Everything IG October 2021October 8th, 2021 by Nathan Hobbs
Next-Level Glass: Is the Industry Capable of Advancing at Breakneck Speed?
By Dave Cooper
The evolution of the central part of windows, namely the part you can see through, is well known. And a crawling evolution of improved performance is reflected in the Energy Star requirements as well as in building codes. But is a slow clock-speed inherent to the glass industry?
It’s worth noting that when the government passed the so-called .30/.30 performance requirements for tax credits in 2009, glass manufacturers rapidly developed and launched new low-E coated glass products at a breakneck pace to meet those demands. This proved that rapid advancement can be achieved in our industry given proper incentive.
The factors determining overall thermal performance of windows as it relates to Energy Star is a balancing act between insulating glass (IG) and thermal performance of frames. In general, center of glass performance is better than the frame. For this reason, window manufacturers often specify IG to achieve the overall window U-factor desired.
But, how good can we get with IG? Current mass-produced IG performance pushes R-8 center of glass measurements with triple pane, high performance low-E coating(s) and a Krypton gas fill. Low-E coatings are tuned for solar heat gain properties. Further enhancement is possible with even more lites or, occasionally, transparent films in the cavity. The current target is mass-produced R-10 windows, as put forth by the Department of Energy’s Building Technology Office roadmap.
Currently, a thin-triple concept promoted and patented by Lawrence Berkeley National Laboratory (LBNL) is seen as a stopgap that allows window manufacturers to adopt a higher performance R-8 glass package compatible with current frame designs, since it is about ¾-inch thick, like a typical dual-pane IG. There is one drawback to thin-triple designs, however: the center lite is 1 to 2 mm thick, which is not a typical soda-lime glass product and presents handling challenges during fabrication.
Focusing on the IG thermal performance, what is the next step-change?
Available now, vacuum insulating glass (VIG) manufacturers can achieve R-15 center of glass with tempered VIG products. Other products on the horizon include IG filled with aerogel-type materials which have super low conductivity and take the place of a gas fill. In those cases, performance is predicted to reach R-10 for a ½-inch cavity fill.
The technologies mentioned push window performance to a whole new level. Of course, there is always the question of cost. These step-change products are still somewhat in the early stage with limited volume. It is likely that the cost for new IG technology will follow the current cost versus performance curve, which is roughly one dollar per R-value per square foot for established products and technologies. For example, R-1 clear glass is roughly a dollar per square foot, whereas a high-volume triple-pane IG at R-8 is in the $8-per-square-foot range. Once greater adoption takes place, such as with VIG, demand will create opportunities for more factories, which will help to bring down costs through volume and efficiencies. Soon Energy Star and building codes can reassess their goals for doors and windows, adding much greater thermal efficiency.
Dave Cooper is a consultant and president of Fenestration Consulting Services LLC.
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