While researching his new report, “Better Window Decisions” about window efficiency, Vision 2020 Chair Alex Wilson of BuildingGreen investigated why the width of the airspace between panes in an insulated glass unit (IGU) for the North American market is narrower than those in Europe.

He found that domestic window testing methods by the National Fenestration Rating Council (NFRC) assume a 70 degree F difference (or delta-T) between outdoor and indoor temperatures, causing insulated glass makers to create an optimal airspace of about 1/2-inch between panes to mitigate convection (heat transfer from air movement between panes) that undermines thermal performance.

“If (the U.S. assumed a 35 degree delta-T for windows, as they do in Europe, chances are we’d be producing windows with thicker airspaces and achieving better overall (window) energy performance,” says Wilson.

While the NFRC agrees that European testing uses a lower temperature differential than the U.S., “The resulting U-factor (for the IGU) is lower/better, but only nominally,” says Ray McGowan, senior program manager at NFRC.

McGowan also points out that North America and Europe use different ISO-certified testing methods on windows, which also muddies the comparison. “It’s truly just a different result from a different test,” he says. If European and North American window products were tested to the same standard, “performance is comparable for a similarly engineering window product.”

Another reason for the difference in the delta-T is that NFRC’s standard covers the U.S. and Canada, the latter increasing the temperature difference than if the standard only covered the States. “Warmer parts of the U.S. could benefit from a gap slightly wider than a half-inch,” says Andy Russo, director of residential glass marketing for Guardian Industries, a global glass manufacturer. “But for North America, it’s the optimal medium,” given the extreme cold in parts of Canada.

Wilson’s larger point, however, is that a wider airspace between panes would result in a thicker and heavier IGU that would, in turn, require a more robust frame and hardware and thus boost the performance of the entire window, if perhaps not just the glass. “I think that explains why U.S. windows aren’t an energy-efficient as their European counterparts,” he says.

McGowan, meanwhile, credits a variety of factors in Europe—higher energy prices, more stringent and better-enforced codes and other energy-use regulations, and climate conditions that allow for more passive ventilation—as reasons why windows in Europe are better built than those in the U.S., though he stops short of calling them more energy efficient. “In the end, the North American window versus European window will likely perform comparably if the same tests are used.”

To that end, he says, the NFRC, DOE, and the State Department are actively working to proliferate the U.S. window testing method (ISO 15099) to achieve global consistency; Europe has been resistant, McGowan says, but Australia and South Africa already use the testing standard and Asian economies, including China, have been receptive. “Most of the global economic growth will occur in the Asia Pacific region for years to come,” he says.

Domestically, Russo envisions a future where regional climate differences are accommodated by wider (or narrower) airspaces in IGUs. “Advances in materials and different combinations of technologies will lead to regional solutions,” he says. “To what degree and how specific will be driven by cost.”

All three men agree that insulating glass technology has yet to reach its peak performance. New technologies such as vacuum-insulated glazing and the application of low-E coatings on the room-facing surface of an IGU, as well as spacer types and width and gas fill options within an IGU, says Russo, “highlight the impact that many variables can play in total window performance.”