Thermal stress is one of many factors that can cause glass to break, even when it is not impacted by a projectile.
Such failures usually are not a testament to the quality of the glass, but rather the result of temperature imbalance within the glass, which can occur when the center of a window or insulating glass unit (IGU) heats and expands while the edges remain cool.
The resulting thermally induced tensile stress on the glass edges can exceed the strength of the glass itself, causing it to break at approximately a 90-degree angle.
Working closely with the IGU fabricator and glass supplier, and conducting a thermal stress analysis at the beginning of the design phase can help architects specify the proper glass for their projects. Here are five thermal stress related rules-of-thumb to consider when specifying glass:
1. Clear or color? Glass can be clear, ultra-clear or tinted in colors such as gray, blue, green and bronze. Tinted glasses absorb solar radiation more than clear glass, which can cause them to collect heat and make them more susceptible to break from thermal stress.
2. Inner or outer surface? Reflective and low-emissivity (low-e) coatings, which improve the solar performance of glass primarily by reflecting solar radiation, can be placed on any one of four surfaces in a dual-pane IGU. While they are usually placed on the inner glass surfaces of the IGU, the coating orientation and the associated risk of thermal stress has to be taken into account.
3. Sunny or shady? Outdoor shading, including overhangs, adjacent buildings, and trees, is one of the most dynamic elements to consider when analyzing thermal stress. Minimizing locations where non-uniform shading of IGUs can occur will help avoid extreme temperature gradients. Interior shading devices, such as blinds or drapes, can increase glass temperature by reflecting solar radiation back through the glass or by reducing the convection and conduction of heat away from the glass. To minimize thermally induced edge stress, the air space around the window glass should be ventilated. There should be a gap of several inches between the glass and shades, blinds or drapes.
4. Where to place a vent? If heating vents, registers and grilles point directly at glass units, warm air will cause the glass to heat up and, under certain conditions, break. Make sure vents are carefully placed to reduce thermal stress risk and avoid placing them between the glass and interior shading devices.
5. How to frame it? Glass framing systems that have low heat capacity also can minimize the chance of a thermal stress break. Structural gaskets and narrow metal framing are favorable because they have less effect on the glass temperature around the edges. Conversely, massive framing – whether metal, masonry or even wood – can have more of an effect, resulting in thermal imbalance and risk of glass breakage.
To learn more about avoiding thermal breakage of insulated glass units and other glass-related topics, visit the PPG Glass Education Center at www.educationcenter.ppg.com.
Related Stories
| Jul 20, 2012
Office Report: Fitouts, renovations keep sector moving
BD+C's Giants 300 Top 25 AEC Firms in the Office sector.
| Jul 20, 2012
K-12 Schools Report: ‘A lot of pent-up need,’ with optimism for ’13
The Giants 300 Top 25 AEC Firms in the K-12 Schools Sector.
| Jul 20, 2012
Higher education market holding steady
But Giants 300 University AEC Firms aren’t expecting a flood of new work.
| Jul 20, 2012
3 important trends in hospital design that Healthcare Giants are watching closely
BD+C’s Giants 300 reveals top AEC firms in the healthcare sector.
| Jul 20, 2012
Global boom for hotels; for retail, not so much
The Giants 300 Top 10 Firms in the Hospitality and Retail sectors.
| Jul 20, 2012
Gensler, Stantec, Turner lead ‘green’ firms
The Top 10 AEC Firms in Green Buildings and LEED Accredited Staff.
| Jul 19, 2012
Renovation resurgence cuts across sectors
Giants 300 reconstruction sector firms ‘pumping fresh blood in tired spaces.’
| Jul 19, 2012
Rental market pushing service, ‘community’
The Top 25 Giants 300 AEC firms in the Multifamily Sector keep four-legged tenants in mind.
| Jul 19, 2012
AEC firms ready to dive into public projects
But the size of the pool keeps shrinking for the Top 25 AEC firms in the Government Sector.