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
AEC Innovators | Aug 27, 2019
7 AEC industry disruptors and their groundbreaking achievements
From building prefab factories in the sky to incubating the next generation of AEC tech startups, our 2019 class of AEC Innovators demonstrates that the industry is poised for a shakeup. Meet BD+C’s 2019 AEC Innovators.
Giants 400 | Aug 16, 2019
2019 Healthcare Giants Report: The ‘smart hospital’ is on the horizon
These buildings perform functions like a medical practitioner. This and more healthcare sector trends from Building Design+Construction's 2019 Giants 300 Report.
AEC Innovators | Aug 15, 2019
Oracle’s replica of a construction jobsite creates an immersive environment for AEC professionals
The Oracle Construction and Engineering Innovation Lab allows visitors to walk through five different stages of construction work, to test new AEC technologies and training techniques.
Giants 400 | Aug 15, 2019
Top 140 Multifamily Sector Architecture Firms for 2019
Humphreys & Partners, KTGY, SCB, CallisonRTKL, and Perkins Eastman top the rankings of the nation's largest multifamily sector architecture and architecture engineering (AE) firms, as reported in Building Design+Construction's 2019 Giants 300 Report.
Giants 400 | Aug 13, 2019
2019 Science + Technology Giants Report: Operational flexibility is a must for S+T buildings
The science and technology (S+T) sector is arguably the industry’s most complex because it caters to a diverse clientele with specific priorities and imperatives, according to Building Design+Construction's 2019 Giants 300 Report.
Architects | Aug 12, 2019
AIA, NCARB help launch coalition to represent complex professions and licensing boards
Architects, registration boards, and others join effort to ensure a unified voice for professions in growing debate around licensure, regulation, and public safety.
AEC Innovators | Aug 9, 2019
Improving architectural designs through iteration
Computational design lets ZGF Architects see patterns that renderings and even models can’t show.
Giants 400 | Aug 8, 2019
Top 200 Office Sector Architecture Firms for 2019
Gensler, AECOM, Perkins+Will, Stantec, and HOK top the rankings of the nation's largest office sector architecture and architecture engineering (AE) firms, as reported in Building Design+Construction's 2019 Giants 300 Report.
Giants 400 | Aug 8, 2019
2019 Office Giants Report: Demand for exceptional workplaces will keep the office construction market strong
Office space consolidation and workplace upgrades will keep project teams busy, according to BD+C's 2019 Giants 300 Report.
Museums | Jul 29, 2019
A new museum debuts inside the Empire State Building
A $165 million, 10,000-sf museum opened on the second floor of the Empire State Building in New York City, completing the second of a four-phase “reimagining” of that building’s observatory experience, which draws four million visitors annually.