Double-Wall And Operable Façade Making Inroads in the U.S.

Quite popular in Europe, the concept of a double-wall (or double- glazed) operable façade is slowly making its way to American shores. Several factors need to be in place to justify the expense of building and calibrating the sophisticated system. The beauty of the technology is its ability to exploit nature’s gifts of natural ventilation and solar heat gain. The façade is made up of two layers: an outer curtainwall and inner glass pane, with an interstitial space that provides a thermal buffer. The outer and inner glass windows/walls, and often interior or exterior shading devices, are opened or closed based upon the time of day and season.

For instance, in winter, the sun heats up the air in interstitial space, which can then be released into the building by opening the internal glazing, providing free heating. During hot summer months, the internal pane stays closed and the exterior glazing is opened, releasing heat and helping to keep the interior cool. On moderate days, both glass panes can be opened to optimize natural ventilation and minimize HVAC requirements.

Josef Gartner GmbH, in collaboration with the Fraunhofer Institute, has developed a closed-cavity façade composed of a triple- glazed panel or insulating glazing unit (IGU) as the interior glass and a single glass pane and shading device on the exterior. The cavity is sealed and conditioned with a small amount of dry air to prevent condensation. Solar heat gain inside the cavity dissipates through the single pane, and the sealed space delivers a high level of thermal protection during winter and summer.

“Double-glazed facades are popular in Europe for several good reasons,” says Altenhofen. “The climate is milder, the occupants accept a broader range of temperatures for comfort, operable windows are required even on high-rise buildings, and energy costs are higher.”

These factors may not apply for some projects in the U.S., where it can be more difficult to make a case for the technology. “Long before we spend that kind of money, we should be doing the basic things like using triple glazing and sun-control devices, and ensuring better control of air leakage,” suggests Keleher.

Gilbane’s Giglio does see double-wall facades starting to take hold in America; his company is currently involved in a number of relevant projects.

Consulting firm Buro Happold has researched double-façade technology, performing dynamic thermal modeling to show estimated energy savings and projected payback periods for varied systems in different climates and orientations (www.cibse.org/ pdfs/8cstribling.pdf). The firm reported  average HVAC energy consumption savings of 27% in Las Vegas and 23% in London, though not all locations were as impressive. The report encourages Building Teams to consider climate, construction type, detailing, and cost when determining the feasibility of double façades. In addition, the report offers the following guidelines:

• A double façade offers the most energy-saving potential on the south and southwest orientations.
• Extreme climates provide more opportunity for energy savings.
• Energy savings can range from 10% to 50% of HVAC energy, and cost payback can range from 30 to 200 years based on today’s energy prices.
• Energy prices will also vary greatly over the life of the building and should be considered.

Altenhofen concedes that double façades may be a good solution in a few cases—notably, when a large amount of glass is desired for a very special view, and where energy savings are very important. “Otherwise, it is possible to deliver better energy performance at lower cost with other assemblies,” he says.