Shadow Conditions That Increase the Amount of Thermal Stress on Glass

By Bill Lingnell, PE - In the June 2013 publication of Glass Canada, Margaret Webb, Executive Director of the Insulating Glass Manufacturers Alliance (IGMA), gave an overview of a technical services bulletin developed by IGMA to assist the industry in providing a proper technical understanding for thermal stress.  The bulletin, TB 1300-13, is titled “Guidelines for Thermal Stress Considerations.”IGMA has recognized the demand for an understanding of this subject matter and has addressed this issue by the guidelines presented in the document that are intended to assist those who design, specify, manufacture and install insulating glass units.

The June article expresses an outline of the many elements that contribute to the condition of thermal stresses that occur in insulating glass units (IGU). This brief article highlights some of the conditions that contribute to thermal stress when shadows occur on the glass in a commercial or residential building. IGMA TB 1300-13 addresses this item in the Building Design section of the bulletin.

The architect or building designer will create a building façade or residence with specific architectural features that include overhangs, extended vertical and horizontal framing members, punched openings, special shade devices and other items that will cast shadows on the glass at certain times of day on elevations of the building that have to be considered in the thermal stress review.

One of the most common examples of thermal stress breakage can occur from conditions arising from a clear cold winter night when the glass cools to an ambient condition over night and then in the morning or when the glass is exposed to solar exposure with shading conditions that allow portions of the glass to stay cold while other portions of the glass are being heated by the solar exposure. The heated areas of the glass tend to expand being forced into a state of compression, while the cool zones that are encapsulated by the framing system and/or shaded area are forced into a state of tension.

The phenomenon resulting from the heated area becoming a state of compression and the cooler areas maintaining a tension state can lead to a thermal stress condition that is in excess of the strength of the glass. This is especially important for annealed glass and will be dependent on the solar properties of the glass with regard to the solar absorptance, reflectance and transmittance.

The edge tension in combination with inherent edge flaws, damage and cutting conditions may create a situation where the glass fracture is initiated at the edge of the glass. Edge flaws and conditions are found to be more predominant for thermal stress breakage than surface conditions. Surface conditions resulting from scratches and surface damage in critical stress regions have been found to cause fracture but this is not as frequent as the edge condition.

Experience and examination of fracture origins has verified that the conditions described can and will occur from thermally induced glass fracture and can occur in summer or winter conditions. This is because the thermal gradient between the heated area of the glass and cooler edges is the critical condition to evaluate.

The shadowing of the glass enhances the thermal stress issue and certain types of shadows have more of an impact than others. The shadows that cast an angular shadow or a “V” shape on the glass have been demonstrated to develop larger thermal stresses that linear shadows.

An example of a thermal stress condition that will impose significant thermal stress is shown in the photo that accompanies this article. This condition is one that causes an angular shadow on the glass. Notice the angular shadow caused by the concrete profile of the building structure. When an annealed glass product with a high solar absorptance outer lite of an insulating glass unit is incorporated in this condition, the thermal stress level is of definite concern.

These guidelines were developed from the collective experiences of insulating glass manufacturers, glass and glazing material suppliers, contract glaziers, design engineers, industry consultants and persons experienced in successful IG manufacturing. The document reflects existing technology and will be subject to periodic review and change when new technologies become available as is the normal process in IGMA.

TB 1300-13 contains various sections to cover the importance of thermal stress conditions. The shadow portion is just one of the many issues that interest the architect, designer, glass fabricator, glazing contractor and others relative to the subject.

To order TB 1300-13, contact IGMA at

600450 Shadow Conditions That Increase the Amount of Thermal Stress on Glass

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