Achieving Thermal Performance in Commercial Windows with Innovation and Design
By Scott Sowers
Boosting the thermal performance of commercial windows is a team effort among the glass companies, component suppliers, and those engineering and manufacturing the frames. The process is shaped by building codes, competition, climate change, and cost factors. As the process continues to evolve, even the experts acknowledge some challenges are difficult to overcome.
“There’s a practical limit to how energy-efficient glass can be,” says Helen Sanders, with Technoform North America in Twinsburg, Ohio. “It is certainly possible to keep adding more cavities, multiple low-E coatings, or vacuum insulating glazing (VIG), but at a certain point, these provide diminishing returns. We must balance these against the relative lifetime of multi-pane constructions. Decreasing the center of glass U-factor won’t achieve the optimum performance if the frame and edge of glass performance are not improved. If not addressed, heat transfer at the edge of the fenestration; frame and edge of glass, can negate these incremental improvements in the center of glass.”
Technoform doesn’t make windows, but the company does make the insulating components that keep them from leaking air and transferring heat in and out. To make windows more efficient, every component and surface is evaluated and tweaked. “The glass and materials around the glass are equally important,” says Sanders. “Once the window has a low-E, insulating glass unit in the infill, dealing with the frame is much more important than incrementally increasing the center of glass performance. The ratio of frame-to-glass determines the relative importance since the U-factor is an area-weighted average of the frame, edge of glass and center of glass. In smaller units, the frame and edge can dominate more than in larger units.”
Weighing strength, performance, and environmental concerns are all formidable challenges that get addressed simultaneously. The industry is also trying to be mindful of climate change concerns relative to adding more panes. “Thermal performance is only as good as the durability of the system. And shorter product lifetimes result in higher embodied carbon because of shorter replacement cycles,” says Sanders. “The glass is the greatest contributor to the embodied carbon in insulating glass. Moving from double- to triple-pane adds approximately 50 percent more embodied carbon.”
The push to develop higher-performance windows comes from multiple fronts. In some cases, healthcare facilities lead the way as hospitals demand specialty windows that can withstand high humidity without producing condensation. Water vapor turning to a liquid form is a constant battle in the window business and typically is traced to a seal gone bad. Sanders says, “One of the advantages of focusing on improving the thermal performance of
the frame first is that both thermal performance and condensation are managed. Thermal bridging in the frame and edge of glass typically drive condensation, and taking the approach of managing the edge helps develop a more balanced performance.” Codifying energy efficiency measures is another major factor.
“Primarily, we see the biggest improvement through building codes,” says Josh Wignall, marketing director at Quaker Windows and Doors in Freeburg, Mo. “But we are seeing areas of the country taking energy codes and performance into their own hands by placing net-zero initiatives in place.” Net-zero—getting a building to produce as much energy as it consumes— requires super tight building envelopes, including the glazing, combined with power generation that is usually achieved through roof-mounted solar or green purchase payment arrangements.
The move toward net-zero and reducing greenhouse gas emissions, in general, is also affecting the design side of the equation in terms of specs. “I believe as more organizations and architecture firms commit to net-zero targets, we will see an increase in glazing performance efficiencies within specification language,” says Gregory Plavcan, sustainability strategist with Gensler, based in Washington, D.C. “I hope that as more building owners and designers ask for efficient glazing, manufacturers will work together to create efficient and cost-effective solutions. We need efficient, low carbon
materials that are widely available and make big strides toward a carbon-free future.”
While residential windows are readily available in a wide variety of framing materials, including wood, vinyl, and fiberglass, it’s not the same on the commercial side. There is one material that rises above all others. “Aluminum has been a popular material for commercial windows because of the structural and sustainability qualities it provides,” says Wignall. “However, it transfers heat and cold extremely well, which can be a problem
for thermal performance.” To compensate for aluminum’s poor insulating qualities, window manufacturers are constantly tinkering with making tighter grooves and channels for the glass to sit in and then insulting those edges to stop heat transfer.
Wignall’s company developed a high-density foam product with Azon, a manufacturing company based in Kalamazoo, Mich. Jerry Schwabauer, Azon vice president, says the introduction of products such as his company’s Azo-Core thermal barrier, can provide significant improvements in thermal efficiency. “The conductivity is up to four times lower than standard polyurethane and up to ten times lower than polyamide,” he says.
The new thermal barriers technologies can help window companies achieve U-factors similar to PVC, wood, and fiberglass without triple glazing, which raises costs, weight, and embodied carbon.
Technoform has also been marketing high performance, low conductivity, thermal barriers by using emerging polyamide (PA) thermal barrier technology, which was originally pioneered in the 1970s. Initially introduced in widths of 10 mm, the new renditions now stretch to 100 mm wide. According to Sanders, the increased widths reduce conductive heat flow. They’ve also developed more complex frame profiles.
At first, legs—which are extensions into the framing cavity—were extended into cavities between the extrusions to slow convective heat transfer. The newer models feature cavities filled with foam. More recently, the firm has pioneered low-emissivity foils combined with PA barriers to reduce heat transfer through radiation, the final frontier in insulting technology.
The latest innovations involve reducing the thermal conductivity of the PA itself. According to Technoform, the lower conductivity PA, sometimes called “low lambda,” allows fabricators to achieve lower assembly U-factors in a given frame design without making the barriers wider.
Adding another lite in a triple-glazed formation is an option that continues to attract designers despite the ecological challenges. The extra layer of glass not only improves insulation, it also offers increases in acoustic performance. But window costs versus saved energy values are always a balancing act. “While a triple-pane glass system may have a greater capital cost than a traditional insulating glass unit (IGU), deeper analysis may indicate that cost can be offset with peak load reductions and eliminating the need for perimeter heating,” says Kyleen Rockwell, design engineer at HKS, based in Dallas.
Specing the Spacers
Double and triple-glazed windows rely on spacers that separate and insulate the edges of the individual panes. Warm-edge spacers are a vital component of IGUs and are typically made from plastic as opposed to aluminum. Adding a plastic layer helps seal things up, and the industry has taken notice.
For example, plastic hybrid stainless steel (PHSS) spacers use a stainless-steel wrapping on the back and sides of the spacer with engineered plastic bridging the top. The design combines the durability of a stainless-steel box spacer with the thermal performance of a non-metal spacer.
According to Sanders, PHSS can improve the assembly U-factor of captured aluminum fenestration by between 0.02 and 0.03 BTU/oF.hr.ft2 over the aluminum spacer, and by up to 0.05 BTU/oF.hr.ft2 for structural silicone systems.
“There is certainly a trend toward warm-edge spacers,” adds David Hewitt, vice president at Tecnoglass/ES Windows, which is based in Barranquilla, Colombia, South America. “Probably the best available for durability, longevity and thermal performance are hybrid plastic and stainless-steel spacers that are specifically engineered for high-performance IGUs,” he says.
In a Vacuum
Another significant window innovation getting a lot of attention is vacuum insulating glass (VIG), which is created by pumping out the air between two separate panes. By some accounts, the original design concept dates to 1913, and the experiments are still going on today.
“The use of VIG within an IGU is on the horizon,” says Sanders. “It may not boost performance significantly over conventional triple or quad glazing, but it will support achieving performance with lower weight and less thick units. VIG used monolithically could also be a good solution for retrofit applications.”
However, cost and return on investment remain the considerable constraints on vacuum technology and other technological advances.
“Costs and aesthetics need to improve to get critical mass and become mainstream with building owners,” says Hewitt. “The industry continues to innovate with new technology, but the market dictates price.”
“Smart” or “dynamic” glass has been making headlines and has even caught the attention of Capitol Hill. In April 2021, a bipartisan group of U.S. Senators reintroduced the Dynamic Glass Act. The original bill was introduced in 2019 by representatives from Mississippi, which is home to an electrochromic glass factory.
If passed, the legislation would make electrochromic glass eligible for federal
energy tax credits. Electrochromic glazing works by passing an electric signal through the glass to change its level of tint depending on sun exposure. This could make a building more energy-efficient as needed in real-time.
However, the downside to these new technologies gaining market acceptance is the high cost versus return on investment. “One of the biggest developments in glass that [could] boost thermal performance is the new VIG launched over the past few years,” says Wignall. “While the technology is not in full-scale production, it will be interesting to see how quickly this glass [is] accepted in the market. Like we have seen with some of the dynamic glass options, getting costs down enough to make financial sense will be imperative for the product [to be] adopted quickly.”
Passive and Active Shading
Adding exterior shading and adjusting building placement relative to the sun
is a big-picture, passive way to increase thermal performance. Architects are also tuned into the possibilities offered by manual and automatic responses. “It is common to use active shading and glare control devices on our projects,” says Plavcan. “I see many instances where we are implementing automatic shading devices in buildings with manual overrides to give users some level of control over the amount of daylight entering the space. Teams also are actively investigating permanent shading devices during the design process. These passive strategies are becoming increasingly more critical as we work to design low carbon buildings.”
The move toward better thermal performance isn’t happening at the same pace or taking the same form for everyone, everywhere. Commercial and residential windows are regulated by different standards, and cost is always a factor. “There is still a prevalence of non-thermal frames in the U.S., especially in the southern regions,” says Sanders. “Non-thermally broken frames limit the thermal and solar control performance of fenestration in hot
Crunching the numbers regarding the cost of the glazed openings against potential energy savings is still knocking a lot of efficient windows off the building plans. “Super thermal performing commercial windows have been available for a long time. However, there is a tipping point to what price the market will bear,” says Hewitt. “Certainly, building codes and competition affect the demand, but the better thermal products are sometimes value-engineered out of the project.”
Scott Sowers is a contributing writer for USGlass magazine.
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