermal shock.
European glazing is more fragile than that in the USA, 90% of glazing in USA is tempered which means it will withstand changes of 150oC against 25oC for un-tempered glazing which is often found in Europe. This means that Europeans should be much more aware of the effects of thermal cracking.
Thermal shock causes cracking on sub standard or damaged windows but these windows could have no problem for ten years, then a window film is installed and the window cracks within a few months. This isn’t a coincidence, although the window film didn’t cause the thermal shock crack, it contributed to it as it increased the heat absorption of the glass which then results in a crack forming, the crack will initiate from a small hidden crack inside the frame due to bad cutting of the glass. The same can also be said when drapes are installed.
Fitting window film to double glazed or triple glazed units isn’t advised due to high heat absorption between the panes of glass, although low absorption film such as a light Reflective Silver 50 or Coolclear will be fine. It isn’t recommended to install a film that has too high absorption onto reflective glazing as this will increase the absorption parameters and could result in thermal shock. Clearer films such as safety film, clear or deluxe UV window film or any of the lighter tints are also perfectly fine to install, alternatively any film can be installed externally.
Things to look out for before fitting window film:
- You should pay particular attention to rounded and split trapezoidal windows or other modern architecture
- Holes or places where fans are used.
- Be aware of old, and large, glazing systems that have a frame that no longer ensures expansion tolerance as this also represents a thermal shock risk.
- Fitting film to wired glass isn’t recommended as the iron wires absorb heat by conducting and the edges of the glass are cut with tongs which leave splinters which can become the source of cracks.
In summary its low quality glass, sub standard cutting of edges, mediocre installation of glass, in-appropriate window frames, external factors and shading which combine to cause a thermal crack on glass. If the glazing is of the correct standard thermal cracking can never occur on any home or office window.