Glass and Thermal Stress

Thermal Stress is created when one area of a glass pane gets hotter than an adjacent area. If the stress is too great then the glass will crack. The stress level at which the glass will break is governed by several factors. Tempered glass is very resilient and not prone to failing due to thermal stress. Laminated glass and annealed glass behave in a similar way. Thicker glasses are less tolerant. Glass containing wire is more vulnerable. The edge quality of the glass can play a part. Glass with damaged edges will take less stress than clean cut glass. A good clean cut edge is the best finish along with fully polished edges. Ground edges and arrissed edges may not be as good. A ground or raised edge is a series of small defects around the glass. The effect brings all the defects to an average level and may at best be only more predictable than a glass with more random damage.

The temperature difference for a location can be calculated and breakage due to thermal reasons eliminated. To assess the thermal risk the following information should be identified:

• Location of the building
• Orientation
• Type of glass being used including details of the insulating glass units
• Size of building overhang if present
• Size of mullion and transom caps if present
• Details of any back up i.e. where a panel makes up a level to a floor or ceiling behind the glass allowing hot air to be trapped and reflecting back at the glass
• The framing material including thermal breaks and frame color
• The window size and if it opens. I.e. changes the angle to the sun
• Details of internal heating systems
• Any other details like other buildings or trees casting a shadow onto the glass

During the year the sun changes its path through the sky. If the glass in a building is subject to excess thermal stress it would be expected to have problems in its first year of use. The most challenging periods are in spring and autumn when the sun angles are low and the nights are cold. Midsummer the edges of the glass will be warmer anyway and the sun's path is more directly overhead putting less direct heat into the glass.