Glass Spandrels and Shadow Boxes
Design and Construction Considerations
Presented on October 13, 2022 at Facade Tectonics 2022 World Congress
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Overview
Abstract
Glass spandrels are a common design strategy utilized to opacify floor levels in building facades. These opaque glass assemblies are integrated into glazing systems, such as curtain walls or window walls, to provide visual continuity with adjacent vision glass, rendering 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, than say, back-painted spandrel glass or glazed-in metal panels. Spandrel assemblies typically include an exterior panel, sometimes 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 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 firesafing, and accumulation of dust/debris in the cavity, to name a few. 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. This paper will provide a summary of these important design considerations and will provide recommendations through lessons learned from the author’s’ experience on projects utilizing glass spandrels and shadow boxes within the building facade.
Authors
John Jackson
Simpson Gumpertz & Heger
jajackson@sgh.com
Keywords
Introduction
Glass spandrels are a common design strategy utilized 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, rendering 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, than say, back-painted spandrel glass or glazed-in metal panels. Spandrel assemblies typically include an exterior panel, sometimes 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 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 firesafing, and accumulation of dust/debris in the cavity, to name a few. 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. This article will provide a summary of these important design considerations and will provide recommendations through lessons learned from the author’s’ experience on projects utilizing glass spandrels and shadow boxes within the building facade.
Definition of Spandrels
The Fenestration and Glazing Industry Alliance (FGIA) and American Architectural Manufacturers Association (AAMA) defines spandrels as “the opaque areas of a building envelope, which typically occur at floor slabs, columns
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Glass Spandrel Types and Design Considerations
Terminology for glass spandrel types can vary widely within the industry, so we will use this opportunity to provide some clarity and definitions for the purposes of this article. Glass
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Other Considerations
In addition to the considerations of the various glass spandrel assemblies we’ve discussed, additional design and construction considerations include thermal performance, edge-of-slab firesafing, glazing system selection, and construction methodology, which
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Recommendations and Conclusions
As discussed in this article, there are a variety of important design and construction considerations when integrating glass spandrels into building facades. A variety of documents have been published on
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To Vent or Not To Vent
The decision “to vent or not to vent” glass spandrel assemblies depends on a variety of factors. Both options carry their own advantages and disadvantages as summarized below:
Vented
Advantages