Online course in Window films, part 8: Thermal stress

Glasses can break for a variety of reasons. Before we take a look at these reasons, it is important to highlight that the vast majority of breakages happen as a result of damages of the glass panel; even a small chipped part below the frame (which is impossible to notice) is enough. Breakages can occur because of the following reasons: tensile stress (the glass breaks as a result of its own weight), mechanical bending stress (e.g. as a result of strong winds), impact stress (e.g. when the window is broken by throwing a piece of stone through it), twisting stress (if the warped frame causes a glass deformity) and thermal stress. Breakage because of thermal stress means that different temperatures and expansions occur in the glass panal, therefore it breaks. This is not an issue with tempered and thermally treated glasses, where all types of window films can be installed, even the most heat absorbant black ones. The next level is that of float glass, where a thickness of 10 mm is critical – you have to take care even with thinner windows, but the majority of films can be installed. Laminated glasses are in danger with a thickness of more than 6 mm, while wire-glass is the worst from this aspect.

Further treatements and the whole structure of the glazing system are also affecting factors. Since the probability of breakage because of thermal stress is directly proportional to energy absorption, dyed glasses and ones treated with a low-e layer are more dangerous than untreated windows. Another aspect affecting thermal stress is the number of glass layers. Nevertheless, factors beside the glass also have to be taken into consideration, such as partial shading of the glass, which also creates temperature differences on the window surface. Interior and exterior shading devices hinder ventilation, and for this reason they are also dangerous. The frame system is also definitive – the more flexible it is, the lower the probability of breakage. The pattern of breakages caused by thermal stress is charactetistic: it runs from edge to edge in a curve, always starting and ending perpendicularly to the edge.

Anyway, if you follow the guidelines of the compatibility tables in SkyFol’s data sheets and install window films accordingly, you do not have to worry about breakages as a result of thermal stress.

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