Spandrel Glass Design and Aesthetic Considerations

By Ellen Rogers

There’s nothing special about spandrel except the critical job it does of hiding unsightly mechanical spaces between floors. That area sometimes uses other materials, but glass is a common choice since many projects call for an all-glass aesthetic.

According to Jennifer Highfield, architectural design associate with Viracon based in Owatonna, Minn., there are a few considerations when fabricating for spandrel applications.

“It is important to know if a contrast is desired between the vision and spandrel areas or if the intent is to have a vision-to-spandrel match,” Highfield says. “If the goal is to match the vision and spandrel, we suggest keeping the outboard consistent and choosing a coating with some reflectivity, creating a more uniform appearance. If a contrast between the vision and spandrel is desired, using a less reflective coating will achieve this.”

Marni Windschitl, an architectural design associate with Viracon, adds that the trend is often to have a vision-to-spandrel match.

“Having the low-E coating on both the vision and spandrel glass allows for this,” she says. “Low-E coatings with lower exterior reflectance and higher visible light transmittance are less effective in blending vision-to-spandrel. Spandrel is intended to be used in non-vision areas with a uniform background behind. Center of glass U-Value is the only performance value that pertains to this condition. Using insulating glass with a low-E coating will meet most energy codes.

Windschitl adds that monolithic glass is rarely used for spandrel.

“An insulating unit allows the low-E coating to be on the #2 surface, providing improved
solar performance,” she says. “The spandrel is then on the #4 surface with a uniform background behind.”

Design Options

Common types of spandrel glass include backpainted (opacified) glass and shadow
box construction. While backpainted glass has been around for decades, some important
considerations remain.

“Selecting the right color enamel is important because certain colors pair better with
different coatings,” says Highfield. “It is also important to know the glass size, as this will
determine what manufacturing process is used,” she says.

The shadow box assembly is similar but uses clear glass instead of opacified. Windschitl says shadow box conditions typically use the same glass make-up as the vision units, combined with a panel behind the glass to provide the opaque background.

“The cavity between the panel and the in-board glass lite provides depth to the spandrel
condition, which can often soften the transition between vision and spandrel aesthetically,” she says. “The inboard lite must be heat treated, and it’s important to know the color of the shadow box material.” Based on the color and design goals of the spandrel condition, fabricators can recommend the appropriate low-E coating.

Since shadow box assemblies use clear glass, potential corrosion and condensation
can be a concern.

“If condensation happens, and you have a backpainted glass, nobody sees it. But if it’s
clear glass, I’d be worried about condensation streaks if that were to happen,” says Christoph Timm, principal with Skidmore Owings and Merrill’s New York office.

Visual Aesthetics

Long spans of floor-to-ceiling glass are a popular design aesthetic in many tall commercial buildings. To achieve the desired aesthetic and still cover the spandrel area, some architects are turning to a kiss mullion design. With this effect, instead of two pieces of glass, one for the vision area and one for the spandrel, only one piece is used to create both the vision and spandrel. This detail can be created using either a shadow box across the spandrel portion or by opacifying the area. “It’s a beautiful detail. I love the detail,” says Timm. The firm has used the aesthetic on many projects, including Manhattan West. “It’s beautiful because you get the floor-to-ceiling glass.”

Plus, he says the overall building performance is better than a traditional spandrel
because you don’t break the glass.

Despite the visual appeal, there can be challenges.

“When the design calls for a single unit to be both vision and spandrel, the enamel must
be applied using a special fabrication process,” says Highland. “Combining vision and
spandrel into one unit, while feasible, creates challenges because the space behind each of
these areas may not be conditioned the same and can cause the glass to act differently.

If the spandrel area is a true spandrel area, it will need to be blocked off from all transmitted light completely,” She continues.

“On a project that requires vision to extend to spandrel in one unit, it can be very difficult
to completely block off transmitted light for only a portion of the glass unit. With more than 50 years of fabrication experience, we believe the option that provides the best aesthetics is using separate vision and spandrel units.”

Timm also points to some challenges with kiss mullions.

“A design like this also means bigger, thicker glass, and that puts you in a different category with cost. So, you must be aware of that. It’s a very attractive detail for architects, but it comes at a price—both financial and in terms of the added carbon.”

Finding Balance

While the design community has many options for filling the spandrel area, glass remains popular.

“Glass is probably the most economical from a design standpoint,” says Timm. “I think
it’s design-driven.”

The use of other types of materials incorporated into the façade is a trend he’s also seeing.

“I think we’re getting past the all-glass building as a trend, right or wrong. I think a beautifully detailed glass box can be extremely high performing, especially if you have the
right window-to-wall ratio,” he says. “And people don’t understand this; they think glass
boxes are bad. But if you insulate it properly, the glass becomes a cladding material.”

Shadow Box Design Resources

“Shadow box spandrels are a popular aesthetic option to give visual depth to opaque areas of the façade,” says Rich Rinka, technical/industry affairs manager with the Fenestration Glazing Industry Alliance (FGIA). “However, shadow box spandrels pose an inherent risk of visible internal fogging and other obstructions under certain conditions, regardless of design.”

To address such concerns, FGIA recently published a new technical report, AAMA TIR-A20-23, Glazed Spandrel and Shadow Box Design Considerations, addressing best practices for the system design of spandrel areas. This includes the use of conventional spandrel glass and vision glass together with shadow box panels.

The report, developed by the Curtain Wall/Window Wall Spandrel Review Task Group, provides an overview of design considerations for glazed spandrel infills, including both spandrel glass and shadow boxes. The document also makes recommendations, where appropriate, for testing and/or analysis criteria considering aesthetics, serviceability and
performance.

Rinka notes that kiss mullions are outside the scope of FGIA’s recently published TIR-A20 document.

“The FGIA consensus process has not yet been applied to these creative spandrel options,” he says.

Speaking of Carbon …

Christoph Timm, principal with Skidmore Owings and Merrill’s New York office, has been outspoken on the need to decrease embodied and operational carbon in buildings.

“We need a carrot, and we need a stick.” That’s what Timm said this past summer when speaking at Glass Performance Days in Tampere, Finland. Timm shared passionate thoughts about what’s needed for a sustainable future, including innovation in products and processes.

So when looking at a spandrel design, his first thought is how much glass must it have?

“And that’s because of the carbon context,” he says. “I also wonder, is it necessary to have a shadow box, which I believe is the most carbon-intensive design because of all the layers–the back pan and usually two layers of glass, sometimes three.”

For Timm, finding ways to reduce the carbon content of architectural designs is an important focus.

“I ask the questions to find out what kind of glass we have to have in the spandrel and how much we can reduce the carbon content,” he says. “The shadow box is the most intensive. The next is insulating glass with ceramic paint on the number three or four surface.”

He continues, “We could go even further and make it backpainted monolithic, but then you have to think about it being safety glass because of thermal stress. This is likely the least amount of carbon, but some fabricators don’t like this because it’s putting fully tempered glass on the outside of a building. Some people just categorically don’t do it.”

Shadow boxes may be a carbon offender but are still frequently used. Timm often challenges their use, asking, “Why are we doing this?”

“Nobody’s ever said, ‘I’m missing a shadow box,’” he says. “When you have a more reflective building or darker glass, then I think it’s beneficial in terms of design not to show any depth [which the shadow box does].”

He adds, “Plus, the shadow box [color] is always so hard to match [the vision area]. I think certain designs do not want the shadow box. I’m trying to educate teams that I work with about that.”

Ellen Rogers is the editorial director of USGlass magazine.
Email her at erogers@glass.com and connect with her on LinkedIn.

To view the laid-in version of this article in our digital edition, CLICK HERE.

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