Skylighting is twice as efficient at letting in light as side lighting, Brown says, but the light is harder to control. Daylighting consultant Matthew Tanterri, president of Tanterri & Associates in New York, agrees. “One of the biggest mistakes I see in residential buildings is the gratuitous use of skylights without understanding the solar heat gain implications.”
As a rule of thumb, skylight size should never be more than 5 percent of the floor area in rooms with many windows, and no more than 15 percent of the room’s floor area in spaces with few windows, according to the Department of Energy Office of Energy Efficiency and Renewable Energy.
Maximize Efficiency
As important as calculating daylighting efficiency is ensuring that all that extra glass isn’t sending energy use through the roof.
In recent years, window and skylight manufacturers have addressed thermal conductivity issues by offering a range of shading options and high-performance technologies such as low-E coatings; dual, triple, and laminated glass; and thermally broken frames.
Energy ratings take some of the guesswork out of specing climate-appropriate glazing. ASHRAE Standard 90.2 establishes minimum residential requirements for U-value and solar heat gain coefficient (SHGC) by climate zone. And the National Fenestration Rating Council labels manufacturers’ products for U-value, SHGC, and visible transmittance (VT), or the amount of visible light transmitted through a window assembly. Most values fall between 0.3 and 0.8; the higher the VT, the more light is transmitted. Manufacturers of translucent solutions such as polycarbonate also typically list VT levels for their products in varying thicknesses and tints.
San Francisco builder David Warner, owner of Redhorse Constructors, takes performance measures a step further. He often sends a physical house model to Pacific Gas & Energy, the local utility, to evaluate daylighting effects. “They will model your project with light all day and give you back a film showing how light penetrates your structure,” he says.
Brown predicts the next decade will deliver more daylighting products with advanced coatings and smart glass that responds to the light level. “These technologies exist now; it’s just a cost question,” he says.
