Considerations When Specifying Jumbo IGUs and Their Components

By Jordan Scott

Architects and owners are continuing to drive demand for large glass sizes and the glass industry has adjusted its products and processes in response. While jumbo insulating glass units (IGUs) are just larger versions of a decades’ old product, there are several considerations users should keep in mind when specifying jumbo IGUs and their components.

Jumbo Definition and Demand

Though Viracon, a fabricator based in Owatonna, Minn., defines jumbo glass as any lite larger than 50 square feet, there is no standardized size that is considered jumbo among all fabricators. The company can fabricate glass sizes up to 130 by 236 inches, which it says is the largest size produced by domestic float manufacturers.

Lance Lawrence, an architectural design associate for Viracon, says the demand for jumbo IGUs is being driven within certain commercial applications. For instance, jumbo, floor-to-ceiling glass is being used for many building podiums. He’s also noticed a trend of combining the vision and spandrel areas and skipping an interior mullion rather than using two units to cover these areas. The benefits include a reduction in metal, which leads to overall improved performance as metal has lower levels of thermal transmission.

Regardless of Size

Helen Sanders, strategic business development for Technoform in Twinsburg, Ohio, says that when architects specify jumbo IGUs they should first focus on the durability of the unit before focusing on thermal performance, strength and sightlines. She says it’s important for architects to understand the durability in regards to moisture ingress and structural performance requirements of the application.

“For both a big unit and a small unit you want your spacer to stay in place. There is pumping action going on within the units due to changes in atmospheric conditions so you want to have good desiccant carrying capacity and a good seal to prevent moisture from coming in and gas going out,” she says. “You’d have the same concerns on any glass size.”

Sanders has noticed that architects are specifying for high thermal performance but says they need to be aware of trade-offs.

“Make sure that as you improve thermal performance you’re not trading that off with reduced durability. That’s not a good trade-off,” she says.

She also points out that while automation of IGU lines creates straighter sightlines, care must be taken to make sure that automated systems meet the durability and structural needs of an application.

The Spacer Factor

At Viracon, an aluminum box spacer can be used up to 120 square feet and a stainless steel box spacer can be used up to 70 square feet. Above those sizes he says a thermoplastic spacer is needed. Viracon’s Thermal Spacer (VTS) has allowed the company to fabricate jumbo glass sizes.

“The reason you cannot use a box spacer for sizes that large is that those metal spacers, due to their weight at that size, are difficult to manipulate. The weight causes the metal to bend and kink during the fabrication process,” says Lawrence.

Sanders points out that it requires more people to handle a larger box spacer frame as a larger frame is more flexible. She says that fabricators tend to use stiff spacers for larger units as they’re easier to handle. Technoform offers its TGI-Spacer M, a bendable warm edge spacer bar which can come with a wire to add stiffness.

A thermoplastic spacer can be applied directly to the glass in an automated process which creates a more uniform look. Traditional box spacers require a PIB sealant, which Lawrence says never fully cures and can flex under windloads.

“VTS forms a chemical bond rather than a mechanical bond. Once it cures there’s no movement,” he says.

Edge of glass U-values are improved when using a thermoplastic or other warm edge spacer compared to a traditional metal box spacer.

“In addition to its other benefits, Argon retention is about 30% better with a thermoplastic spacer compared to traditional box spacers,” says Lawrence.

He has noticed a movement toward thermoplastic spacers driven by increasingly stringent energy codes. Unless a project requires stringent U-values or the IGU size dictates it, metal box spacers are still a popular option.

Steve St. Pierre, design and application engineer for Ensinger North America of Washington, Pa., says large glass requires a larger frame, but frames with large hollows inside them can work against performance energy.

“Many people think that the air in a big hollow is good insulating material. Unfortunately, it really isn’t in some cases. Large hollows in aluminum create convection. The exterior of the aluminum is heated by the sun and the interior is cool from the inside of the building. Now, as the air rotates in the cavity, it will grab the hot air from the exterior and transfer it to the cool side and heat that up,” he explains.

St. Pierre points out that those hollows can be broken up with flag, or fins that protrude into the hollow, to prevent convection.

Another important design consideration is to keep the glass centered over the thermal break and to have the thermal break at least the width of the glass itself, says St. Pierre.

Impact on Glaziers

Different IGU configurations don’t make a significant difference in how glaziers install jumbo IGUs, according to Lawrence. However, the added weight of large units is something glazing contractors should be prepared for when working with jumbo IGUs. They need to have equipment that can load, offload and manipulate the glass.

Lawrence’s advice for glaziers is that if they see a spec calling for ¼-inch over ¼-inch glass for lites more than 90 square feet, that they point out that the configuration will likely not be strong enough to support a unit of that size.

“They need to be knowledgeable enough to know that a traditional 1-inch construction won’t work for large units,” he says.

Other Considerations

As jumbo IGUs continue to grow in popularity, Lawrence points out that the low-E coating options available may be more limited. Viracon doesn’t offer every low-E coating at its maximum sizes but he says the majority of the most popular coatings are available.

“Make sure that what you want is available before you start to design and specify glass this large,” says Lawrence.


Fabricating jumbo insulating glass units (IGUs) requires machinery that can handle the size and weight of large glass lites.

Forel, based in Treviso, Italy, developed its No Limits IG line to meet the demand for jumbo glass. Its line is able to handle sizes up to 228 by 588 inches with a single glass thickness of 40 mm and an overall IGU thickness of 100 mm. The weight load capacity for assembled IGUs is up to 330 pounds per linear foot.

Marco Schiavon, CEO of Forel North America, says his company developed a continuous sealant extrusion system that doesn’t stop to refill and then restart. It can also handle up to three different materials with minimized human interaction.

“The level we’ve achieved should cover architects’ glass demands for the next ten years. Our line is oriented on the future demands of architects,” he says.

Lisec, another European machinery manufacturer, offers an IGU line that can fabricate glass up to 130 by 787 inches.

“The production of jumbo IGU glass is now a standard procedure on the lines. Decisive for the quality of the final product is the handling of the glass before and after the line as well as the selection of the frame system. Lisec also offers systems for the pre-sorting of the panes and for the removal of the panes in the semi-automatic area. Lisec’s portfolio also includes application systems for automatic frame setting for fixed, flexible and thermoplastic frame systems,” says Clemens Macarei, general manager of Lisec Germany in Kassel, Germany.

Jordan Scott is an assistant editor for USGlass magazine. She can be reached at

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