I’ll spare you the chemistry lesson, but suffice it to say that until a couple of years ago it wasn’t feasible to apply a low-emissivity coating to an exposed glass surface without the treatment eventually tarnishing from contact with moisture and oxygen.

Then, thanks in part to touch-screen mobile devices like the Apple iPad, the impossible became possible, as a tougher conductive tin-oxide formula emerged that allowed glass manufacturers to apply an effective and durable low-E coating to the surface that faces the interior of a room, commonly called the #4 surface among window wonks.

Big deal? With low-E coatings applied to both the #4 and the #2 (the inside face of the outer pane) surfaces, an insulated (two-pane) glass unit can achieve a center-of-glass rating as low as U-0.20, or about R-5—a 20% improvement over a standard low-E-coated IGU.

“Until now, you had to go to triple glazing to achieve that level of performance,” says Alex Wilson of BuildingGreen and co-chair of the Vision 2020 Products & Performance focus area.

And, instead of reducing the IGU’s clarity (or visible transmittance/VT) by adding an extra low-E coating, glass manufacturers applying the technology claim to maintain or even improve the VT of such assemblies, which in turn can improve daylight harvesting and reduce lighting energy demand.

“The coating is thicker, but has fewer layers and becomes integrally bonded to the glass,” says Andy Russo, director of the Residential Market Segment for Guardian Industries in Auburn Hills, Mich., which helps explain the ability to preserve a unit’s VT rating in the mid to high 80% range. The new coating may also make the glass more scratch-resistant, he says, a welcome by-product for an exposed surface.

The ability to improve the U-value and solar heat gain performance of a two-pane IGU, says Russo, pushes the window and housing industry closer to the European standard of triple-paned glass, which can achieve R-6 or U-0.16 performance, albeit at a cost that’s still too rich for the U.S. market. “This takes conventional two-pane units as far as possible,” he says, while preserving standard frame and hardware specs (and costs) that would have to be beefed up for triple-pane IGUs.

Low-E coatings on the #4 surface even trump the effectiveness of new-age airspace fills such as krypton, which are not only costly, but also problematic. “Krypton gas is lighter than argon and air, so it can escape the airspace more easily through a broken seal,” Russo says, nullifying its benefit as an insulator.

To date, only Guardian’s ClimaGuard IS-20 and IS-15, Cardinal Glass’ LoE-i89, and PPG’s Sungate 600 units feature the #4 low-E coating technology and are available to residential window manufacturers serving the North American market.

Looking ahead, advances including vacuum-insulated, dynamic (or electrochromatic), and building-integrated photovoltaic glazing units—the latter with the ability to convert direct sunlight into energy—remain on the cusp of mass-market commercialization and poised to push glass and overall building energy efficiencies to a new level.

For Russo, however, the real impact will have to come from a change in how windows are regarded among housing industry professionals, window makers, and homeowners. “We need to go from a ‘feature’-oriented market to one that values performance over everything else,” he says, a transition that Europe made more than a decade ago. “It’s a fundamental shift in priorities.”