Light and Cool
Jun 1, 2010 12:00 PM, By Mike Kennedy (mkennedy@asumag.com)
Window selection and placement can keep classrooms properly illuminated without unnecessary heat gain.
Smith Middle School, Chapel Hill, N.C., is filled with daylighted spaces. Architect: Innovative Design. Photo courtesy of Innovative Design
When the subject of buildings and daylighting arises, most people’s thoughts will turn first to windows. To the uninitiated, it seems a simple formula: more windows, more daylight; fewer windows, less daylight. But designers know that effective use of daylighting in a building design involves more than just letting in light to otherwise darkened spaces. Too much daylight, especially the glare of direct sunlight, can be as undesirable as too little daylight. Daylighting also affects how much heat enters a facility; the benefits of natural light quickly can be outweighed when the heat that comes with it creates a space uncomfortable to occupants. When the building in question is a school or university, the importance of effective daylighting strategies becomes even more critical. A classroom with good daylighting design can help students boost their academic performance, studies have shown. So for designers of education facilities, decisions about windows go beyond the question of quantity. Where in the space should windows be placed? In the ceiling? Along the walls? How high on the wall? Which walls? What kinds of window glazing should be used? How large should the aperture of each window be? Will the windows need shading devices? Will louvers or baffles be necessary to help control how much light enters the space? How does the size and shape of each space, and the footprint and orientation of the building as a whole affect where windows should be situated? By determining how each of these decisions affects the lighting in learning spaces and the energy costs of heating and cooling the building, schools and universities can construct energy-efficient education facilities illuminated with ample daylight.Taking another view
The windows that provide views from a classroom to the outdoors are not the ones that bring about effective daylighting. Schools should develop their daylighting designs with "roof monitors; high, south-side light-shelf apertures; or high, north glass transom windows," according to the Guide for Daylighting Schools.
Daylight Dividends, a national research program of the U.S. Department of Energy and other agencies, advocates for the incorporation of daylighting strategies in building designs. It commissioned Innovative Design, an architectural firm in Raleigh, N.C., that specializes in sustainable design, to put together the Guide for Daylighting Schools.
Roof monitors—raised sections of a roof with vertical glazing—should be considered first in putting together a daylighting design, as long as the school that is being planned is a one-story facility. Monitors with south-facing glazing, properly sized overhangs and interior baffles to diffuse the light provide the most daylighting benefits. Roof monitors create uniform lighting throughout a space, eliminate contrast and glare, bring natural light to spaces far from the perimeter of the building, and allow more heat to enter the building in colder months.
South-facing light shelves are considered the next best daylighting option, according to the guide. Light shelves are horizontal surfaces that deflect sunlight coming through windows toward the ceiling to spread diffused light throughout a room. Light shelves can be used in multi-story buildings and typically cost less than roof monitors, the guide says. But because the light is coming from just one side of a space, achieving uniform lighting is more difficult.
North-facing monitors provide daylighting benefits, but are not as energy-efficient as those that face south. They typically require at least 25 percent more glazing to achieve the same annual daylighting contribution, the guide says.
"The best way to design the size of daylighting apertures is to size the glazing and overhangs so that just the right amount of radiation is brought into the school during the summer peak cooling condition," the guide says. "If the glazing is south-facing, this strategy will allow more and more radiation to enter the space as fall becomes winter."
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