Exploring the Options: How Insulating Glass Can Be Used to Meet Energy Star 7.0

By Dave Cooper

There is a great desire (or push, depending on your perspective) to further improve the thermal performance of doors and windows, especially as
the pending release of Energy Star version 7.0 raises the bar. Why? Because we can offset building more power plants if we simply do a better job at conserving the energy already being produced. Energy Star aside, the technology exists for making advancements. So, what’s stopping us? The answer might be investment, which is needed up and down the channel to make the necessary improvements.

Window fabricators have depended upon glass manufacturers to introduce and commoditize better performing low-E coatings and products that can be incorporated into standard ¾-inch dual-pane insulating glass (IG). That’s partly because upgrading glass is the easy path for a window fabricator to adopt a higher performance system with little to no thermal improvement to the frame. This avoids the time-consuming and expensive process of window redesign, procuring components, retooling and subsequent product launch. Unfortunately, we are reaching the point where dual-pane IG cannot keep up with Energy Star or building code advancements. The continuous march forward in whole window thermal performance leads us down a new path in IG.

A Look at the Menu

Energy Star 7.0 does include a trade-off path with solar heat gain in the Northern climate zone. For this analysis, we will simply consider the base whole window thermal performance requirement of less than 0.22 Btu/h-ft2 -F for the Northern Zone—the most stringent requirement.

First, consider a standard baseline IG—center of glass (COG) thermal performance for a ¾-inch-thick dual-pane IG with a double silver coating on surface two and ½-inch cavity with 95% argon. A quick calculation in Lawrence Berkeley National Laboratory’s (LBNL) Window 7.8 program shows this IG has a Ucog of 0.238 Btu/h-ft² -F. This is considered a good performance glazing by most and is used throughout our industry. At 3/4-inch thickness, this IG is applicable to most window glazing cavities.

Option One: Maxing Out

The first option for improvement includes going all-out with a dual-pane design. Optimizing the low-E coating(s) and gas filling may allow a window to achieve needed performance requirements, so long as the frame is “good enough.” A 3/4-inch-thick IG may include a high-performance triple-silver low-E coating on the inner cavity (surface two, for our calculation), along with 95% argon gas filling. Adding a surface-four interior coating and finally krypton fill in an optimized 3/8-inch width cavity improves the performance to a very respectable 0.175 COG.

Option Two: Tripling Up

The second option is to incorporate a standard triple-pane IG at 13/8-inch thickness, but this will likely require frame and/or hardware redesign to handle the thicker and heavier glazing, along with subsequent retooling in the factory. With a triple silver low-E coating on surface two and 95% argon fill, a Ucog of 0.18 Btu/h-ft² -F is achieved— slightly worse than the highest performing dual pane IG above. However, adding 95% krypton fill with optimized 3/8-inch cavities, along with a surface-four coating, pushes this triple-pane Ucog to 0.138 Btu/h-ft²-F.

Option Three: Thinning Out

The third option includes what’s described as “thin triple.” This is a triple-pane IG that is thinner than a standard triple-pane IG at 1-inch thick or less. This IG has 1/8-inch outer lites (surface two with triple silver coating) and a very thin, non-standard 3/64-inch center lite. Thin triple IG can be made even thinner (3/4-inch wide) with a slight penalty in thermal performance. Ucog ranges from 0.162 to .0138 Btu/h-ft²-F, which is very similar to option two above, with the benefit of easy adoption to standard window glazing pockets. But there are challenges, namely handling and cutting such a thin center lite, along with the inability to temper for safety glazing applications.

Option Four: Vacuuming Out

The fourth option is a break from the traditional. This path leads to incorporating a 3/8-inch vacuum insulating glass (VIG) panel into an existing frame, either alone (two lites of glass, or monolithic VIG) or combined with a third lite and traditional spacer, desiccant and gas filling, known as a hybrid VIG (HVIG). There are exciting pathways that unfold. Incorporating a VIG into a traditional vinyl frame yields an overall fixed window Uw of about 0.18 Btu/h-ft²-F, approaching R-6 performance.

Combining a VIG-specific frame with VIG yields a remarkable improvement in thermal performance. If this “VIG Frame” were readily available (we are closing in on this as a viable option), whole window thermal performance approaches R-10. An inspiring aspect of this includes the fact that a vinyl window manufacturer could simply replace standard IG lineals and utilize existing tooling and hardware to manufacture a breakthrough window product that will remain compliant to Energy Star for many years to come.

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

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