“Let the sun shine in” used to be a pretty big no-no for grocery stores. Strong, glaring sun increases air conditioning loads, shortens product life and causes discomfort for both customers and employees. In addition, it can blind bar-code readers, rendering them inoperable during certain parts of the day.
In recent decades, the trend was toward cutting down the amount of light admitted to retail buildings. But the result — the virtually windowless “black box” retail building — turned out to have its own problems. Such buildings require high levels of artificial lighting, which generates substantial heat, making air conditioners work harder in an era of rising energy costs.
The popularity of the black box was largely fueled by a desire to control the presentation of product under conditions set by lighting designers. Then came some studies that found consumers shopping longer and spending more in stores that were lit with natural light.
A 1999 “Skylighting and Retail Sales” study done by the Heschong Mahone Group for California's Pacific Gas & Electric looked at a chain that had some stores with skylights and others without. All other things being equal, the stores would experience 40% higher sales if they were fitted with skylights. A second “Daylight and Retail Sales” study by H-M-G in 2003 of another chain in a different sector came up with a range whose upper bound was 40%. During the 2001 California energy crisis, when stores were running on reduced power, that chain's daylit stores booked sales that were 5.5% higher than non-daylit units.
The 2003 study estimated that energy savings from daylighting were 24 cents per square foot per year for the chain, thanks to photocontrols that dimmed the artificial lighting during periods of strong sun, with a potential to reach 66 cents per square foot with state-of-the-art controls.
But the big benefit was seen in sales increases. In the 2003 study, the dollar benefit of increased sales was determined to be at least 19 times the value of the energy savings, and the study said the actual ratio was likely to be 45 to 100 times.
Outpost Natural Foods, a three-store co-op in the Milwaukee area, opened a new store in 2000 that was designed from the ground up to include daylighting as part of an overall “green building” approach. “We saw a big difference in sales per customer,” said Pam Mehnert, Outpost's general manager. “That store continues to be our highest sales per shopper of the three we have. And it was that way right from the beginning.
“People are taking more time and spending more money. Our sales per customer at the State Street store is around $30; in our other stores, it's 10% to 15% less.”
Daylighting a retail space, and especially a supermarket, is not a simple matter of adding a few windows and skylights, though.
“It's not that easy — you don't just punch a bunch of windows in a store,” said Doug Milburn, founder and president of Advanced Glazings, Sydney, Nova Scotia, which makes high-performance engineered glazings. “You've got to do more than that to do it successfully.”
Advanced Glazing's Solera glazing is designed to diffuse natural light, project it into interior spaces and eliminate glare and hot spots while maximizing natural illumination levels.
“The story starts with distribution,” Milburn said. “If you want to properly light a space, you've got to get light in and up. We human beings are used to having bright above us, so creating a bright floor makes an unnatural-feeling space, and if it's dark above you, you'll tend to want to turn the lights on to get that lighting distribution proper. By properly distributing that light, you create a space that a light meter will show has a proper amount of light in it — but it'll also feel good to a human being.”
Outpost's skylights bring in diffused light. “As daylight comes into the skylight, the light bounces off reflective chambers inside, and there's a filter beneath,” Mehnert said. “It looks like a 4-by-4 fluorescent fixture in the ceiling.”
In addition, the daylight in the Outpost store is spread throughout the store.
“To replace artificial lighting, daylighting has to be evenly distributed,” said Matt Tendler, a member of the Outpost co-op and consultant on the design of the State Street store. “The right approach is a series of small skylights laid out in a regular grid in a similar fashion to artificial lighting.”
A Lighting Design Strategy
The use of skylights and clerestory windows, especially when a diffusing glazing is specified, provides a basis level of interior light, which lighting designers then augment with fixtures and lighting elements.
“I look at daylighting design as a subset or part of lighting design,” Tendler said. “You're using daylight in conjunction with artificial lighting.
“Where a lot of daylighting falls down is, they don't really consider the artificial illumination and the controls of that illumination,” he continued. “If you don't have some kind of design to either turn off or dim down artificial lighting, or design it to lower levels in the first place, you don't save any energy.”
“The whole system is on a light-sensitive mechanism,” Mehnert said of Outpost's 14,000-square-foot State Street store, of which 8,700 square feet is retail space. “A third of all our lights are not in use when the sun is out. It's not too sensitive, so when a cloud goes over, it doesn't switch on and off.
“The efficiency of all our stores is good,” Mehnert added. “But the State Street location is about a third less costly to run than the others. The changes we made to make it a ‘green’ building in terms of energy efficiency and daylighting cost about 10% more than if we had not done those things.”
“The nice thing about daylighting is that it changes throughout the day,” Tendler noted. “If it's done right, where you have fixtures spaced at one to two times the height of the ceiling, we're getting a nice, soft daylight, which has a little different feel, because daylight has a very high color temperature — 6,000 to 7,000 Kelvin, vs. most grocery store lighting. They typically want to do it at warmer colors, because people's skin and the product tend to look better.”
Sunlight also delivers half the heat to a store's interior for the same level of illumination.
“Sunlight is twice as cool as high-efficiency lighting,” Milburn said. “HE lighting gives you about 50 lumens per watt, whereas natural sunlight is about 100 lumens per watt. It's twice as much light for every unit of heat, so it's twice as ‘cool’ as artificial light. People think of sunlight as ‘hot,’ but the reason they think of it as hot is because you've got this extremely concentrated patch that falls on the floor.”
Glazing products can diffuse sunlight to prevent this effect.
“You see grocery stores with a lot of glass on them,” Milburn said. “Particularly if that glass happens to be east- or west-facing, you'll get direct-beam sunlight in, and that is going to cause visual problems for customers and staff, it'll interfere with scanners trying to read bar codes and it will overheat product on the shelf. That's about the concentration of light; fix the concentration and you actually have light that's cooler.”
The Little architectural firm, based in Charlotte, N.C., is currently doing store design consulting for several chains, including Wild Oats Markets, Safeway's Eastern Division, Vons, Ingles Markets, Tesco and Publix Super Markets.
“All of them are looking at the opportunities with skylighting,” said Tim Morrison, principal. “Some to a higher degree than others.
“Supermarkets have been considering skylights for a long, long time, but their first concern is the safety of their product and its appearance,” he said. “In certain parts of the store, such as with wine, sunlight can affect the product. That was a concern years ago, before people introduced the filters and other devices that could help enhance the experience.”
Natural light delivers optimum color rendering to put product, especially fresh product, in the best possible light.
“Natural sunlight has, by definition, a perfect color-rendering index, so what you see color-wise is much nicer than if you're under artificial light,” Milburn said.
Mehnert is quite happy with the color rendering in her store. “Love it,” she said. “It's absolutely amazing. That was the response we had from customers — you get such a natural color from it.”
A main way supermarkets can justify the expense of implementing daylighting is by taking advantage of energy credits and other incentives made possible by government and utility efforts to increase energy efficiency. Natural lighting's potential to cut the need for electric lighting during daylight hours and reduce air conditioning loads makes it an energy-saving technology that's eligible for certain tax credits and utility rebates.
In addition, federal, state and utility funding supports resource groups that provide daylighting expertise. Among the resources around the country is the Daylighting Collaborative, Madison, Wis. It was started in 1999 at the Energy Center of Wisconsin, when Wisconsin utilities got together and decided that “daylighting was one of the single most important ways to reduce energy use in buildings,” according to the Collaborative's program director, Abby Vogen Horn.
However, “a lot of times when we're talking about daylighting, we won't talk about energy savings, because that's not everyone's goal or interest,” she said. “In a supermarket, it's display lighting; it's ambience; it's good color rendering, so that produce looks very fresh and appealing. And then we say, ‘Oh, and then you're still going to save all this energy if you include a control system that works with your daylighting to reduce your electric lighting usage.’
“There's a potential [for energy savings] simply because if you're using daylighting, it should be used in place of electric lighting, because the quality of light is so much better for display purposes. In a supermarket there are a ton of lights, whether it's strip fluorescents or HIDs or whatever, and being able to turn those lights off is a very real energy savings during daylight hours.”
Daylighting is much more amenable to new construction, where it can be part of the design process, designers told SN. But several limited opportunities do exist with many existing buildings.
“It's a lot easier to design the daylighting concepts in new construction,” said Mohammad Ismail, project manager and senior associate at Little. “Remodels, it's hard to penetrate the roof. It's limited on existing buildings.”
An existing roof truss system may not be adequate for the increased weight of skylights, and the building's diaphragm — the building's skin, which must respond to wind loads and the like — may be weakened by having too many holes cut in it.
“We had a customer who was interested in adding some skylights to a project, and at one time they only wanted 20 skylights,” Morrison noted. “Then they wanted to double that scheme. It wasn't a simple matter of just doubling the skylights, we had to look at whether the components of the building were doing what they needed to do structurally.”
Also, since direct sunlight can cause problems with product, skylight specification and placement are key considerations. It's important to use skylights and glazings that filter out harmful rays and diffuse natural light in areas where a view through clear “vision” glass is not needed. According to Mehnert, “We had talked with a couple of other co-ops that had done some [clear] skylights in their stores and had heard some horror stories about it, where they ended up covering the skylights with blue tarps because there was so much direct light coming in it was melting the chocolate, molding the bread much quicker, things like that.”
The viability of a skylighting retrofit will often depend on an individual store and its customer base. In upscale markets, where stores compete for customers on ambience and a commitment to “green” building, the cost may be justified.
“Demographics change,” Ismail said. “You have urban areas that were distressed in the last 20 to 30 years, and now they're coming back, so from a developer's standpoint, an existing grocery store 20 years ago was not generating revenue, but now you have more sophisticated customers, so it makes good business sense.”
According to Vogen Horn, daylighting is really nothing new, but something that's being rediscovered. “We used to do daylighting all the time, and we used to be very effective at it,” she said. “We didn't always have electric lighting and air conditioning. So a lot of this is re-teaching ourselves how to do what we always knew how to do. It's a very real strategy that anyone should be able to incorporate in their very next project.”
For owners, the ambience that daylighting brings into a store is the greatest rediscovery. “When we first opened, we got comments from customers like, ‘I can't believe how good it feels in this store,’” Mehnert said. “They just knew that the ambience in the store was so much different than in our other stores, and so much different from other stores they'd been in.”
“Studies show that customers react well to daylighting, that generally they stay in the store longer, they're more comfortable,” said Morrison. “Ten years ago many supermarkets were designed as a black box, where the inside was painted dark — the idea was to create a theater. This was not the easiest thing to pull off, in that the ambient to highlighted or featured areas would have to be a 3:1 ratio, so the overall store might appear dark when you walked in, but targeted areas would have a brighter, higher concentration of light. Daylighting is going to change all that.”
The Daylighting Collaborative
Lighting Research Center, Rensselaer Polytechnic Institute
Daylight in Buildings: A Source Book on Daylighting Systems and Components
Lawrence Berkeley Laboratories
Heschong Mahone Group daylighting studies
Advanced Glazings Ltd.
Outpost Natural Foods
Lightfair International Trade Show and Lightfair Daylighting Institute