On Window

Thermal Fracture of Glass – Risks

Thermal fracture occurs in windows due to temperature differences in the glass. Some areas heat up faster to a higher temperature while other areas remain cool, causing stress to build at a molecular level at the intersection of these areas. Thermal fracture of glass can occur without window film but the chances increase depending on the type of window film together with a number of other factors. The risks of glass breakage are negligible for perforated window film posters and are still extremely low when full window wraps are deployed.

How is thermal stress caused?

The main cause of thermal stress is when the center of the window is heated by solar energy (or heating units inside the window) while the edges of the window (encased within a frame at the very edges) remain cool. Figure 1 shows how the stress builds up as the heated glass in the center of the window expands outwards until it reaches resistance from the cooler glass at the edge. The glass will crack when the stress becomes too great and this usually occurs at a point where there was already a weakness.

There are a number of contributing factors including the heat absorption properties of the glass which are impacted by window film, the heat resistance of the glass to breakages, the amount of heat applied to the glass from both inside and out, the extent to which parts of the glass are in shade at different points of the day, the nature of the window frames and how they conduct heat, the nature and color of any internal blinds and finally the manufacturing quality of the glass and in particular the edge treatment of the glass.

What does a thermal fracture look like?

A thermal stress fracture is easy to spot. One end of the fracture will be in contact with the edge of the window, in a line of about 20-30mm (1 inch), which is a right angle to the edge of the glass. The crack can then deviate in any direction from there.

What is the risk that Contra Vision® Perforated Window Films™ will cause glass to break?

There is an extremely low probability that Contra Vision® Perforated Window Films™ will exacerbate existing imperfections in glass and cause it to break. We receive one reported case for every 250,000 square meters (2,500,000 square feet) of perforated window film we sell. This does not mean that people should be complacent and a number of factors should be considered before installing a window wrap.

Solid window films including printed graphics, tint and reflective products are more likely to cause breakages because they cover 100% of the surface and therefore attract more heat. We still advise against using dark colors in hot climates if the glass is not toughened. Please read the advice below and consider all the factors which increase the risk.

Table 2 should be consulted if the glass is not toughened and you are considering a a window wrap covering at least 50% of the glass area (posters are not a significant risk). If any contributory factor in Table 2 is assessed to be high risk then we recommend action to reduce risk, e.g. very light/bright graphics and high transparency. It has also been recommended to leave up to 50mm (2 inch) gap between the edge of the window film and the frame. This might allow the heat to dissipate before it reaches the very edge.

If there are 3-4 medium risk factors  in Table 2. then the overall risk becomes high.

Disclaimer: Contra Vision is not liable for any glass breakage and the information provided here is in good faith.

Thermal Glass Fig 1
Thermal Glass Fig 2
Thermal Risks Glass Type
Table 2