In the quest to improve a home’s energy efficiency, windows often take a backseat to flashier technologies such as solar panels or radiant heating. But they’re more than just dressings: Today’s sophisticated glazing systems not only offer low U-factors, but they also respond automatically to lighting and heating conditions.

According to the U.S. Department of Energy, inefficient windows can account for up to 15 percent of a home’s heating and cooling losses—and up to 25 percent of a home’s energy bill. Recent years have seen marked improvements in window performance, says Stephen Selkowitz, senior adviser for building science and leader of the Windows and Envelope Materials Group at Lawrence Berkeley National Lab. He estimates that about 80 percent of the residential market now consists of relatively high-performing windows, or “standard low-E, double-glazed Energy Star windows.”

Low-E windows use microscopically thin coatings to lower the glass’s U-factor, or ability to transfer heat. They cost about 10 percent to 15 percent more than non-coated windows, and can reduce energy flow by 30 percent to 50 percent. (See a breakdown of low-E glass production methods on page 56.)

Architects and homeowners looking for even better efficiency can choose windows with triple glazing, argon or krypton gas fills, or vacuum glazing, where panes are separated by a vacuum instead of inert gas, to reduce heat transfer.

Besides Energy Star, the National Fenestration Rating Council offers a comprehensive rating system with specifications in five categories: U-factor, solar heat gain coefficient (SHGC), visible light transmittance (VT), air leakage, and condensation resistance.

In colder climates, the biggest energy consideration for window specification is heat loss. Architects and builders should select glazing systems with low U-values and high R-values. In warm and sunny climates, however, a product with a low SHGC that transmits less solar heat into the home is more desirable.

However, it gets more complicated, Selkowitz says. Northern climates typically require air conditioning during three to four months of the year, so many architects and homeowners opt for insulated, triple-glazed windows that sacrifice passive heating opportunities in the winter.

Dynamic glass is one solution to the paradox, since it responds to changing light and heat conditions. Three major types of dynamic glass are photochromic, thermochromic, and electrochromic. When exposed to sunlight, photochromic glass darkens or becomes opaque—similar to the transition eyeglass lenses that gained popularity in the ’90s—but it doesn’t control heat gain and may darken more in the winter when the sun’s rays are more direct. Thermochromic glazing blocks out sunlight in response to rising heat, but the inability to control the glass’s response is a major disadvantage.

Electrochromic glass, on the other hand, contains a clear nano coating of an electrochemical substance, such as tungsten-oxide, which darkens in response to a low-voltage electrical current. These “smart” windows can be controlled by the homeowner or a building-management system. For example, one electrochromic IGU by Sage Glass, based in Faribault, Minn., has a VT that ranges from 0.62 in its clear state to 0.02 at full tint, and a SHGC ranging from 0.48 to 0.09. “It’s basically a light and heat valve for your home,” says Helen Sanders, Sage’s vice president of technical business development. Meanwhile, typical VT and SHGC values for low-E glass may be around 0.75 and 0.70, respectively.

The potential benefits go beyond energy efficiency, Sanders says. “If you’ve just spent millions of dollars on a stunning view of the ocean … you’re going to have significant issues with sun and glare most of the year,” she says. “What you’ll have to do is have window treatments that block that view or, alternatively, you won’t put as much glass in. [This technology] provides homeowners and designers a way of achieving the architectural design they want without compromise.” Electrochromic glass also blocks more UV light than typical low-E glass in a clear state: When fully tinted, it blocks most wavelengths of visible light, protecting artwork and furnishings from fading in sunlight.

A number of architects are taking advantage of the technology. RKD Architects of Edwards, Colo., recently placed H Windows fitted with electrochromic glass in a south-facing glass wall for a hyper-efficient home in Edina, Minn. The VT of the 11 panels of 1 3/8-inch triple-pane Sage Glass ranges from 0.62 to 0.034, while the SHGC transitions from 0.48 to 0.09.

In a bid to make the technology more accessible, Boulder, Colo.–based startup US e-Chromic secured funding from the National Renewable Energy Laboratory to develop electrochromic technology, which may be used for retrofits. Loren Burnett, CEO and founder of US e-Chromic, estimates that this technology will cost consumers about $20 per square foot, as opposed to $50 to $100 per square foot for new electrochromic windows.

As glazing becomes increasingly high-tech, it’s important not to lose sight of the other window components, notably the sash and frame. “If you take a highly insulating glass and stick it in an aluminum or steel frame, it will lose a lot of heat,” Selkowitz says.

A recent window technology that gives windows an unprecedented U-factor is the Deceuninck Genius Window. With interlinked dual sashes separated by a 50-millimeter insulating space, the window—when combined with triple low-E glass and krypton gas fills—achieves an astonishingly low U-factor of 0.07. Lower-cost options with low-E glazing and argon gas–filled panes achieve a still-noteworthy U-factor of 0.10.

Consumers may not yet be in the market for the cutting-edge—and costlier—options such as electrochromic glass or interlinked dual-sash frames, but as these new window systems become more widespread, that could change.

“It’s not just about cost,” Selkowitz says. “It’s about comfort.” Beyond access to views and natural light, “you also have the confidence that if energy prices soar, or if you lost heat at your house, the house would retain heat for a few days. There are lots of reasons besides energy efficiency that you’d want to [maximize performance].”

Of course, the simplest option remains open for the taking: fine-tuning window placement to maximize passive solar design and maintain daylight; accounting for local climate conditions; and addressing potential issues of glare. “People often don’t take advantage of this,” Selkowitz says. “And it’s basically free, assuming you take the time to do it.”