Daylighting Design Testimony

Costco understands the value of skylights

One of 13 such stores in New York State, the Costco Warehouse in Lawrence, New York, has been operating since 1993 with full daylighting and photocontrols. More than 200, 4′ x 8′ skylights illuminate the 148,000 square foot store. Interior daylight levels commonly reach more than 150 foot-candles, measured horizontally in the aisles, and 30 to 60 foot-candles vertically on the shelves. When the 400-watt metal halide light fixtures are turned off in groups of one third by the photocontrols, customers don’t even notice. Even with all the electric lights turned off during the sunniest days, the store remains bright and attractive.

The Costco Corporation operates a vast international chain of membership wholesale stores. With more than 250 skylit stores in the United States, Costco’s commitment to skylighting has been exemplary. “We have been using skylights in all our new facilities since 1985. We believe daylight is good for our customers, good for our business, and good for the environment,” says Keith Thompson, Vice President of Facilities.

The company has been evolving the basic design over those years, finding small areas for improvements and then standardizing on those elements. For example, metal ceiling joists are now ordered with a special light colored primer rather than dark gray in order to increase the reflectance of the ceiling. Similarly, white vinyl faced insulation is chosen specifically to increase ceiling reflectance. Skylight curbs and security bars are included in the standard metal framing contract, and skylight installation and warrantee are part of the standard roofing contract.

Skylighting Design Costco has used a variety of skylight products, depending on local availability. Products are chosen for durability, ease of installation, light diffusion and energy efficiency. The most common selection is a 4′ x 8′ double-glazed bubble skylight, made from a combination acrylic-fiberglass material that provides very good light diffusion. The skylights are typically placed about 20′ apart, aligned down the center of each aisle. With ceiling heights of 30 to 40 feet or more, the skylights provide very even illumination throughout the store.

With over 75,000 skylights nationwide, Costco could have a big problem on its hands if it had to worry about leaks or security problems. Metal bars spaced 6″ apart in the skylight well prevent forced entry or accidental falls through the skylights. Painted white, they are invisible from below. Skylights are installed with tamper proof screws and standard flashing details. “We have a variety of roofing types. We’ve had more problems with parapet flashing or cable penetrations than with any of our skylights. The key to avoiding leaks is a good, long term workmanship warranty from the roofing contractor,” explains Keith Thompson. “We always overlay our plans to make sure that the skylight locations are coordinated with HVAC, structural, and lighting locations. A skylight might be moved in plan to provide more light at the center of the store, or avoid a refrigeration area. We also want to make sure there is sufficient working room around the HVAC units and perimeter areas for safety reasons. Once located, skylights are rarely moved, even when the store layout changes. If we need to adjust to a new plan, we are more likely to re-circuit the lights, or occasionally cover up a skylight or two. Our shelving racks might be re-located, so flexibility in layout is important, ” explained Keith Thompson.

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