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
| Mar 27, 2013
RSMeans cost comparisons: college labs, classrooms, residence halls, student unions
Construction market analysts from RSMeans offer construction costs per square foot for four building types across 25 metro markets.
| Mar 26, 2013
Will Google Glass revolutionize the construction process?
An Australian architect is exploring the benefits of augmented reality in the design and construction process.
| Mar 24, 2013
World's tallest data center opens in New York
Sabey Data Center Properties last week celebrated the completion of the first phase of an adaptive reuse project that will transform the 32-story Verizon Building in Manhattan into a data center facility. When the project is completed, it will be the world's tallest data center.
| Mar 22, 2013
8 cool cultural projects in the works
A soaring opera center in Hong Kong and a multi-tower music center in Calgary are among the latest cultural projects.
| Mar 22, 2013
Earn $500 as a DOE proposal reviewer
The DOE'S Building Technologies Office this morning put out a call to the AEC industry for expert reviewers for its new energy-efficiency initiative for small commercial buildings, which make up more than 90% of the commercial building stock.
| Mar 21, 2013
Best Firms to Work For: Enermodal Engineering is green to the core
At Enermodal Engineering, there’s only one kind of building—a sustainable one.
| Mar 21, 2013
Are charter schools killing private schools?
A recent post on Atlantic Cities highlights research by the U.S. Census Bureau's Stephanie Ewert that shows a correlation between the growth of charter schools and the decline in private school enrollment.
| Mar 20, 2013
Folding glass walls revitalize student center
Single-glazed storefronts in the student center at California’s West Valley College were replaced with aluminum-framed, thermally broken windows from NanaWall in a bronze finish that emulates the look of the original building.