Cracking was observed in the exterior curtain wall glass on an early-1970s mid-rise building in the Midwest. As originally constructed, the curtain wall included single-glazed 5.5-mm (7/32-in.) thick bronze-tinted glass at all floors—except the uppermost level, which featured single-glazed 9.5-mm (3/8-in.) thick clear polished plate glass.
The glass units ranged from 2184 to 3454 mm (86 to 136 in.) in height, and in width from 838 mm (33 in.) adjacent to the projecting building structural columns, to 1372 mm (54 in.). The original specifications for the glass were not available for review. However, several decades ago, a film was added to the interior face of the tinted glass as an additional sunlight-control measure.
During a recent maintenance program, the existing film was removed from the tinted glass, and a new film was applied. This film was also used on the clear glass at the topmost floor, on the south and west elevations. It was applied in the late fall, followed by sealant work in early winter. Within a month of the installation’s completion, cracking of clear (untinted) glass units in the narrower lights on the south elevation occurred, followed by cracking of both clear and tinted glass at several other locations.
Research into the film used in the retrofit installation revealed it was anticipated to result in a significant decrease in solar heat gain coefficient (SHGC) of the single-glazing (from 0.82 to 0.39). Retrofit films of this type both reflect and absorb energy from the sun’s rays; the absorbed heat energy can cause higher edge stresses in the glass than existed before film application. Glass cracking related to thermal stresses typically originates at, and perpendicular to, the edge of the glass, as seen in the photo.
Further, thermal stresses in a glass lite are increased by differential shading. For example, solar heat gain varies between shaded and unshaded areas of the glazing. Glass that cracked adjacent to the exterior projecting columns is in full shade during the early morning and full sun by the afternoon.
In contemporary construction, it is common to use heat-strengthened glass when films or tinted glass are specified. Although documentation is not available to confirm the glass type used here, the glazing appears to be plain annealed glass due to the lack of any markings indicating heat-strengthening.
Based on the likelihood the observed cracking is attributed to thermal stresses related to the application of the new film, removal of the film will be necessary to prevent the risk of further cracking over the next few years. Also, because the cracked glass presents a safety hazard, protection is required until remediation is completed. Among the lessons learned from this example: When improving any performance attribute of a building enclosure, care must be taken to understand the effect on the entire assembly’s behavior.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at email@example.com.
David S. Patterson, AIA, is an architect and senior principal with the Princeton, New Jersey, office of WJE, specializing in investigation and repair of the building envelope. He can be reached at firstname.lastname@example.org.
Kenneth M. Itle, AIA, is an architect with the Northbrook, Illinois, office of WJE, specializing in investigation and preservation. He can be reached at email@example.com.