Glass breakage can occur for many reasons not related to window film. However, the application of window film can exacerbate the situation if a dealer is not aware of the association between glazing systems, window film and weather-related factors. Understanding the potential glass breakage risks will lead to better window film application decisions.
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An example of thermal stress
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Understanding Thermal Stress
Thermal stress occurs when there is an uneven distribution of heat (solar radiation and/or interior heat) on a glass surface. If glass is heated uniformly, the whole panel will expand.
If uneven heat distribution exists, the glass expands in different spots causing excessive stress. Thermal stress fractures normally occur on the glass edge where the stress exceeds the strength of the glass. Current glazing systems add to this problem since the glass edge is usually inset approximately 1/2 inch into the window frame. When sunlight strikes the glass, the center of the glass absorbs more heat versus the glass edges, causing a temperature imbalance. Since window film is applied
to only exposed glass, this temperature imbalance widens, and thermal stress increases, because more heat is absorbed into the window. Thermal stress also occurs when there
is a defect in the glass during the manufacturing process. Damaged or chipped glass weakens the panel and increases the risk of fractures.
Thermals stress is normally not a problem with heat-treated or tempered glass because the glass is constructed to withstand more severe temperature imbalance pressure. Johnson Window Films provides a “Film to Glass” chart to help dealers understand the risks of thermal stress.
Tinted Glass
The amount of Total Solar Absorptance (TSA) is a major factor in understanding thermal stress. The TSA is higher for tinted or coated glass, since more heat is absorbed (30% to 75%), resulting in a greater chance of thermal stress fractures. Additionally, metal vapors or deposits are incorporated during the manufacturing process of coated glass, which increases heat absorption.
Glass Thickness
The thickness of the glass can also determine whether thermal stress may occur. Here are a few general rules provided by the window film industry:
• Solar control window film is not recommended for annealed glass thicker than 3/8”.
• Solar control window film is not recommended for tinted or coated annealed glass thicker than 1/4".
• Solar control window film is not recommended for all laminated glass.
Window Size
The size of the window can also contribute to thermal stress. Here are a few general rules provided by the window film industry:
• Solar control window film is not recommended for monolithic annealed glass in excess of 100 sq. feet.
• Solar control window film is not recommended for annealed IG units in excess 40 sq. feet.
• Solar control window film is not recommended for annealed laminated glass of any size.
Exterior Shading
Glass temperature imbalances can be caused by exterior shading of sunlight. There are several rules of thumb to apply when faced with exterior shading issues.
It’s acceptable if a straight-line shading pattern covers a majority of a clear annealed glass panel. However, straight-line shadow patterns may cause problems on tinted glass.
It’s acceptable if an angled or L-shaped shading pattern covers a moderate perimeter of the glass. This pattern is sometimes acceptable for tinted glass.
Narrow shading patterns around the perimeter are marginally acceptable for clear annealed glass, and not acceptable for tinted or coated glass.
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Some harmful shading examples are illustrated below:
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Interior Shading
Window blinds, drapes and other window treatments trap heat if placed too close to a glass window. This can cause glass temperature imbalances as well. To avoid this problem, there should be a 2” clearance between the window treatment and the glass,
and a minimum 1.5” clearance at the top and bottom of the window treatment.
