Getting Beyond Double Pane: The Question Isn’t If, but When

By Dave Cooper

What will come next for door and window glazing to achieve improved thermal performance?
The limits of double-pane insulating glass (IG) are well known. (High-end, low-E coatings and gas
filling options, along with best-in-class warm edge spacers, can only go so far.) At the same time, a carrot-and-stick analogy and the steady march to improve thermal performance is easily understood. The carrot includes tax rebates, such as the IRA and U.S. tax code 179D (commercial buildings energy efficiency tax deduction), or utility rebates; the stick represents codes, stretch laws and Energy Star requirements.

So how can a door and window company prepare for the future? Historically, change has been gradual, but steps have been taken with an eye toward what lies ahead. The gradual widening of the glazing pocket in frames to accommodate a standard triple-pane IG assembly is an example. Even though it may not be absolutely necessary at this time, the frame is ready. Simply glaze it with double-pane IG and a wider glazing stop for now. Beyond standard triple-pane, what’s next for glazing? There are three options that have become widely known.

Thin Triples

With thin-triple glass, thermal performance approaches that of standard triple-pane IG units (IGUs), but this is a glazing at ¾-inch wide with three pieces of glass. The center lite is quite thin—between 1 mm and 1.8 mm. However, thin-triple assemblies result in narrower widths for the gas cavities, where Krypton performs best. Krypton is quite expensive right now. Additionally, there are handling concerns with very thin glass on standard fabrication and IG equipment. Another issue with thin glass involves its ability to meet safety glazing requirements. The testing of thin
glass in a standard ANSI Z97.1 impact device may not be possible. Presuming the thin, soda-lime glass can be tempered (not easy) or is a borosilicate type, testing, rating and certifying it for safety glazing is problematic.

Then there is aerogel. For this type of IGU, a water-clear aerogel material is sandwiched between the glass lites. U-factor ratings at or better than triple glazing are possible, depending upon the width of the IGU. At this point, there have been demonstrations of low-haze product coupons from a few start-ups, but unfortunately the process for scale up is presenting significant challenges. For this reason, this technology is likely several years from becoming commercially viable.

Vacuum Insulating Glass (VIG)

VIG is a step change in center-of-glass (COG) thermal performance, with a U-factor rating of 0.08 or lower, irrespective of width. There are over two dozen manufacturers of VIG, with the best-known having been in business for two decades (demonstrating very low failure rates). The product may seem innovative, but it has been available for quite some time. VIG units as large as 126 inches by 67 inches are currently available. Widths range from ¼ inch (6 mm) to ½ inch (12 mm). Unfortunately, all VIG is currently imported, often with an escalated 25% tariff rate. Fortunately, VIG equipment is available from several manufacturers, and it will be produced in the U.S.
within the next three to five years.

There is a path to get to much better thermal performance with the glazing alone. It is not a question of if, but when.

Dave Cooper is a consultant and president of Fenestration Consulting Services LLC.
dcooper@fenconsultant.com

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