Recommendations and conclusions
There are a variety of important design and construction considerations when integrating glass spandrels into building façades. A variety of documents have been published on glass spandrels over the years within the industry; at the time of this article’s publication, FGIA and AAMA are in the process of publishing a new technical document titled “Spandrel and Shadow Box Design Considerations.” Although much of the technical design of glass spandrels can be analyzed and calculated, thoughtful judgement is required based on the results of analyses, empirical testing, real-world experience, careful consideration, balancing, as well as prioritization of project-specific requirements.
Industry guidance, architects, engineers, consultants, manufacturers, contractors, and even building owners, often have varied recommendations and opinions for glass spandrel design based on their collective and individual experiences. For example, some insist on vented shadow boxes, others insist on sealed shadow boxes, while others recognize the decision ‘to vent or not to vent’ depends on the project-specific conditions. Some wonder why they should spend extra money on metal back-pans whereas others may never use foil-faced insulation again. The final detailed technical design of a glazing system, including the glass spandrel, is typically delegated to the glazing system subcontractor. It is their responsibility for the system’s design and performance. However, early collaborative discussions should be had amongst all parties, including the owner, architect, consultants, and the various manufacturers and contractors, to help set appropriate expectations and ensure project success.
To vent or not to vent
The decision ‘to vent or not to vent’ glass spandrel assemblies depend on a variety of factors. Both options carry their own advantages and disadvantages as summarized below:
| Vented | |||
| Advantages | Disadvantages | ||
| ● Ventilation allows air pressure relief of solar heat buildup, reducing cavity temperatures and reducing potential for glass and/or metal panel bowing, reducing potential of material off-gassing, and reducing differential glass thermal stress. | ● Dust/debris and condensation/streaking are visible in transparent/translucent glass (shadow boxes). | ||
| ● Ventilation allows cavity condensation to evaporate/dissipate in-service. | ● Ventilation requires the additional cost (if not already included in the design) of a sealed metal panel (intermediate or back-pan). | ||
| ● A ventilated design accommodates realities of construction imperfection (which will allow some air leakage through the system), providing a means to dissipate potential moisture. | ● Incorporating vents and additional seals into the air-water-vapor design of field-glazed systems (e.g., stick-built curtain wall) presents additional challenges. | ||
| Non-vented | |||
| Advantages | Disadvantages | ||
| ● Fully sealing the assembly prevents dust/debris and other contaminants from the outside environment from entering the cavity in-service. | ● Sealed design does not allow for air pressure relief due to solar heat buildup, increases cavity temperatures, increases potential for glass and/or metal panel bowing, increases potential of material off-gassing, and increases differential glass thermal stress. | ||
| ● Does not necessarily require a sealed metal panel (intermediate or back-pan) and foil-faced insulation (often more cost-effective, albeit with other durability concerns) can be used instead. | ● Properly designed and constructed ventilation allows cavity condensation to evaporate/dissipate in-service. | ||
| ● Sealed designs can be incorporated into both factory-glazed (e.g., unitized curtain wall) and field-glazed (e.g., stick-built curtain wall) systems. | ● A perfectly, fully sealed design is not realistic; realities of construction imperfection will allow some air leakage through the system, creating potential risks for trapping moisture (without a designed means to dissipate) and trapping dust/debris within the cavity. | ||
John Jackson is a senior project manager with Simpson Gumpertz & Heger’s Building Technology group in Washington, DC. His work focuses on the design and engineering of innovative enclosure systems, with particular expertise in curtain walls, custom glazing systems, structural glass, and glass investigations. Jackson can be reached at jajackson@sgh.com.
