Will Window Film Cause My Glass To Break?

Window Film and Thermal Stress 

So many San Francisco Bay Area customers ask: Does window film cause glass to break? We explain below.

“Will window film cause my glass to break?” This is one of the most frequently asked question we receive from our potential customers. As mentioned in our recent post about window film myths, this is a common concern. Many in the San Francisco Bay Area have been lead to believe that window film causes thermal stress glass breakage. Why does this misconception exist? It is based on the fact that window film can increase the thermal stress on the glass due to sunlight.  However,  the misconception does not take into account a few factors. Factors like the relative strength of glass and the heat tolerances of different types of glass need to be considered.  Also, there are techniques that window film manufacturers and installers take to avoid the risk of thermal stress breakage. 

Although glass can always break, thermal stress breakage of residential and commercial windows is rare. Window films come with film-to-glass thermal stress compatibility guidelines that account for different glass types. As long as these guidelines are followed, it is extremely unlikely that window film will cause thermal breakage. To help you better understand the dynamics of thermal glass breakage, and how we account for windows’ thermal strengths, read on.

What is Thermal Stress

Thermal stress is caused when glass is exposed to excessive temperature variation. The greater the temperature difference, the greater the thermal stress. Temperature differences in window glass are generally caused by the way sunshine affects a window. Different parts of the window receive different amounts of sunlight and heat. For instance, the center of a window gets direct sunlight and the edges get less. The sun’s energy warms the center of the window, causing it to expands. This puts pressure on the cooler edges to also expand. If this pressure exceeds the breaking strength of the glass, it can fracture. 

Different Glass, Different Strengths

Different types of glass have different strengths. For instance, tempered glass is so strong that it will only suffer thermal breakage due to fire.  Because of this, it is often used in many architectural applications. This includes storefront windows and doors. Heat-strengthened (also known as “thermally toughened”) glass is also exceptionally resistant to thermal stress. It amazingly has a temperature difference threshold of almost 500 degrees Fahrenheit! Temperature difference thresholds for other glass types include:

  • Annealed and float glass—about 105 °F 
  • Roughcast glass—about 86 °F
  • Wired, polished or cast glass—about 77 °F

In general, thermal breakage is not a concern with tempered and heat strengthened window glass. However, it does need to be considered in relation to other window glass types.

Window Film and Thermal Stress 

Window film experts like ClimatePro are very familiar with thermal stress. We conduct a detailed film-to-glass compatibility evaluation. This happens prior to beginning any window film installation. This evaluation considers many factors including, glass type, glass color, pane size, and glass thickness. We also consider external shading, window coverings, frame type, altitude, and solar energy intensity. The film-to-glass compatibility evaluation also needs to review other important window film properties. This includes factors such as solar energy transmission, reflection, and absorption. Absorption is especially important, as it is key in determining the risk of thermal stress breakage. Lastly, the evaluation has to account for how solar-optical window film properties. These vary depending upon the specific film and distinct glass properties on which it will be applied.

Window film manufacturers include film-to-glass thermal stress compatibility guidelines with their products. Installers can quickly assess the compatibility of a specific film for your window. Professionally assessed Window film applications reduce the risk of thermal stress breakage. Remember, glass can naturally break. Hence, we can never provide an absolute guarantee against thermal breakage. However, thermal breakage is typically covered under the warranties we offer with our window film work.

ClimatePro: Your Window Film Specialist

Looking for privacy, security, UV protection, glare reduction, decorative, or heating and cooling window? Interested in adding the benefits of window film to your home or business, but still concerned about thermal breakage? Contact us today for a free consultation. We have offices in North Bay, San Francisco, and San Jose, we can be reached at 707-755-7337.     


Learn About This Author

Brett Torrey Haynes

Brett Torrey Haynes is the marketing and content manager for ClimatePro. He currently lives in a small house on the side of a mountain.

Comments 2

  1. I live in NZ. A few weeks ago we put 3M UV film on living room windows in the house I have lived in for 20 years. The largest cracked within two weeks. They have no overhang, it happened in full sun and the windows are hot to touch, while the others on the same wall, not with film, are cool to touch. Film does cause windows to crack.

  2. If you install a darker tint film it will usually have a higher absorbtion rate and is more likely to crack your glass. The specs on each film you are considering should always be looked at and the absorption rate on the spec sheet for that film should not really be more than 50% for most glass. Tempered or heat strengthened glass is more tolerant so that # can be a bit higher. UV has nothing to do with the amount of tint … all films have UV reducing capabilities… nearly 100 percent… Even with a totally clear film. We used to carry 3M film and they never printed the absorption rates on their spec sheets. And even with 3Ms approval we used to still have glass crack quite often. With our current product line we might see one instance about every 5 years and that’s usually something wrong with the window not because of the film.

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