This article was originally published in the January/February 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online January/February 1997
Tubular Daylighting for Sun LoversMany energy auditors have been getting questions from sun-deprived homeowners about tubular skylights. Also called light tubes or light pipes, tubular skylights admit light from a roof-mounted acrylic dome and channel it through a reflective tube to a ceiling-mounted light diffuser within the home. They bring natural light to interior spaces, but it remains to be seen whether, on balance, they provide a cost-effective lighting retrofit.
Light tubes were first developed in 1990, and they were introduced to the North American market in 1992. At least seven brands are now manufactured and sold in the United States and Canada (see Light Tube Brands). Prices start at about $200 to $300. Light tubes provide more natural lighting and color rendition than artificial light sources, although the type of reflective material and the diffuser color and design may cause variation.
Their advantage over traditional skylights is that they can be installed without cutting into ceiling joists or roof rafters, and the tubing can be angled to fit around obstructions. Some models come with other options that increase their versatility; these include ventilation fans or electric lights behind the ceiling diffuser for nighttime use.
Light tubes cannot completely take the place of artificial lights, since they are ineffective at night and inadequate on cloudy days. But they have found a market niche for interior locations in the home such as bathrooms, hallways, and even closets. They can also add light to living rooms, kitchens, and other rooms that are used during daytime hours.Performance In evaluating the performance of light tubes, most manufacturers' research efforts have focused on determining their light output compared with electric lights. Since there are no standard test procedures, various techniques have been used.
In 1995, Scott Phillips, then a design engineer with the Alberta Research Council in Calgary, tested a 13-inch-diameter SunLite on behalf of its manufacturer, Patterson Enterprises. Phillips said that the greatest light intensity from a light tube is measured just below the interior ceiling diffuser, whereas an incandescent bulb puts out the greatest light intensity in a horizontal direction, around the circumference of the bulb. He measured overhead brightness of the SunLite in late August in Calgary at noon, 4 1/2 feet directly below the unit, and concluded that it would require a 1,300W frosted incandescent light bulb to produce the same brightness.
Phillips also devised a method to measure total light put out in all directions by the light tube. What he calls equivalent total light from the same tube was 420W. Further calculations showed that these values are cut almost in half in winter at the same latitude. Clouds will, of course, also reduce the light output.
Light tubes are not all alike. In general, one will get more light from a tube that is shorter, wider, and more reflective. Latitude, local climate, and the number of sunny days per year make some locations more suitable than others. Roof orientation and the degree of roof slope above the room where the light tube is needed will also affect lighting efficiency.
Most models have curved reflective plates within the roof dome that are positioned for maximum solar exposure to catch sunlight and reflect it downward into the tube. Most manufacturers offer standard products or kits with a 4-ft tube length. Some offer standard tube lengths ranging from 2 to 6 ft, and extension tubes can be added as needed. Tube diameters of brands on the market range from 8 to 21 inches. The largest diameters are generally designed for commercial applications, but they can be used by homeowners willing to pay the higher cost.
Light tubes use two primary types of reflective material: an anodized aluminum tube with a highly polished reflective interior surface, or laminated silver film over a rigid aluminum tube. The Sun Tunnel is the only flexible model on the market. Its flexibility makes it more versatile for installation, but tests indicate that it transmits less light than other brands.Energy Savings How much lighting energy a light tube displaces depends on the wattage of the electric lighting and the length of time that it would have been used were it not for the light tube. If a light tube replaced the daytime use of a single bathroom light used one hour per day, it might save the homeowner a few dollars per year. In a windowless home office used eight hours per day, however, the light tube might save $30 or $40 per year in a sunny location.
The big unknown with light tubes is their effect on house heating and cooling needs. This may vary slightly from one brand to another based on type of material used; it also depends on proper roof sealing and proper installation of the product. Manufacturers claim that the tubes have lower heat gain than traditional skylights in summer because the glazing surface area is much smaller. However the tubes themselves are often uninsulated, and sometimes they are ventilated to prevent condensation, so they may cause added winter heat loss.
As with windows and traditional skylights, condensation can be a problem with light tubes. Cold weather condensation on the interior of the ceiling diffuser has been reported in some homes. Some manufacturers use an air-permeable gasket to seal the roof dome to the reflective tube, allowing moisture to evaporate out the top. The types of plastic, roof seals, and gaskets used in light tubes will affect their longevity and efficiency. Warrantees range from 5 years to lifetime.
Light tubes make the most sense in areas where electric lights would otherwise be used for most of the day. They are currently being tested in office and commercial buildings, where they have the most potential for displacing electricity use. Light tubes in home applications that involve displacing only an hour or two per day of electric lighting use (such as bathrooms and closets) may take decades to pay back. But for some people, the idea of using sunlight instead of kWh is more significant than the payback.
Light Tube Brands Skypipe, manufactured by Skydome Industries in Australia, distributed by Pacific Sun Lite, 694 Pleasant Valley Road, Suite 8, Diamond Springs, CA 95619. Tel:(916)621-4963 or (800)217-1212; Fax:(916)621-3675.
Solar Bright Corporation, 3665 E Bay Dr., Suite 204-256, Largo, FL 33771. Tel:(800)780-1759; Fax:(813)581-1391.
Solatube, 5825 Avenida Encinas, Suite 101, Carlsbad, CA 92008. Tel:(619)929-6060 or (800)773-7652; Fax:(619)929-6067.
SunPipe, made by the SunPipe Company, P.O. Box 2223, Northbrook, IL 60065. Tel:(847)272-6977; (800)844-4786; Fax:(847)272-6972.
Sun Tunnel, made by 2H Systems Incorporated, 103 Godwin Ave., Suite 155, Midland Park, NJ 07432. Tel:(201)612-1143 or (800)369-3664; Fax:(201)612-9642.
Tubular Skylight Incorporated, 5704 Clark Rd., Sarasota, FL 34233 (800)315-TUBE.
Vista EZ Light, made by ODL Incorporated, 215 E Roosevelt Ave., Zeeland, MI 49464. Tel:(616)772-9111 or (800)253-3900; Fax:(616)772-9110.
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