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
Giants 400 | Sep 9, 2019
Top 70 Industrial Sector Architecture Firms for 2019
AECOM, Stantec, Ware Malcomb, FSB, and Macgregor Associates top the rankings of the nation's largest industrial sector architecture and architecture engineering (AE) firms, as reported in Building Design+Construction's 2019 Giants 300 Report.
Giants 400 | Sep 9, 2019
2019 Industrial Sector Giants Report: Managing last mile delivery
This and more industrial building sector trends from Building Design+Construction's 2019 Giants 300 Report.
Codes and Standards | Sep 9, 2019
Free app calculates maximum allowable heights and areas for buildings
A free app that calculates the maximum allowable heights and areas for buildings of various occupancy classifications and types of construction has been released.
Retail Centers | Sep 6, 2019
Another well-known retailer files for bankruptcy: Here's the solution to more empty anchor stores
Where can you find the future of retail? At the intersection of experience and instant gratification.
Giants 400 | Sep 5, 2019
Top 110 Hotel Sector Architecture Firms for 2019
Gensler, WATG, HKS, HBG Design, and Steelman Partners top the rankings of the nation's largest hotel sector architecture and architecture engineering (AE) firms, as reported in Building Design+Construction's 2019 Giants 300 Report.
Architects | Sep 5, 2019
AIA launches landmark initiative to drive climate action
AIA Board of Directors approves member-led resolution to rally architects in mitigating and adapting the built environment.
Giants 400 | Sep 3, 2019
Top 140 K-12 School Sector Architecture Firms for 2019
DLR Group, PBK, Huckabee, Stantec, and VLK Architects top the rankings of the nation's largest K-12 school sector architecture and architecture engineering (AE) firms, as reported in Building Design+Construction's 2019 Giants 300 Report.
Market Data | Sep 3, 2019
Nonresidential construction spending slips in July 2019, but still surpasses $776 billion
Construction spending declined 0.3% in July, totaling $776 billion on a seasonally adjusted annualized basis.
Giants 400 | Aug 30, 2019
2019 K-12 School Giants Report: 360-degree learning among top school design trends for 2019
K-12 school districts are emphasizing practical, hands-on experience and personalized learning.
Giants 400 | Aug 28, 2019
Top 35 Data Center Architecture Firms for 2019
Corgan, HDR, Gensler, AECOM, and Page top the rankings of the nation's largest data center sector architecture and architecture engineering (AE) firms, as reported in Building Design+Construction's 2019 Giants 300 Report.