Until recently, daylighting was one of the most misunderstood sustainable building product categories, mostly because the amount of daylight entering a skylight is not uniform or constant. While radiant heat can be measured by indoor air temperature, it's much more difficult to measure the effect of sporadic daylight on workers, students, and other commercial and institutional building occupants.
Neall Digert, PhD, technical director of lighting manufacturer Solatube International, says that, in his past experience as an energy consultant, "50% of my time was devoted to educating the design team about energy efficiency, partially through daylighting. That was a very nebulous process. It took a lot of hard work to find ways to provide the rigorous documentation the industry requires."
Lighting systems, not skylights
Solatube, based in Vista, Calif., makes tubular daylighting devices, a new product category that uses skylight openings, lens technology, reflection, and ceiling-mounted diffusers to bring a more uniform stream of daylight to interior spaces. Patented in Australia in the mid-80s, the first Solatube devices were sold there in 1991. Since then the company has expanded across the globe.
Solatube and its competitors refer to their products as "daylighting fixture systems," not skylights.
Such systems penetrate the ceiling plane and, using proprietary optical technology, provide a more constant distribution pattern of daylight, as opposed to the shifting patterns of skylights and windows. "There are no distracting shifts in light patterns," Digert said.
Solatube devices use a roof-mounted acrylic dome—usually 10–21 inches in diameter—that filters out UV radiation and sends the remaining sun's rays down a narrow tube, made of a proprietary material called Spectralite Infinity. Spectralite is not a mirrored product, but rather a multilayer acrylic material bonded to an aluminum substrate for structure. Micro-fine layers of acrylic create a surface that sends 99.7% of spectral-reflective light down the shaft, while still being transparent to infrared rays. Only visible light makes it through the device. Photocells can also be installed to save light for days with an earlier sunset.
Solatube devices utilize a roof-mounted acrylic dome that filters out UV radiation and sends the remaining sun’s rays down a narrow tube. Lined with an acrylic surface that retains 99.7% of spectral-reflective light, the tube directs the daylight to a diffuser mounted in the ceiling that spreads the rays to provide even distribution.
Another Solatube innovation is its Light Intercepting Transfer Device, a patented reflector that collects low-angle sunrise, sunset, and winter-angle light. The LITD works in conjunction with Solatube's Ray-Bender, a Fresnel lens attached to the housing of the device's dome that sends all angles of sunlight down the tube device to insure a constant stream of daylight.
The manufacturer claims that, through these two technologies, the system can meet or exceed ASHRAE ambient illumination requirements for a commercial space, even on a highly overcast day.
By using angle-adapters, the Solatube device can be routed through multiple floors and run horizontally or vertically. It was this innovation that allowed tubular daylighting devices to be used in many commercial applications that had previously been off-limits.
"It's something where you want to be careful to have as direct a path as possible," Digert cautioned. "We can duct the daylight around interior obstructions, usually around mechanical systems or parapet walls, then put the diffuser in the ceiling plate. We finesse the light through at least 60-degree angles. Some vertical clearance is required."
Multistory systems with larger domes (some as wide as 41 inches in diameter) can be used in warehouses and educational buildings, particularly in the sunny climates of California and Florida. By daylighting interior spaces, building owners can expect to save 23–50% of their energy costs, according to Solatube users. Tubular daylighting devices can also contribute to the daylighting credit in the U.S. Green Building Council's LEED rating system. By using photometrics with detailed lumen outputs specified for the site, engineers can accurately calculate the energy savings provided by a tubular daylighting device.
Del Mar (Calif.) Elementary School cut its energy costs by 24% after installing Solatube devices. Then there's the 43,235-sf Stater Bros. grocery store in Chino Hills, Calif. "The annual lighting energy costs for this store were nearly cut in half," said Mike McCasland, property development manager for Stater Bros. "In addition to saving energy, the Solatubes keep the store lighted in the event of a blackout. So it's a significant savings."