Toughened energy-efficiency regulations have prompted the use of windows with better insulating properties. Although thermal glass is more efficient and durable, it is not impervious to damage. Thermal, or tempered, glass can withstand cold or hot temperature extremes, but under the right set of circumstances, it will crack or even shatter. Understanding what factors cause fractures in thermal glass can help you avoid damage.
Thermal Stress and Thermal Shock
Thermal stress can be caused by a number of factors, both environmental and structural. Thermal shock is always caused by rapid and extreme temperature changes. Heat causes glass to expand. If the glass is then suddenly exposed to extreme cold, the shock will cause the glass to break. To demonstrate the difference between thermal stress and thermal shock, heat a glass marble in a frying pan, then drop it into ice water. Depending on how hot the glass became, it will either develop many tiny cracks or shatter. The tiny cracks represent low-energy thermal stress fractures. The more extreme and rapid the temperature change is, the more likely the glass marble is to shatter completely. The shattering results from high-energy thermal shock.
With panes of thermal glass in windows, environmental factors are the most common cause of breakage. When the indoor temperature and the outdoor temperature differ too much, it puts stress on the glass. This is demonstrated most dramatically by a house fire on an extremely cold day. The glass is cold on the outside, but the fire on the inside suddenly superheats the interior area of the window, causing it to crack or even explode. When warm outdoor temperatures drop suddenly to extreme cold temperatures, thermal stress fractures can appear due to supercooling of the heat-expanded glass. The opposite extremes, from cold to hot, can also cause cracking.
Most thermal glass stress fractures begin at the edge of the pane. In windows, the frame protects the edge of the glass, keeping it cooler than the rest of the pane. When sun hits the pane, the edge does not heat as rapidly as the center of the window. Blinds that reflect heat back onto the glass and trap heat between the blinds and glass cause the window to become even hotter than if the blinds were not there.
Heat-absorbing elements placed on the glass, such as labels or sun-catchers, can cause hot spots in the glass, making it vulnerable due to uneven heating. Even something as simple as a constant shadow on a window can cause temperature differentials that cause breakage.
Indoors, a heat or cooling vent aimed directly at the windowpane can cause excessive heat differentials and thermal stress.
The color and material of the window frame affects thermal pane temperatures by influencing the temperature of the edge of the pane. Reflective colors and materials keep the edge much cooler than the exposed area of glass, which can cause thermal fractures starting at the edge.
Since most fractures begin at the edges, you must inspect thermal panes before putting in installation to ensure that the glass has no cracks or edge damage. Don't glaze panes with damaged edges.
While high-energy thermal shock is rare, you can avoid it by having a thermal assessment performed to determine risk. A thermal assessment should always be done during design and is recommended for double-glazed and solar-control glass.