The availability of window options spanning upwards of R-5, 8, and even 11, and the diversity of coatings, films, and insulated glazings allows immense flexibility to customize each window according to its exposure. As reported in EcoHome’s “Optimizing Windows” (March/April 2010, page 31), for the all-day exposure of south-facing windows, cold climates need a greater U-factor (ideally 0.20 or less) but less-efficient SHGC (perhaps 0.50 or higher) to boost heat gain in the winter and offset heating energy, along with overhangs or other shading to reduce gain in the summer. In hot climates, south-facing windows should combine shading with U-factors and SHGC ratings of 0.30 or better.
Orient the floor plan away from the hard-to-control west-facing exposure as much as possible, the report continued. If windows are needed, select units with efficient U-factors and SHGC ratings. If more glass is desired for views, increase the SHGC and use shading inside and out. The eastern exposure also receives lots of daylight but is cooler, allowing for an SHGC rating of 0.40 or more, especially in cold or mixed climates, and a U-factor of 0.30 is sufficient to retard thermal transfer. For northern exposures, which offer the least opportunity for solar gain, dual-pane units with a standard low-E coating are sufficient.
Windows in the sitting area overlooking the first floor funnel light into the interior of this Levitch Associates home in Berkeley, Calif.
Windows above the sink and clerestory windows at the ceiling bring natural light into the kitchen workspace of Anderson Andersons B-House in Shimasaki on Kyushu Island in Japan.
Interior and exterior shading strategies can mitigate solar gain and energy loss, as well. Designs can make use of exterior elements such as pergolas, battens, overhangs, and natural landscape features to shade or fragment overly strong direct sunlight. When exterior design options are limited, interior shades are also available. “They’re not always as efficient,” Fougeron notes, “but they give you infinite variability—you can move them up and down, or sideways, when you want to block the light.”
Home renovation specialist Maurice Levitch of the California design-build firm Levitch Associates notes that site orientation, particularly in an existing structure, may limit—but does not eliminate—design options for daylighting. Historic homes that are required to maintain original facades and homes restricted by housing community covenants can be especially challenging when it comes to changing a home’s exterior to allow more daylight in, he says.
Levitch’s strategy is to “look for opportunities to borrow light from other rooms” within the existing structure. Skylights over stairwells, tubular skylights, glazed transoms above interior doors, removing interior walls, and placing windows in interior walls are some of the methods he uses to accomplish this. Daylighting “is more aesthetic,” Levitch says. “A brighter space with natural light gives you more of a feeling of space during the day.”
Still, as attractive as the idea of sun-filled spaces might be to the homeowners who clamor for it, he cautions clients that the benefits don’t always outweigh the cost or difficulty of tearing down bearing walls or rearranging an interior just to increase natural lighting.
Michael Morris is a freelance writer in Ossining, N.Y.