Glass spandrels and shadow boxes: Design and construction considerations

August 11, 2021
Publication: The Construction Specifier
Author(s): John Jackson

Abstract

Glass spandrels are a common design strategy used to opacify floor levels in building facades. These opaque glass assemblies are integrated into glazing systems, such as glass curtain walls or window walls, to provide visual continuity with adjacent vision glass to render an all-glass look to the building facade. Certain glass spandrel designs, such as shadow boxes, can also provide a greater expression of visual depth in comparison to back-painted spandrel glass or glazed-in metal panels. Spandrel assemblies typically include an exterior panel (sometimes they include an intermediate panel) and are often backed by insulation, either foil-faced or with a metal back-pan.

Although simple in concept, the physics and technical design considerations of glass spandrels and shadow boxes can be quite complex and must be carefully considered. These include the potential for heat build-up in the cavity, glass differential thermal stress, risks of cavity condensation, thermal performance, air-water-vapor seal, edge-of-slab fire safing, and accumulation of dust/debris in the cavity. Project-specific exterior climate and interior psychometric (i.e. temperature and relative humidity) conditions must also be considered. In addition, glass spandrel design must consider the method of construction of the glazing system, whether factory-glazed (e.g. unitized curtain wall or window wall) or field-glazed (e.g. stick-built curtain wall). Temporary protection during storage, transportation, and installation, as well as in-service considerations such as glass replacement must also be factored into the equation.

Like many issues with glazing systems, design considerations for glass spandrels must be balanced carefully against the project-specific performance requirements, including considerations for construction, schedule, cost, and maintenance in-service. A summary of these important design considerations will be provided, along with recommendations through lessons learned from the author’s experience on projects using glass spandrels and shadow boxes within the building facade.

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