How Hot should Glass Get? Comparing Thermal Loading Criteria in Spandrel Glass

Comparing Thermal Loading Criteria in Spandrel Glass

Abstract

This paper presents various methodologies for calculating the temperature differential in glass spandrel panels. Some methodologies have been derived from direct measurements in case studies.  Others from international standards or research papers.  As energy codes for buildings continue to demand more efficiency within the exterior walls of buildings gradually, the ‘we always did it this way’ approaches will no longer work and may potentially lead to unsafe conditions for spandrel glass. Spandrel glass was developed to preserve a continuous glazing aesthetic while covering the perimeter structure with glass coatings and a layer of insulation directly behind the glass, which is not visible. While this makeup provides benefits, such as reducing the overall u-value of the exterior wall system, it often does so by significantly increasing the temperature differential across the glass surface. When this temperature difference is not accounted for, spandrel glass is often prone to fracture. Fractures have become common enough in the construction industry that the National Glass Association has published technical notes to educate the building design and construction community about the phenomenon. In this paper, we present various methodologies for predicting the design surface temperature of glass and compare them with forensic data we gathered from a building’s spandrel panel with a history of thermal fracture. We conclude the paper with our recommendation of the thermal temperature determination process that a design team can use to address this phenomenon responsibly.