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 centre 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 several factors that can influence thermal stress risk, including:
- The heat absorption properties of the glass (influenced by window films)
- The glass’s resistance to thermal breakage
- The intensity of heat from internal and external sources
- Shading patterns throughout the day
- Heat conductivity of the window frame
- Type and color of internal blinds
- Glass manufacturing quality, particularly the edge treatment
What does a thermal fracture look like?
A thermal stress fracture typically originates at the edge of the glass and extends approximately 20–30mm (about 1 inch) at a right angle from the edge. From there, the crack may branch off in various directions.
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. Reported cases are extremely rare but 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 absorb 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.
Figure 1: Build-up of thermal stress in a glass window
Figure 2: How to recognize a thermal fracture in glass
Table 1: Glass Types
| Glass type | Advice | Summary |
| Tempered/Toughened | This is very resistant to heat stress and should never break. The temperature differential in the glass must exceed 250°C/482°F which is unlikely. | Very Low Risk |
| Heat Strengthened | Very unlikely to break as it can withstand temperature differences which are 3 or 4 times higher than other glass types below. However, we recommend that you do not combine printed perforated graphics with close fitting blinds or other shading devices on the inside of the window, especially an additional reflective window film. | Low Risk |
| Annealed glass, single pane or laminated with worked edges | We recommend you consider Table 2 below to assess the combined risk of other contributory factors. | Low/Medium Risk (see Table 2) |
| Rough cast glass, clean cut edges | As above with slightly increased risk. See Table 2 for more advice. | Medium Risk (see Table 2) |
| Low E coated Double Glazing | Most modern double glazing comes with a low E coating. In this case we strongly recommend against using inside applied window graphics This is because the low E coating reflects back heat which is coming from inside. The perforated graphics on the inside face of the window can help to trap the heat in the glass It is also advised to restrict use of perforated graphics if the window area is greater than 40sqft/4m2. See Table 2 below for an assessment of combined risk of other factors. | Medium Risk (see Table 2) |
| Triple Glazing | We do not recommend using any window films on the inside of triple glazed windows. Heat may become trapped inside the window unit, increasing the internal temperature, heat buildup can create thermal stress and could lead to glass cracking or seal failure. It is common for Low-E coatings to be applied and this also increases the risk. Triple-glazed windows are highly effective at preventing heat transfer. Adding a film to the inside may further increase the temperature differential between the interior and exterior panes. This could lead to condensation forming between the layers of glass. | High Risk |
Table 2: Risk Factors
| Factor | Low Risk | Medium Risk | High Risk |
| Graphics | Most perforated graphics including medium/darker prints with higher transparency | Dark print on 20% (80/20) transparency perforated. Medium/dark prints on non-perforated window film | Non-perforated film, black |
| Glass treatments | No treatments to increase solar absorption of glass | Solar control or tinted glass with absorption less than 50% | Annealed glass with >50% total absorption. |
| Frame type | Dark colored aluminium is best option. It absorbs heat and transmits to glass edges. | Wood: vinyl; light coloured aluminium; frames which are in thermal contact with masonry: | |
| Exterior shading | Minimal/moderate shadows which partially cover the window | Shadows covering up to a third of window during parts of day. | |
| Interior shading | No internal shading devices used during daytime (these should not be needed in addition to perforated window film which blocks at least 50% of light) | Shading devices such as blinds or curtains can reflect solar energy back onto the window and also trap hot air close to the window | Closely fitted (<50mm/2”); lightly coloured. e.g. reflective/metallic blinds, which are used during daytime |
| Glass thickness | Glass thickness less than 10mm/0.4” with no solar treatment to increase absorption | Clear annealed glass thickness >=10mm/0.4”. Tinted glass thickness >6mm/0.24”. Thicker glass absorbs more heat. | Tinted glass thickness >10mm/0.4”. Or medium risk elements combined with dark graphics, internal shading etc. |
| Glass size | Smaller windows provide less area for heat absorption | Largish windows around 100sqft/10m2 single glazed windows and 40sqft/4m2 for double glazed | 120sqft/2m2 for single glazed windows and 50sqft/5m2 for double glazed |
| Solar exposure | Very low risk of thermal breakage for NE/N/NW windows in northern hemisphere and SE/S/SW in southern. | South facing windows in northern hemisphere. north facing in southern. Climates with high average temperatures. | As medium risk but with additional reflective surfaces around window which concentrate heat in particular areas |
