They're expensive, hard to design and tricky to install. A debate rages over whether they work. And they don't seem very American-although that might be changing.

They're double-wall façades-also known as dual skins and "active envelopes"-and for some green building specialists, the Holy Grail of exteriors. They can slash energy costs, say advocates, while allowing in lots of daylight and big, unfettered views. Dual-skinned buildings can be naturally ventilated, too, in some climates without need for any air conditioning.

For the uninitiated, dual-skin means more than double-glazed. A typical system has two independent, metal-framed glass walls with an air space in between as narrow as 6 inches or as wide as 3 feet or more. The chosen depth depends on local conditions and specified performance. And the air within the cavity travels, either naturally (passive) or by means of fans and HVAC systems (active). The walls can also have operable panels or windows, as well as shades or louvers within or outside the cavity to control sunlight and heat gain.

"It's a step farther than the Trombe wall, which was big in the late '70s," says Gregory A. Mella, a sustainable architecture principal with SmithGroup, working out of the Washington office.

Trombe walls use glass and masonry to collect and evenly distribute solar warmth, Mella explains, and today's dual glass walls also trap heat in winter and counter heat gain by means of air movement in summer.

While the concept has backers, some professionals (and most U.S. curtain wall manufacturers) remain unconvinced. A few outspoken critics even hail from Germany, where strict energy codes and workplace regulations for minimum daylight levels and views make it a Mecca of dual-skin structures. Such rules and incentives are more rare in North America. So, while European dual-skins number into the thousands, only a handful exists here.

Band of builders

Still, a growing band of building designers and owners swear by them. For an engineering building at the University of Pennsylvania, for example, a ventilated double wall was shop-fabricated as large, unitized sections to speed installation.

The design obviated the need for fritting or tinting of the floor-to-ceiling glass, says Richard Maimon, an architect with KieranTimberlake Associates, of Philadelphia.

"You can be comfortable next to the glass on the hottest days of August and coldest days of February," he says. "And the energy use has been modest."

The engineering school was smitten with the high-tech image, yet equally concerned with energy use. The same factors attracted Manulife Financial to a comparable unitized product for its new U.S. headquarters in Boston. There, more than 200,000 square feet of clear glass, framed in stainless steel and aluminum, protect a cavity vented by fans-and integrated into the HVAC system.

"It's the first active wall system using the air-pressure differential between the ceiling plenum and the tenant space to circulate air," says Bernie Gandras AIA LEED AP, an associate partner with Skidmore Owings & Merrill, of Chicago. "So it has no ductwork on the perimeter."

Passive double walls, on the other hand, rely on convective currents in the "thermal flue" to move hot air up and out. The 14-story, LEED-NC Silver-rated Seattle Justice Center, which houses the city's police headquarters and municipal courts, has a double-skinned southwest exposure. This passive wall's 30-inch-wide air gap essentially serves as "thermal buffer," says architecture firm NBBJ's LEED-accredited Kerry Hegedus. Unlike the factory-built unitized systems used elsewhere, NBBJ designed the inner wall of storefront spanning from floor to floor, and the outer screen of steel-tube-supported, single-glazed curtain wall.

"We wanted to keep it simple, using conventional materials that wouldn't scare contractors away," says Hegedus.

Yet the wall section is packed, harboring a catwalk and automatic louvers to regulate heat and light. "Lots of glass often means you need some type of operable shading system," says Maurya McClintock, a façade engineer with San Francisco-based Arup who consulted to the engineering school and SJC. "The primary reason you use a double-wall façade is that they allow you to provide solar control while maximizing glazed area."

This was true in all three projects, in fact. The building owners and occupants wanted as much daylight as possible without wasting energy. Moreover, they were all committed to using this rare breed of envelope-in part because they had personally inspected similar installations in Europe. In countless other cases, however, the complexity and the costs-hovering at $300 per square foot-prove too much for project teams to bear.

"The payback cycles are very long, and the systems are hard to model with standard energy software," says KTA's Kevin B. Pratt AIA. "It's a tough nut to crack. Many projects don't get out of the value-engineering stage."

In America, at least, these premium façade packages often send specifiers hunting for simpler greening strategies, says SmithGroup's Mella. "Like growing deciduous vines on the outside of their buildings."



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