Piecing it together: understanding thermal breakages

Piecing it together: understanding thermal breakages

Featured Article
21 Nov 2018

As featured in Glass Times magazine - 2016

When a pane of glass in a window breaks spontaneously, the cause can often appear to be a mystery. Phil Brown, European regulatory marketing manager at Pilkington United Kingdom Limited, examines what causes glass breakages and explains how those across the construction supply chain can mitigate the risk.

Why do glass breakages occur?

Thermal overstressing remains a common cause of glass fractures. Product knowledge training courses feature thermal safety as a topic and there are a number of calculation programs available that can assess the risk during the specification stage. Despite this, instances of thermal glass breakages still occur – so what causes them?

During the installation process, it is common to install glass by retaining the edges of the pane within a frame with a gasket or glazing bead. The area of glass exposed to solar radiation absorbs heat, rises in temperature and expands. In comparison, the edges of the glass, which are shielded from solar radiation, remain cooler than the exposed area. The resulting differential expansion (the difference between the hottest and coolest part of the glass) causes tensile stress at the edge of the glass pane. If this exceeds the breakage strength of the glass, a thermal fracture will result.

Thermal fractures tend to start from the edge of the glass and at 90 degrees to it. For this reason, it is often difficult to identify a thermal fracture without deglazing the system, as the evidence can be hidden within the rebate of the frame.  

What increases the risk of thermal fracture? 

There are many factors that can influence the risk of thermal overstressing in glass. These include where the glass is located, its orientation, whether it is in a vertical or sloping position, the frame type used, to the absorption of the glass and the size of the pane. All of these factors can influence the temperature difference between the centre and edge of the glass, affecting the risk of thermal breakage. For example, indoor shading devices such as blinds or curtains can increase the temperature of the glass by reflecting solar radiation back through the glass. 

The condition of the edges of the pane of glass is also extremely important. As the tensile stresses are located at the edges, the breakage strength of the glass is generally related to the extent and position of flaws in the edge. Damaged edges can significantly reduce the thermal resistance of glass; to avoid issues later, glass with damaged edges should not be installed. 

How can the risk be mitigated? 

As is in the case with any project, communication is key. As a project progresses, the specification of glass can change. Unless the implications of these changes are taken into account, however, breakages can occur. For this reason, communication between architects, specifiers, suppliers and installers, as well as an understanding of how glass behaves in different settings, is crucial. 

Most glass manufacturers offer an assessment service for predicting the thermal safety of glazed installations subjected to solar radiation, usually on the completion of a checklist. Some manufacturers even make the calculation programs available to customers. For example, a thermal stress calculator is available to customers signed-up to My Pilkington™, our on-line business resource. 

If the result of a thermal safety check shows that glass is at risk, the most common solution is to change the glass specification to a heat treated form (e.g. toughened) of the selected glass. Toughened glass can typically resist a temperature differential of more than 200°C, which is much greater than that of annealed glass. If a heat treated version is unavailable or impractical for a specific application, then it may be possible to specify an alternative product or change the design. 

Assessment methods vary across Europe, but technical committee CEN/TC129/WG8 (Glass in Building – Mechanical Strength) is developing a European standard which could see publication as early as next year.  This can only help further raise awareness of the importance of thermal safety checks to design-out potential problems during the specification stage, helping to reduce the number of occurrences of thermal fractures on site. 

Discovering that a pane of glass is at risk of thermal overstressing and in need of treatment too late in a project can result in considerable unforeseen costs. However, the consequences of replacing a broken IGU that has fractured in-situ are worse still.  

Today’s advanced glazing solutions have given specifiers and fabricators the power to create ever more visually impressive and high-performance windows and façades. With this power, however, comes a responsibility to ensure that buildings and occupants will be completely safe in every environmental eventuality.