Thermal stress fracture occurs when temperature differences across a glass pane create internal stresses that exceed the glass's tensile strength. Understanding what causes thermal stress—and how window film affects it—is important for anyone selecting film for existing glazing. Glass expands when heated and contracts when cooled. When different parts of the same glass pane are at different temperatures, the warmer areas expand while the cooler areas do not. This creates internal tension at the cooler edges, which are constrained by the frame and cannot expand freely. If the temperature differential between the center of the glass and its edges becomes large enough, the resulting tensile stress at the edges can exceed the glass's strength, causing fracture. This type of fracture typically originates at the edge of the glass and propagates inward at roughly a 90-degree angle—a pattern that distinguishes thermal stress fracture from impact fracture. Several factors determine whether thermal stress is a concern for a specific window: Glass type — Different glass types have different thermal stress tolerances. Annealed glass has the lowest thermal stress resistance. Heat-strengthened glass has greater resistance. Fully tempered glass has the highest thermal stress resistance among common glass types. However, tempered glass is not immune to thermal stress fracture. Glass thickness — Thicker glass generally has greater thermal mass and may be more susceptible to temperature differentials in certain conditions. Edge condition — The condition of the glass edge affects its strength. Chips, scratches, or poor edge finishing reduce the glass's resistance to thermal stress fracture. Shading patterns — Partial shading of a glass pane is one of the most common causes of thermal stress. When part of a window is shaded while the rest receives direct solar radiation, the temperature differential can be significant. Solar orientation and climate — Windows with high solar exposure receive more solar energy and are more susceptible to thermal stress than windows with lower solar exposure. Frame and installation — How the glass is retained in the frame affects how thermal expansion is accommodated. Frames that grip the glass edge tightly can increase thermal stress risk. Window film affects thermal stress because it changes how solar energy is absorbed and distributed across the glass. Solar control films absorb a portion of the solar energy that would otherwise pass through the glass. This absorbed energy heats the film and, by conduction, the glass surface. The result is that the glass surface temperature increases more than it would without film. If the glass edges are cooler, the temperature differential between the center and edges increases. This is why solar control film—particularly darker, more absorptive films—can increase thermal stress risk on certain glass types, particularly annealed glass with poor edge condition or in partially shaded configurations. Safety/security films (clear films) absorb less solar energy than tinted solar control films, so their effect on thermal stress is generally lower. However, any film that increases the glass surface temperature relative to the edges can contribute to thermal stress in susceptible configurations. Key principle: The thermal stress risk from window film depends on the specific film's solar absorption characteristics, the glass type and condition, the shading pattern, and the installation environment. It is not a universal outcome of applying film to glass. Annealed glass — Standard float glass with no additional heat treatment. Lowest thermal stress resistance. Most susceptible to thermal stress fracture from window film, particularly in partially shaded configurations or with high-absorption films. Heat-strengthened glass — Glass that has been partially heat-treated to increase its strength. Greater thermal stress resistance than annealed glass, but not as high as fully tempered glass. Fully tempered glass — Glass that has been fully heat-treated to significantly increase its strength and change its breakage pattern. Highest thermal stress resistance among common glass types. However, tempered glass can still fracture from thermal stress in severe conditions. Laminated glass — Two or more glass layers bonded with an interlayer. Thermal stress characteristics depend on the glass layers used. The interlayer does not significantly affect thermal stress resistance. Insulating glass units (IGUs) — Sealed double- or triple-pane units. Film applied to the interior surface of an IGU changes the thermal dynamics within the unit, which can affect both the glass and the IGU seal. Consult the film manufacturer's guidance for IGU applications. Before installing solar control film on existing glazing, a thermal stress assessment is appropriate in the following situations: Solar Gard provides technical resources for film-to-glass compatibility. For applications where thermal stress risk is a concern, contact The Window Place USA for product guidance. How do I know if my glass is annealed or tempered? Can I apply solar control film to annealed glass? Does safety/security film cause thermal stress? What does thermal stress fracture look like? Questions about film compatibility with your specific glass? Contact The Window Place USA for product guidance. For information on film compatibility with tempered and laminated glass, see our article on Window Film on Tempered or Laminated Glass.How Thermal Stress Develops in Glass
Factors That Affect Thermal Stress Risk
How Window Film Affects Thermal Stress
Glass Types and Thermal Stress Tolerance
Assessing Thermal Stress Risk Before Installing Film
Frequently Asked Questions
Tempered glass typically has a small etched or printed mark in one corner indicating it is safety glass. Annealed glass typically has no such marking. If you are unsure, look for an etched or printed mark in one corner of the glass — tempered glass is typically marked. If you still cannot determine the glass type, contact The Window Place USA for guidance before ordering.
In many cases, yes—but thermal stress risk must be evaluated. The specific film, glass condition, shading pattern, and orientation all affect the risk. Lighter films with lower solar absorption carry lower thermal stress risk.
Clear safety/security films have lower solar absorption than tinted solar control films, so their thermal stress contribution is generally lower. However, any film that increases glass surface temperature relative to the edges can contribute to thermal stress in susceptible configurations.
Thermal stress fracture typically originates at the glass edge and propagates inward at roughly a 90-degree angle. This pattern distinguishes it from impact fracture, which typically shows a radial pattern from the point of impact.