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
Industrial Facilities | Apr 13, 2016
Ford begins 10-year plan to centralize Dearborn, Mich., campus
The company said that it will rebuild 7.5 million sf of work space over a 10-year period, which will shift 30,000 employees from 70 buildings now into two primary locations.
Building Tech | Apr 12, 2016
Should we be worried about a tech slowdown?
Is the U.S. in an innovative funk, or is this just the calm before the storm?
Multifamily Housing | Apr 7, 2016
Multifamily and Specialized Housing projects honored in 2016 AIA Housing Awards
A San Francisco low-income mixed-use complex, a Los Angeles homeless veterans housing facility, and a series of student residential buildings at UMass were among the winners.
Green | Apr 4, 2016
AIA report analyzes 20 years of the best green projects
"Lessons from the Leading Edge" is a study of the 200 Committee on the Environment (COTE) Top Ten Award winning projects since 1997.
Architects | Apr 1, 2016
Adrian Smith earns UIC’s Legacies and Leaders Award
The Chicago architect graduated from the school and created a scholarship for aspiring architects.
Architects | Mar 31, 2016
Zaha Hadid dies at 65
Often credited as being a pioneer for women architects for her work in a male-dominated field, Hadid had designs commissioned around the world from London to Hong Kong to Cincinnati.
Architects | Mar 31, 2016
Auto-shading windows and point-of-decision design are among the research projects to receive AIA funding grants
Firms represented by the projects and initiatives receiving grants include HKS, DO|SU Studio Architecture, and McClain + Yu Architecture and Design.
Hotel Facilities | Mar 30, 2016
The Usonian Inn, a Frank Lloyd Wright-inspired motor lodge, is on the market for $665,000
The Usonian Inn proudly displays many Wright-inspired characteristics, the most prominent of which is the use of cantilevered overhangs.
Designers | Mar 30, 2016
A technical pen for the modern age
Morpholio’s new ScalePen feature dynamically sets line weight depending on the scale or zoom level of the drawing.
Architects | Mar 29, 2016
Why drawing remains relevant in the design process
Hand-drawn concepts allow ideas to emerge and build stronger connections between the design and the audience, as Gensler Principal and Design Director Alex Fernández writes.