Novel Glass Technologies.

By C.C. Sullivan and Barbara Horwitz-Bennett

Tried and True Building Approaches

Along with lighting and MEP systems, glass technology represents one of the most dynamic areas of technological change in building design. Driving these advances are such end-user concerns as enhancing interior daylighting, reducing facility maintenance, and expanding view corridors for occupants. In recent years--and especially today, as fuel prices spiral upward--controlling building energy use continues to dominate the choice of glass enclosure products.

In response, a broad array of low-emissivity, insulated, and hybrid glass systems are available to Building Teams, Yet despite the fact that the industry has come a long way developing these energy-efficient window glazing and building facade products, efforts are continuing with what seems like an even a greater push to reduce the carbon footprint of buildings.

"Windows in the United States use approximately two quads a year in heating energy, which is approximately one-third of all building space heating energy used," states Dariush Arasteh, a researcher with the Lawrence Berkeley National Laboratory (LBNL) Window and Daylighting Group, Berkeley, Calif. "Because heating loads are strongly tied to conductive losses, technologies which lead to lower window U-factors are the key to reducing heating energy," he says. "U-factor" refers to the standard measure of how well a product keeps heat from escaping.

So when reduced operating cost and environmental impact matter, it just makes sense to focus on glass. In fact, green building guru Jerry Yudelson, P.E., LEED AP, principal of Yudelson Associates, Tucson, lists improved glazing and enclosure systems as one of the most effective ways to control energy use in buildings. "The real action in buildings today is on façades," he observes. "Façade design and engineering is a rapidly growing international movement."

Recent building design trends, however, have tended to make this specialized endeavor even harder, as architects and owners press for more glazed area per square foot. "There's an architectural trend toward greater transparency. People want a good visual connection to the outside, but the thermal requirements kill that transparency," says Erin McConahey, P.E., a mechanical engineer with Arup's Los Angeles office. McConahey offers one solution: "With double-facade systems, one can improve thermal performance and gain transparency."

While double- and triple-glazed enclosures are common in Europe, the use of double walls is still limited in the U.S. Although the market and environmental drivers seem to be falling into place, American buildings generally lag behind their European counterparts when it comes to incorporating innovative façade systems.

"U.S. developers and clients often demand a payback on energy operation alone of less than three to five years. Compare this to European buildings, which are often justified over a 20-30 year payback, and paying anywhere from three to six times the energy prices of the U.S.," explains Maurya McClintock, façade engineer, Arup, San Francisco. "We just cannot make a convincing argument for such U.S. clients." Other factors mentioned by building professionals active on both continents that drive this trend in Europe include a difference in cultural mentality--and even health regulations. Occupant expectations have created a greater market demand for "healthy" buildings, and more progressive codes regulating daylight and fresh air requirements.

However, with the green movement ramping up in the U.S. to meet more sophisticated owner expectations, the tide is turning. For example, the number of double-envelope façades seen in American projects is rapidly growing. Recent examples include the Seattle Justice Center, designed by NBBJ for the city of Seattle. The vented, double-skin enclosures maximize the positive qualities of glazing while minimizing its negative energy impact and its potential for thermal discomfort, especially on easterly or westerly glazed exposures, according to the Center for Sustainable Building Research at the University of Minnesota (www.commercialwindows.umn.edu/case_sjc.php ).

Explaining how these enclosure systems function, Nils Petermann, a research associate with the Alliance to Save Energy, Washington, D.C., the sponsor of the Efficient Windows Collaborative, says: "Double-envelope facades consist of two glazed assemblies, often with a single-pane envelope on the exterior, double-pane windows on the interior, and a vented air cavity and shading devices in between. The shading devices within the double envelope can absorb solar heat gain, which is then re-radiated or vented to the outside. In the heating season, the ventilated cavity air can be recirculated into the building for heat recovery."

IS GLASS GETTING SMARTER?

Another area where significant technological advancement is now taking place is in the realm of electrochromic glass materials, often referred to as switchable glass and "smart glazing," says the renowned building consultant Richard Keleher, AIA, CSI, LEED AP, Concord, Mass. "The coming trend, supported by the U.S. Department of Energy, is the use of variable/electrochromic glazings that change their properties in response to varying outdoor conditions," he explains. Smart glass can reduce or control HVAC loads and solar heat gain without blinds or curtains, and also block most ultraviolet (UV) rays, which damage some building contents.

In fact, LBNL researchers claim that windows using these intelligent glass products "could reduce peak electric loads by 20% to 30% in many commercial buildings and increase daylighting benefits throughout the U.S., …