Managing thermal stress in glass can be tricky as there are various factors that have an influence on it. Walls & Roofs explores the causes and prevention methods that architects can use to design thermal stress-free glass systems.  

Thermal stress is a term that describes the internal stresses created within a pane of glass when it is subjected to temperature variations across its area, relating particularly to solar-control glass.

Hulde Jones, marketing support manager at PG Glass, says that glass in the vision and spandrel areas of a facade expands in response to the solar radiation striking it. “The more absorbent the glass, the greater and quicker its response is to solar radiation.”

She explains that if the safe temperature difference between the exposed glass area and the edge is exceeded, the glass may crack. “This crack is referred to as the thermal breakage.” According to Jones, thermal breakage is generally recognised by the fact that the crack always starts perpendicular to the glass edge, proceeding straight for about 30 to 50mm, then altering to a singular or double meandering crack.

Jones further explains that damaged edges may cause thermal breakages at a lower than expected temperature differential. “This lower figure is virtually impossible to accurately predict as it is dependent on the severity of the edge damage.” She adds that where the risk of thermal breakage exists, the glass should be toughened to ensure thermal safety.

The causes of thermal stress
Thermal stress can be caused by a number of aspects, all of which have to do with the absorption rate of the glass. “A thermal stress evaluation must be requested when glass with high heat absorption is utilised, whether in single glazing or in a silicone-insulated unit,” Jones adds.   

She says the cause of cracking due to thermal stress is often a direct result of the glass not having been properly polished on a straight-line polishing machine. “It is imperative that edges of all heat-absorbing glass, like a solar-performing or energy-efficient glass, must be properly polished and free of vented damage, prior to installation, to reduce the possibility of thermal breakage.”

According to the thermal warranty on PG Glass products, glass will be safe from thermal stress fracture when subjected to a list of conditions. One of the conditions is that glass should be glazed in accordance with recognised industry practice and as specified in SANS 10137. Any glass with damaged edges must be rejected prior to installation. “Only glass having good-quality polished edges should be installed,” says Jones.  

On the website www.pilkington.com, the European glass company Pilkington states that edge quality of the glass can play a part in creating thermal stress. “Glass with damaged edges will take less stress than clean-cut glass. A good clean-cut edge is the best finish along with fully polished edges.” Pilkington states that ground edges and arrissed edges may not be as good.  “A ground or arrissed edge is a series of small defects around the glass. The effect brings all the defects to an average level and may at best be only more predictable than a glass with more random damage.”

Jones says no paint or signage material should be applied onto the glass; otherwise the thermal warranty will be breached. “If internal blinds or drapes are fitted to the vision lites these must be installed at a minimum of 50mm off the inside glass face, open top and bottom,  to afford adequate ventilation,” she adds. 

She explains that it is therefore advisable to request a thermal stress evaluation from Glass South Africa (GSA), a division of the PG Group, for each application prior to specifying and/or purchasing the glass. She says this is particularly important where a glass with high heat absorption is utilised, whether in single glazing or in a silicone-insulated unit.  

Pilkington states that the sun changes its path through the sky during the year.  “If the glass in a building is subject to excess thermal stress, it would be expected to have problems in its first year of use. The most challenging periods are in spring and autumn, when the sun angles are low and the nights are cold.” In midsummer the edges of the glass will be warmer anyway and the sun’s path is more directly overhead, putting less direct heat into the glass.
Jones says external shading devices and louvres also influence thermal stress. “The distance between the shading device and the glass facade, as well as the type of blinds that are used on the inside of windows, may have an effect on thermal stress.”

Another factor to consider when managing thermal stress, is the space between the windows and the blinds in millimetres. She says architects should also take note of other buildings that will create static and/or mobile shadows on their building. “Even the ventilation method applied to a glazed area, the type of aluminium framing system  used and the type of glass that is selected to design a glass system will have an influence on thermal stress.”
 
According to Pilkington, glass is also sensitive to thermal stress produced by fire. Special design considerations need to be taken where fire protection is a key concern, either for property protection or life safety.

The different types of glass that affect thermal stress
Laminated glass is more susceptible to thermal stress, since it is compiled of two pieces of annealed glass that have been laminated together with a polyvinyl-butyrale  interlay. This type of glass is available in a variety of tinted colours, which adds to the complexity of the absorption percentage of heat within the lamination. “PVB colours, such as grey and bronze, have a higher absorption rate and are therefore more susceptible to thermal stress,” Jones says. According to her, toughened (Armourplate) glass is thermally resistant.

Pilkington mentions that toughened glass is very resilient and not prone to failing due to thermal stress. “Laminated glass and annealed glass behave in a similar way. Thicker glasses are less tolerant. Glass containing wire is more vulnerable.”

Jones concludes that in terms of frames it is preferable to use a darker frame, since this will absorb heat at a more even pace with the glass. “Light-coloured frames will reflect the heat and will heat up at a slower rate, thus increasing the chances of thermal stress.”

Managing thermal stress in glass can be tricky as there are various factors that have an influence on it. Architects should carefully select each element when designing a glass system for a specific project to prevent thermal stress.

Full acknowledgement and thanks are given to PG Glass for the information given to write this article.