Everything IG January/February 2022

February 16th, 2022 by Nathan Hobbs

Rising to the Occasion: In the Quest for Better Windows, Can Insulating Glass Be the Hero?

By Dave Cooper

When considering the entirety of a window— which portion contributes most to thermal insulation? Many technically trained experts might answer, “It depends.” That doesn’t mean no one has a clue; rather, it means there are always special circumstances and exceptions to the norm.

With the costs for launching a new window design reaching $1 million, possibly the easiest way for a window fabricator to improve the thermal performance of a given product is to “upgrade” the IG, where several elements can be altered to improve U-factor. The short list of options includes number of panes, gap width and gas-fill properties, low-E coating(s) and edge systems (generally meaning spacer, desiccant, and sealants). At the same time, optimizing around one performance factor may lead to trade-offs in aesthetics, costs and durability. For the purposes of this mental model, where the baseline IG is upgraded, we will not consider those trade-offs.

Number of Panes

Increasing the number of panes in the insulating glass unit (IGU) will improve center-of-glass thermal performance for windows. For comparison, the Department of Energy states that a window having double pane IG with inert air filling and no low-E coating has a typical U-factor of roughly 0.5. Adding a double silver low-E coating improves this to 0.35, while adding Argon fill takes it to 0.3. Triple pane IG windows with a U-factor of 0.2 have been around for a long time and are common in many parts of the world, namely northern Europe. The main considerations in adding panes are overall thickness and weight. Recent developments in “thin triple” reduce overall width and weight, but also rely on a Krypton fill.

Cavity Gap and Gas Fill

The IG cavity gap width is important, as it influences thermal performance. Using a fill gas of Argon or Krypton improves thermal performance through lower conductivity. For air-filled units, ½-inch is the optimum gap width, having a center-of-glass U-factor of 0.3. For 90% Argon fill, the optimum gap is in just slightly narrower, with a U-factor of 0.25. With 90% Krypton fill, the optimum gap is about 5/16-inch with a U-factor of 0.22. Interestingly, for a triple pane IG, the optimum gap width increases by about 2 mm in each case. For triple pane construction, the air-filled unit improves center-of-glass U-factor 0.16, while 90% Argon improves to 0.13 and the 90% Krypton unit achieves 0.11.

Low-E Coating

Low-E coatings influence thermal performance by reflecting IR energy. Best thermal performance is achieved with a triple-silver coating (at the expense of visible transmission). For dual-pane IG, the relative improvement for a triple-silver over single-silver coating is most effective with Krypton fill, then Argon, followed by air. Differences typically are less than 0.1 Btu/hr ft2 F among single-, double- and triple-silver coating types in a dual IG. For triple pane IG, the improvement in performance versus number of silver layers is greatly diminished.

Edge Seal System

Taken as a system, the edge seal consists of spacer, desiccant and sealants. Thermal performance is always best at center of glass with its gas filled cavity. As the measurement approaches the edge, thermal conductivity takes over due to heat transfer through solid edge components. Obviously, improving the thermal insulation properties of the edge seal improves the edge of glass thermal performance. This has led to a large class of spacers known as warm-edge.

Combining the above thermal improvements can lead to better overall window U-factor performance, meeting Energy Star or code requirements. However, if the frame is allowing excessive heat energy conduction, conventional IG may not be enough. In that case, vacuum insulating glass (VIG) or hybrid VIG should be considered.

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

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