After a hiatus of more than 20 years, dual-façade systems, commonly used in European construction, are slowly reemerging in U.S. buildings. Recent examples include the Seattle Justice Center; Levine Hall, on the University of Pennsylvania campus; Genzyme's corporate headquarters in Cambridge, Mass.; and the U.S. headquarters of Canadian-based financial services firm Manulife, in Boston.
Double-wall systems come in several basic types: externally or internally ventilated; internally vented, internally extracted, and coupled to the building's HVAC system; and internally vented and externally exhausted.
Some systems rely entirely on natural ventilation, while others work in concert with mechanical equipment. In general, naturally vented systems are less well suited for use in humid or highly polluted conditions.
The U.S. Green Building Council's LEED rating system and California's Title 24 energy regulations are helping to focus greater attention on dual-wall systems, says Maurya McClintock, associate principal and façade engineer in the San Francisco office of engineer Arup. Both establish a relationship between glazing performance and the amount of glass that can be used.
By virtue of their construction and their integral shading devices, dual-wall systems can provide a greater amount of glazing without violating overall envelope energy requirements. By maximizing the use of glazing while controlling solar heat gain, daylight can be allowed to penetrate more deeply into the building's interior. Without integral sun shading, says McClintock, a dual-wall system cannot achieve its full potential.
McClintock says U.S. dual-façade applications are conceptually similar to what is being done in Europe, but European developers view project costs from the perspective of a much longer life span—30 years or more—than the 3–5 years typically accorded U.S. buildings. Thus, they are more willing to invest in energy-saving options. U.S. clients also want a validation of the system's performance, as well as assurance that the wall system will interface properly with other systems, such as daylighting controls, she says.
Dual-wall systems can be purchased off the shelf or customized for a specific building use and climate. Before a system can be designed, simulations must be created to confirm expected performance. The process is complex, and requires the use of a number of different simulation tools. For example, simulations must be developed in one program in order to obtain input for computational fluid dynamics studies.
"We don't have the kinds of tools that allow us to quickly model a number of variable conditions on a continuum," says Edward Lyon, a senior staff engineer with engineering firm Simpson Gumpertz & Hegar, Waltham, Mass. "We don't have that great model that says, 'I can start at any point in time and model the entire year.'"
A number of U.S. building product manufacturers are reportedly looking at getting into the dual-wall market. However, they would have to compete with Italy's Permasteelisa, whose preeminent market position is due in no small part to its significant research activity on dual-wall construction. Over the past decade, it has spent about $8 million on R&D in this specialty area.
