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4 Factors That Affect Thermal Stress Breakage

A Glass
Window glass has an inherently contradictory nature. On one hand, glass panes offer enough strength to withstand decades of use. On the other hand, that same glass can be fragile enough to break easily. Window glass can crack and shatter even without being struck by an object.

Windows often crack as the result of thermal stress. Unfortunately, few homeowners understand just how thermal stress affects a window, let alone how they can avoid it. If you would like to improve your knowledge of windows and window glass, keep reading. This article takes a closer look at four factors that affect the likelihood of thermal stress breakage.

1. Uneven Shading

Thermal stress breakage occurs as the result of changes that take place in the glass as its temperature rises. Sunlight plays a huge role in the likelihood of a thermal stress break, especially if the glass does not receive equal amounts of sunlight. Parts of the window in the sun expand faster than those in the shade.

For the most part, however, window glass has enough internal strength to resist uneven rates of expansion. Yet if the difference in the rate of expansion becomes great enough, it creates stresses that cause the glass to crack. Be aware of how sun falls on your windows, especially those that receive relatively more intense afternoon sunlight.

2. Edge Exposure

Thermal stress breaks may occur even when all portions of the exposed glass receive equal amounts of sunlight. In that case, the problem may relate to the part of the glass covered up by your window frame. Such glass remains permanently isolated from exposure to sunlight. As a result, edge glass does not expand as rapidly.

Most window manufacturers account for this difference by selecting glass with an appropriate ratio between surface and edge strength. Yet when replacing glass in older windows, edge exposure can present an issue. If too much of the glass remains shielded by the window frame, the difference in the rate of expansion can lead to cracks.

3. Type of Glass

Once upon a time, window glass was just window glass, with very little to differentiate one pane from another. Today, however, window glass comes in a wide array of styles, each of which possesses a very different level of strength. Two of the most common varieties of glass — float glass and tempered glass — display very different levels of resistance to thermal stress.

The principal difference between float and tempered glass lies in their method of production. Specifically, manufacturers create tempered glass by cooling it at a much faster rate than float glass. This rapid cooling produces glass with mechanical strength as much as five times greater than float glass.

This strength difference has a huge effect on thermal stress resistance. Standard float glass can withstand thermal differentials as great as 40 degrees Celsius. Tempered glass, by contrast, can withstand thermal differentials as great as 250 degrees Celsius. This difference makes tempered glass virtually immune to the sorts of stresses experienced in the day-to-day life of window glass.

4. Film Compatibility

Windows allow a lot of sunlight to enter a home, often driving up the cost of summertime cooling bills. To promote better energy efficiency, many homeowners choose to install heat-blocking films on their windows. While effective at reducing solar transmittance, in a small number of cases, such films may promote thermal stress breakage.

The problem occurs especially with films that absorb the sun's radiation, rather than reflect it. If the film absorbs too much heat too quickly, it may outpace the absorption of the glass itself. The resulting temperature difference leads to thermal stress cracks.

Sunlight and shifting temperatures place a lot of stress on a window and can even result in unwanted cracks and fractures. For more information about how to keep your windows in good shape, contact our glass experts at Monterey Glass Specialists Inc.