John Straube’s presentation, “Building it Right: Fenestration for Net Zero Buildings,” at the Architectural Record/Green Source 2009 Innovation Conference proved to be, shall we say, enlightening, if not a little chilling. Straube’s talk (peppered with witty remarks) focused on the energy problems with glass. I thought we were through with that old modernist hobgoblin—in these days of ceramic frit, double and triple-glazing, low-emissivity glass, and automatic shades. But Straube, an associate professor at the University of Waterloo, Canada and principal with Building Science Corporation, says not so.
Fully glazed buildings are still highly inefficient in reducing heat flow and mitigating solar radiation. While Straube conceded in his talk that the technology of glass has improved greatly since the 1960s, it’s all relative. In terms of heat loss, today’s curtainwalls with say, low-e, argon-filled double-glazing might end up with an R-value (resistance to heat flow) of 3. For Straube, this qualifies them as “the Hummers of today.” And, he added, R4 “is still embarrassing.” With triple-glazed, gas-filled systems, you can get an R5 to R8 range, he noted, which is, at least, “serious.” But for cold climates architects should specify glazing with R10 value to keep from expending too much energy. Even better, of course would be quadruple-paneled windows with low-e—that would get you up to R14.
However, too often what he called “PTB—Pathetic Thermal Break” still characterizes glazed fenestration, owing to the high conductivity of aluminum frames. You need a good thermal break to separate the exterior frame from the interior one and lessen the conductivity of heat and cold, explained Straube, adding that an aluminum frame has all the thermal resistance "of a frying pan," or as he also put it, “Aluminum insulates nada.” Straube also pointed out that the seals in window frames need to be replaced every 30 or so years for the highest insulation value.
As for reducing solar loads in hot weather, Straube maintained that "about the same peak solar energy, 250 BTU/per hour/per square foot, hits the side of a building in New York and Phoenix" on west-facing glass facades. One thing that could help, but isn’t used as much in the U.S. as elsewhere, would be shades on the exterior of the glass curtainwall. They actually cut solar gain three times more efficiently than interior shades do, he said, adding interior shades mainly help cut the glare.
While people naturally desire daylight in the work environment, its penetration needs to be coupled with automatic light control that continually adjusts to the angle of the sun. And, Straube argued, you don’t need glass to go from the ceiling to the floor to get effective daylight penetration: daylight can effectively enter a room without solar gain, he maintained, using a ratio of 1.5 to 2 times the height of the window’s head. Glass below sill height is “useless,” he added.
To shed more light on the topic for architects (and magazine editors) Straub suggested going to his web site, www.buildingscience.com.