In conventionally glazed windows, temperature gradients normally occur between the covered glass edge and center of glass as a result of the window framework. In a conventional glazing system, the glass edge is generally captured up to approxiamately ½";(13 mm) and, therefore, somewhat insulated from the sun's rays. The glass edge temperature will increase when the glass is sunlit (as a function of solar absorption by the glass panel) much more slowly than the center of the glass. When the glass edge is also shaded, greater thermal stresses occur because the shadows inhibit a temperature increase at the glass edge. As a result of a "colder" glass edge, or greater temperature difference between the center and glass edge, higher tensile stresses occur.

Certain types of body tinted glass and coatings are inherently at risk of thermal stress breakage simply because of the temperature imbalances which occur in the glass due to solar absorption at the sunlit glass area versus the captured (covered) glass edge. These include Viracon's standard reflective coatings as well as the lower light transmitting "high performance" low emissivity coatings applied to tinted glass. These products generally require the base glass substrate to which the coating is applied to be heat treated (heat strengthened or tempered).

In some cases, even non-coated, tinted glass products, i.e: Azurlite and EverGreen may require heat treatment simply due to the exterior wall design. contractors and/or suppliers will evaluate these applications upon request.

Preventing Thermal Breakage

Prior to award of the contract, the contractor should provide the necessary submittals for a project. These submittals generally include a shop drawing review and glass strength analysis from a proposed supplier. The glass strength analysis will validate the glass performance to design wind and snow load pressures as well as a thermal stress analysis. To properly evaluate the anticipated thermal stress for a given project, contractors and/or suppliers requires drawings which accurately depict the conditions which will be present on each building design.

These drawings must show any horizontal or vertical building projection which may cast shadows on the glass. In addition, framing type and color along with window treatments such as interior blinds should also be clearly indicated on the shop drawings for proper consideration. In the event that interior blinds are positioned such that the minimum clearances cannot be achieved or ventilated, heat treated glass may be required. Heating and cooling registers should be located to the room side of any shading device. Specific information regarding minimum blind clearances and HVAC registers' locations can be found in the "Specifying Glass" section.