Can Fully Glazed Curtainwalls Be Green?
When i see a fully glazed, floor-to-ceiling building, I see an energy-consuming nightmare a building that requires lots of heating and cooling at the perimeter just to maintain comfort. On a cold winter day, offices exposed to the sun require cooling, while those in the shade need heat. Most of the tremendous performance gains in glazing technology over the past 25 years have been squandered on increased window area. The reasons for poor performance are simple: glazing systems have very little ability to control heat flow and solar radiation.
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Good-quality, thermally broken commercial windows typically have U-values of about 0.5, corresponding to an R-value of two. By using the best available low-e coated, argon-filled double-glazed units (with a center-of-glass R-value of four), overall window R-values of three might be approached. If the goal is low-energy buildings, why cover large portions of any building with such a system, particularly in cold-and-hot climates? Spandrel panels are not much better in this regard. Although often packed with three or more inches of insulation (providing a nominal R-12), the metal back pans short-circuit the insulation so their overall R-value is typically closer to four.
But the thermal qualities are only part of the story. Glazing lets light in. That, after all, is the primary reason we use glazing. The solar heat gain that results is the reason many buildings require air-conditioning. It is a testament to the miracles of modern glazing, which selectively allows more visible light than infrared heat radiation, that many buildings can have such large window areas and remain comfortable in the summer. Nevertheless, very good commercial glazing still allows about one-third of the sun’s heat to enter. The size of a building’s air-conditioning plant is almost always defined by the glazed area.
It is true that daylight can offset the need for electric lighting and provide a psychologically healthy connection to the outdoors, but you don’t need floor-to-ceiling glass for that. In most occupancies and building types, there is no benefit to vision glass installed at floor levels (unless the occupants spend much of their time lying on the floor near the window), but there is a substantial energy penalty. Good daylighting design can reap all the benefits of glazing using vision glass covering less than half the enclosure. Numerous studies have shown there are no daylight benefits with window-to-wall ratios over 60 percent, and in many cases 40 percent is optimum. Even at these ratios, windows in a low-energy building should generally be triple-glazed, with large thermal breaks and some form of exterior shading (preferably operable).
In the end, glazing is a classic design problem that requires one to balance the desire for thermal comfort, energy efficiency, and light quality (all of which require small window areas) with the equally important desire for view, daylight, and connectivity with the outdoors (all of which benefit from large vision-glass areas). Many designers have shown that beautiful and high-performance buildings can result from a proper balance. All too often, however, it seems architects choose all-glass curtain walls because they make it easy to create a sleek impression while leaving all the tricky details in the hands of the manufacturers. How much longer can we afford to pay the energy bills that result from that choice? It’s high time to revive the craft of designing beautiful facades that don’t cost the earth.
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