Designing spandrel glass

Condensation
When there is a large difference between lower outdoor and higher indoor temperatures, shadow box spandrel designs are sometimes prone to creating moisture/vapor pressure differences across the insulation. Insulation, which is included to act as a barrier to the unconditioned space behind the shadow box, does not necessarily help the glass perform better, although it can enhance the performance of the entire package. It can also act as a vision-blocking medium if no opacifying material is used on the glass.

Rips, tears, or holes in the insulation foil backing, the aluminum securing tape, or other unsealed areas (as well as improper tape installation) can create paths for moisture to migrate through the insulation into the shadow box air space. When this happens, and the temperature of the indoor glass surface (i.e. Surface 4) reaches dewpoint, the resulting moisture condenses on the glass. Under certain viewing conditions, it becomes visible from the outside, marring the appearance of the spandrel.

Even when spandrel boxes are fabricated with a perfect vapor barrier, moisture can infiltrate the air space through weep holes, breaches in the pressure-equalized glazing system, or leaks in the glazing system joints.

Over time, repeated cycles of condensation and drying may cause moisture residue to accumulate and permanently stain the glass. In addition to ruining the appearance of the spandrel, condensation cycling has the potential to irreversibly damage the glass, the coating, or both.

Debris and glazing lubricants
Between the time spandrel glass is glazed and the time insulation is installed, windborne dirt, fireproofing materials, and other construction debris can accumulate on the indoor surface of the glass. In addition, during construction of a building, water can collect on concrete decks and become infused with concrete dust and other alkalis.

When these waterborne materials come in contact with uninsulated spandrels, the spandrels may be contaminated with chemicals that can damage the glass or coating and create difficult-to-remove stains. Even when cleaning and stain removal on exposed units are successful, a faint residue may remain on the glass, becoming visible under certain viewing conditions.

Also, in dry glazing systems, lubricants are typically required to effectively install the indoor glazing wedge. If these lubricants are not carefully and thoroughly removed, they can leave visible deposits on shadow box spandrel glass.

VOC accumulations
Sealants, paints, and other materials used to manufacture and install insulation materials, glazing sealants, and glazing gaskets in shadow box spandrels often contain volatile compounds and chemicals that can be discharged under certain atmospheric conditions, such as exposure to high temperatures.

In shadow box air spaces on sunlit elevations—where temperatures can easily surpass 71 C (160 F)—these materials can be released, then condense on spandrel glass when temperatures fall. The resulting deposits can accumulate over time, causing stains and risking permanent damage to the glass and coating.

For this reason, specifiers should ensure all materials used in a spandrel assembly are:

• tested by participating suppliers for VOC releases; and
• approved/warranted for shadow box spandrel applications.

Improper insulation installation
When insulation is installed so it contacts spandrel glass, even in a small area, increased glass temperatures or ‘hot spots’ are likely to occur. This increases the probability of thermal stress-related glass breakage, as extreme differences in temperature gradients on a glass surface may force tension stresses to occur.

In addition to increasing potential for glass failure, hot spots can create localized color distortion, damage the coating, or cause water to accumulate where insulation touches the glass, forming residue and staining the glass.

Conclusion
Architects favor vision glass for the transparency and energy-efficient beauty it bestows on buildings. Glass spandrels provide a versatile, colorful, non-transparent tool for hiding the machinery and mechanicals that make designs functional.

By understanding the available spandrel glass options and their technical challenges, architects can help ensure the glass spandrels they specify will maintain functional integrity and attractiveness throughout a building’s lifespan.

Steven D. Marino has more than 30 years of experience with Vitro Architectural Glass (formerly PPG Flat Glass), in both automotive and architectural glass. He has held positions in manufacturing, quality, and other technical roles, including his current post as manager of technical support in Vitro Glass’s technical services department. Marino has a degree in mechanical engineering from Youngstown State University. He is an active member of both the Insulating Glass Manufacturers Alliance (IGMA) and Glass Association of North America (GANA), and sits on multiple committees and task groups for both organizations. Marino currently sits on the GANA Board of Directors as secretary and chairs the GANA Tempering Division Technical Services Committee. He can be reached at smarino@vitro.com.

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