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
Resiliency | Feb 15, 2022
Design strategies for resilient buildings
LEO A DALY's National Director of Engineering Kim Cowman takes a building-level look at resilient design.
Products and Materials | Feb 14, 2022
How building owners and developers can get ahead of the next supply chain disaster
Global supply chain interruptions that started at the very beginning of the pandemic are still with us and compounding every step of the way. Below are a few proven tips on how to avert some of the costly fallout should we be faced with similar commercial disasters at any time in the future.
Urban Planning | Feb 14, 2022
5 steps to remake suburbs into green communities where people want to live, work, and play
Stantec's John Bachmann offers proven tactic for retrofitting communities for success in the post-COVID era.
Urban Planning | Feb 11, 2022
6 ways to breathe life into mixed-use spaces
To activate mixed-use spaces and realize their fullest potential, project teams should aim to create a sense of community and pay homage to the local history.
Senior Living Design | Feb 11, 2022
Design for senior living: A chat with Rocky Berg, AIA
Rob Cassidy, Editor of MULTIFAMILY Design + Construction, chats with Rocky Berg, AIA, Principal with Dallas architecture firm three, about how to design senior living communities to meet the needs of the owner, seniors, their families, and staff.
Architects | Feb 11, 2022
How computer simulations of vision loss create more empathetic buildings for the visually impaired
Here is a look at four challenges identified from our research and how the design responds accordingly.
Healthcare Facilities | Feb 10, 2022
Respite for the weary healthcare worker
The pandemic has shined a light on the severe occupational stress facing healthcare workers. Creating restorative hospital environments can ease their feelings of anxiety and burnout while improving their ability to care for patients.
Architects | Feb 8, 2022
Perkins Eastman and BLT Architects merge
Expanding services in hospitality, education, and mixed-use sectors to better serve clients.
Architects | Feb 3, 2022
SmithGroup elevates Mark Adams to lead workplace practice
In his new role, Adams leads the firm’s practice devoted to the design of corporate and commercial facilities.
Architects | Feb 2, 2022
Steven L. Pliam joins LEO A DALY as Design Technology Leader
Pliam will oversee the deployment and use of technologies such as computational design, parametric design, digital practice, reality capture, visualization, virtual/augmented reality, GIS and AI/Machine Learning.