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
| Jun 7, 2013
First look: University of Utah's ‘teaching hospital for law’
The University of Utah broke ground on its cutting-edge College of Law building, which will facilitate new approaches to legal education based on more hands-on learning and skills training.
| Jun 7, 2013
First look: Austin breaks ground on 'light-filled' Central Library
The design scheme by Lake|Flato and Shepley Bulfinch incorporates reading "porches" and a light-filled, six-story atrium.
| Jun 5, 2013
Survey of AE firms shows profits, hiring on the rise
A recent survey of more than 40 Boston-area architecture and engineering companies by consulting firm DiCicco, Gulman & Company confirms continued growth in business volume.
| Jun 5, 2013
USGBC: Free LEED certification for projects in new markets
In an effort to accelerate sustainable development around the world, the U.S. Green Building Council is offering free LEED certification to the first projects to certify in the 112 countries where LEED has yet to take root.
| Jun 4, 2013
SOM research project examines viability of timber-framed skyscraper
In a report released today, Skidmore, Owings & Merrill discussed the results of the Timber Tower Research Project: an examination of whether a viable 400-ft, 42-story building could be created with timber framing. The structural type could reduce the carbon footprint of tall buildings by up to 75%.
| Jun 4, 2013
Notification reinvented: SimplexGrinnell introduces revolutionary family of intelligent notification appliances
Simplex TrueAlert ES uses addressable technology to improve protection, simplify installation and reduce operating costs.
| Jun 3, 2013
6 residential projects named 'best in housing design' by AIA
The Via Verde mixed-use development in Bronx, N.Y., and a student housing complex in Seattle are among the winners of AIA's 2013 Housing Awards.
| Jun 3, 2013
Trifecta of awards recognize Vision/Rubenstein campus, Bayer Healthcare HQ
When Vision Equities, LLC and Rubenstein Partners purchased the 200-acre former Alcatel-Lucent campus in Whippany a little more than two years ago, the partnership recognized the property’s potential to serve as a benchmark infill revitalization for the State of New Jersey.
| May 31, 2013
Nation's first retrofitted zero-energy building opens in California
The new training facility for IBEW/NECA is the first commercial building retrofit designed to meet the U.S. Department of Energy’s requirements for a net-zero energy building.
| May 30, 2013
The Make It Right squabble: ‘How many trees did you plant today?’
A debate has been raging in the blogosphere over the last few months about an article in The New Republic, “If You Build It, They Might Not Come,” in which staff writer Lydia DePillis took Brad Pitt’s Make It Right Foundation to task for botching its effort to revitalize the Lower Ninth Ward of New Orleans.