How Low Can We Go?
Reducing U-Values to Meet Future Performance Targets
by Ric Jackson

The window industry faces constant questions about future performance regulations. When are the next standards coming? How stringent will they be? How will they affect window designs?

These are all legitimate questions, especially as the U.S. Department of Energy speculates about establishing R-10 windows—those with 0.10 U-values—as a potential future benchmark.

This possibility raises another important question about window U-values: How low can we go?

R-10?
Don’t fret if you don’t have R-10 windows in your plans yet. It may not even be possible to produce a window of this caliber using today’s components. A triple featuring the best performing spacer and insulating frame, the lowest-emissivity glass and argon gas filling can achieve a 0.17 U-value (R-5.9). The same window using krypton gas yields a 0.15 U-value (R-6.7). Adding a pyrolytic low-E coating (hard coat) to the interior-most surface (surface 6) in the krypton-filled triple nudges the U-value down to about 0.14 (R-7.1). That is about the lowest U-value achievable using conventional window designs with existing materials.

More Glass?
Adding glazing to an insulating glass unit (IGU) is a potential means of reducing U-values. Each additional glass lite, combined with a spacer, provides another insulating airspace to reduce heat transfer. For example, adding a fourth lite and moving the hard coat to surface eight in the krypton-filled triple, noted previously, reduces the U-value to 0.13 (R-7.7). Add one more lite—with the hard coat now on surface 10—and the U-value drops to 0.12 (R-8.3). Continuing in the same fashion, it is conceivable that a unit with seven layers of glass—a septuple-glazed unit—could meet the 0.10 U-value (R-10) target.

Each new glazing layer pushes the unit closer to the 0.10 target. However, each layer also takes manufacturers further away from feasibility. Consider how each lite of glass adds 15 pounds or more to a 2.5-foot by 5-foot window. The added weight requires stronger framing and hardware.

Another issue with adding multiple layers of low-E glazing to an IGU is the progressive loss of visible light transmittance (VT) and solar heat gain (SHG).

"Each new glazing layer pushes the unit closer to the 0.10 target.
However, each layer also takes manufacturers further away from a feasible reality."

Better Frames?
Adding more glazing to an IGU has a significant impact on center-of-glass (COG) U-values. However, conductivity associated with the frame and the edge of glass overrides some of those efficiency gains. When modeling and weighting components, the frame contributes to about 20 percent of the total window U-value, and the edge of glass effect—the cumulative effect of the spacer, glass edge and frame being in contact—makes up another 20 percent. The COG represents the remaining 60 percent.

In the examples described above, the quadruple-glazed 0.13 U-value window has a 0.08 COG U-value, and the 0.12 U-value window with five lites has a 0.06 COG U-value. With such low COG U-values, it’s clear that the 40 percent contribution of the frame and edge of glass effect is detrimental to a window’s total U-value. Therefore, the industry needs to improve frame designs to further reduce their conductivity.

A variety of energy-efficient vinyl, fiberglass and engineered thermoplastic frames are available, with the best performing designs utilizing low-conductivity filler materials with low K-values in framing cavities.

Another potential consideration to reduce frame conductivity is to add additional chambers within the framing cavity. Smaller, narrower chambers can reduce convection currents and provide better insulating properties compared to wider, unfilled chambers.

While no magic combination of components exists today to produce a 0.10 U-value (R-10) window, the industry is inching closer to reaching this target. With each new advancement, we’ll know how low we can go.

Ric Jackson is the director of marketing for Quanex Building Products Engineered Products Group. He can be reached at RJackson@QuanexEPG.com. His opinions are solely his own and do not necessarily reflect those of this magazine.

DWM
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