In November of last year, we moved into our newly renovated third-floor office in downtown Brattleboro. On the top floor of an unusual, slate-shingle-sided building constructed in the mid-1800s by the Estey Organ Company, the
EBN offices are wonderfully illuminated with natural light. Built before electric lighting existed, the building is long and narrow with tall windows. At one end of our floor is the original drafting room of the company, where perimeter windows are augmented by a roof monitor, bringing in lots of additional daylight and giving the space an almost outdoors feel. It was the natural daylighting that first attracted us to this space, and—though it necessitates some special measures in regulating the visual and thermal comfort of our offices—that daylighting is appreciated each and every day.
This article takes a look at the advantages of daylighting, including energy savings potential and new research showing productivity benefits. Daylighting is most applicable to non-residential buildings—offices, schools, manufacturing plants, and other spaces with largely daytime occupancy—though some information should prove relevant to houses as well. The accompanying checklist offers strategies for enhancing daylighting with conventional windows and skylights. In our next issue we’ll look at more high-tech options for daylight collection and distribution.
What is Daylighting?
Daylighting is the use of natural lighting in a building through perimeter windows, roof windows (clerestories and roof monitors), skylights, or specialized light pipe (daylight distribution) systems.
Capturing the environmental benefits involves more than simply the use of natural lighting, however. The practice of daylighting involves the
integration of daylight with electric lighting, overall building design, mechanical systems, and interior design.
Daylighting is both a science and an art. On the one hand, new glazing systems, lighting controls, and sophisticated computer modeling are opening up tremendous new horizons for daylighting, yet on the other hand, we are only now returning to the integrated daylight designs that architects and builders were routinely implementing in the 1800s. Prior to the widespread use of electric lighting in the early 1900s, daylighting was a necessity. Most commercial buildings were either designed to be narrow (like ours) so that daylight could penetrate from both sides, or they were kept to one story and illuminated from above with skylights or roof windows.
With electric lighting, dependence on daylight for illumination began to diminish. It was not until fluorescent lighting came into widespread use in the 1950s and ’60s, however, that daylighting consciousness all but disappeared. Glass was still widely used in commercial buildings in the 1970s—indeed its use was greatly increased—but often with such dark tints that we essentially had all-glass buildings with no windows.
But daylighting is coming back. Today, spurred by new awareness about health and productivity, and aided by better computer modeling tools and technical advances in glazings and controls, we are surpassing the daylight systems of a hundred years ago. And we’re saving energy to boot.
While it is generally assumed that daylighting will save energy, this is not necessarily the case. To succeed as an energy-saving measure in a commercial building, the daylighting strategy must be well planned and implemented, including full integration with the electric lighting scheme. In principle, daylighting can reduce electrical use both for lighting and for cooling. These benefits will occur if—and only if—electric lighting is switched off or dimmed when daylight provides adequate illumination. “The best daylighting scheme in the world won’t save energy if you don’t turn the lights off,” notes daylighting expert Greg Franta, FAIA of the ENSAR Group in Boulder, Colorado. Conveniently, daylight is typically most available at times when electrical usage in commercial buildings is at its peak (hot summer afternoons). Thus, use of daylighting can save money not only by reducing electrical usage but also through savings in demand charges.
When electric lights are on daylight-sensing dimmers, energy savings can be very significant. Nationwide, lighting accounts for 20% to 25% of total electricity consumption, according to E Source, Inc., publisher of the highly regarded
Technology Atlas series of reports (see
Vol. 6, No. 7); and in commercial buildings, lighting accounts for 37% of electricity consumption (34% if just interior lighting is considered). E Source says that 40% to 60% lighting energy savings can be achieved using daylighting strategies. Even conventional office buildings can often benefit from daylight-controlled dimmers since a significant amount of space is in the perimeter zone, adjacent to windows or a glazed façade.
The other primary energy benefit of daylighting—reduction of cooling energy use—is more complicated because adding windows or skylights to provide daylight will usually
increase cooling loads.
A net reduction in cooling energy use will occur only if heat from the electrical lighting is reduced enough to offset this increase. In reality, the situation is even more complicated, as conductive heat gain and loss through fenestrations also plays a part. Overall, only a well-executed daylighting design will reduce cooling loads.
In daylighting’s favor is the fact that, compared with electrical lighting, daylight delivers more of its energy as visible light and less as heat. This is illustrated in the figure below. Note that the better the electric lighting (the higher the efficacy), the less significant this cooling-load-reduction benefit from daylighting. T-8 fluorescent lamps release a far lower percentage of their energy as waste heat than incandescent lamps, for example. With daylighting, spectrally selective glazings introduce less heat per unit of light than clear glass or tinted glazings. The latter are the worst because they absorb and re-radiate heat from the sun yet transmit little visible light.
Daylighting Performance and Design cites a rule of thumb that each unit of electricity used for commercial lighting adds another half-unit of electricity use for cooling. If we replace that fluorescent lighting with daylighting on a lumen-for-lumen basis, we can reduce the cooling load through daylighting. However, it is very tricky to distribute daylight effectively throughout a space, and many daylighting strategies provide significantly higher illumination levels than the fluorescent lighting systems they are displacing. If a daylighting design generates only half as much heat per thousand lumens of useful light, but twice as much light is provided, the resultant cooling load will be the same at that point in time. The amount of available daylight fluctuates throughout the day and year, so designers must find a balance between the most hours of useful daylight and the potential for overheating during peak daylight hours. “We find with skylights that optimum building performance is seen when daylight levels at peak hours are two to three times the target illumination levels,” says daylighting consultant Lisa Heschong, AIA. Also, if direct beams of sunlight are allowed to penetrate to the occupied zone, they can create uncomfortable thermal hot spots (in addition to glare).
Optimizing energy benefits (and minimizing energy penalties) of daylighting necessitates integrated design and careful modeling during the design process. Physical models can be studied with a moveable light source or be placed on a
heliodon, which tilts the model to simulate different times of day and year, allowing designers to examine interior light under varying conditions (by looking through a small opening or using a camera). While still useful, physical models are not as necessary as they once were for daylighting analysis because of advanced computer daylight modeling tools now available.
Daylighting Performance and Design, author Gregg Ander, AIA, Chief Architect and Manager of Design and Engineering Services for Southern California Edison, provides detailed simulation results for the incorporation of daylighting features in a typical two-story office building in six representative U.S. cities. Using DOE-2 energy modeling software, Ander simulated a base case and two daylighting options for the building in each climate. The first option assumes the addition of automatic daylight-driven lighting controls in the building’s perimeter zone (33% of the floor space), and the second adds skylights and controls to daylight the core of the second story. The capital cost of the first option was assumed to be $7,530, while the second option was priced at $20,145.
Both strategies saved energy in every climate tested, with simple payback for the first option ranging from 1 to 1.5 years and for the second from 4.7 to 7.1 years. Peak electrical demand was also reduced in all climates, though reductions in demand costs were not included in the economic analysis. Peak demand for the second option (adding skylights) was only slightly lower (and in one case slightly higher) than for the first option. Interestingly, overall natural gas usage (for heating) was higher in every climate due to the reduction in heat output from the lamps. In some cold climates (Chicago, Seattle) this increase was enough that total HVAC energy use was slightly higher with the daylighting. In no case, however, was this increase enough to offset the savings from reduced energy use for lighting.
Productivity, Health, and Well-being
Daylighting offers an important energy-saving opportunity for commercial buildings, but it is actually the productivity benefits that are generating most of the excitement about this lighting technology. There is a growing body of evidence suggesting that people in naturally daylit spaces will be more productive—whether keypunching data in an office, stamping out widgets in a factory, buying clothes in a department store, or learning the three R’s in school. They’ll work more efficiently, they’ll miss less work (or school) due to illness, they’ll buy more, they’ll even be more creative, proponents claim.
In commercial office buildings, to follow that example, even a small improvement in worker productivity can mean much more to a company’s bottom line than a dramatic reduction in energy consumption. Consider that (in very round terms) a typical company annually spends about $2 per square foot for energy in an office building, about $20 per square foot for rent, and about $200 per square foot on labor ($22, $220, and $2,200/m
2, respectively). Thus, a one-percentage-point improvement in worker productivity could save as much money annually as the company’s entire energy budget!
Green building proponents have long suggested that such features as natural daylighting, natural ventilation, and healthy building materials can boost productivity. In a landmark paper in 1994, the Rocky Mountain Institute profiled eight buildings in which productivity improvements from a few percent to more than 15% were measured (see
Vol. 4, No. 3). Architects Mike Nicklas and Gary Bailey of Innovative Design in Raleigh, North Carolina—a firm noted for its daylighting designs in schools —presented a paper in 1997 indicating that students in schools with daylighting performed better in standardized tests (see
Vol. 5, No. 3). Neither of these papers, however, used scientifically rigorous data collection and statistical analysis.
Now, two studies commissioned by Pacific Gas and Electric have taken the statistical analysis of daylighting benefits in retail stores and schools to a new level, significantly bolstering the claim that daylighting boosts productivity. The studies, examining daylighting effects on retail sales and on school performance, were conducted by the Heschong Mahone Group under the direction of Lisa Heschong, with statistical analysis provided by RLW Analytics. They are part of PG&E’s Daylighting Initiative.
The first study, reported in
Skylighting and Retail Sales, compared the sales performance during an 18-month period of 108 retail stores operated by a large (unidentified) chain retailer. Two-thirds of the stores had skylighting; one-third did not. Otherwise, the stores were very similar, with the same basic interior design, the same merchandise, and all management and advertising handled by headquarters—thus making it possible to separate out and measure the effect of a relatively small number of variables. The researchers used multivariate regression analysis to identify variables that had statistically significant effects on sales performance (which was simplified into a “sales index” so that proprietary information about the retailer would not be divulged).
The significant variables were the absence or presence of skylighting, the number of hours the store was open per week, the population of the zip code where the store was located, the average income in the zip code where the store was located, and the number of years since the store had been remodeled (see figure above).
Skylighting was found to have the largest impact of these five factors, boosting the sales index by an average of 40% (range of 31% to 49%). In other words, if a non-daylit store had average sales of $2 per square foot ($21.50/m
2) over a given period, sales would be expected to increase to between $2.61 and $2.98 per square foot ($28.09–$32.08/m
2) if skylights were added. Statistical analysis found a 99.9% certainty that this is a true effect associated with daylighting.
The second study looked at student performance in daylit and non-daylit schools. In this study, reported in the paper
Daylighting in Schools, student performance was tracked in three different school systems that had both daylit and non-daylit classrooms.
The most comprehensive analysis was carried out in the Capistrano School District in Southern California, a district with 40,000 school students in 44 schools. Test scores in reading and math were examined from second through fifth grade classrooms in 27 elementary schools. The classrooms had a variety of lighting configurations, ranging from total absence of daylighting, to extensive window area, to large central skylights in rooms. The
rate of learning during the 1997-98 school year was examined by measuring improvement in standardized test scores from fall to spring. By using the rate of learning rather than absolute test scores or grades, the researchers controlled for demographic variables among students as well as for many other variables.
Students in classrooms with the most daylight had a 20% to 26% faster learning rate than students in classrooms with only artificial light. Students in classrooms with the largest window area learned 15% to 23% faster than students in classrooms with the smallest window area. Students in classrooms with central skylights that diffused the incoming light had 19% to 20% faster learning rates than students in rooms with no skylights, while students in classrooms with
non-diffusing skylights (where glare from direct sunlight may have been a problem) had a 21%
lower improvement in reading than students in rooms with no skylights. Finally, classrooms with operable windows were associated with 7% to 8% greater improvement in test scores than students in classrooms with fixed windows. The improvements in test scores correlated with highest daylighting to a 99.9% statistical certainty and with presence of diffusing skylights to a 99.7% statistical certainty.
While the other two studies of daylighting in schools—Seattle, Washington and Fort Collins, Colorado—were less detailed, they reinforced the basic findings of the Capistrano study. The researchers concluded: “We found a uniformly positive and statistically significant correlation between the presence of daylighting and better student test scores in all three districts.” By looking at both classrooms with skylights and classrooms with large window areas, they were able to conclude that “the positive effect of daylighting was distinct from all the other attributes of windows.”
Although both of these studies demonstrated with a high degree of certainty that daylighting can have real, measurable benefits, the reports do not explain why.
Among possible explanations listed by the authors for the retail effects are: customer loyalty that causes a higher percentage of customers to return to the daylit stores; more relaxed customers (like the effect of easy-listening music); better visibility of the merchandise; more attractive products because of improved color rendition; and better employee morale in the daylit stores, which might translate into better customer service. Possible explanations for the school test score improvements include: better visibility of work due to higher light levels or better light quality; improved health; improved mood; higher arousal levels (through the suppression of melatonin); improved behavior; and teachers being more attentive to student needs.
Lead author Lisa Heschong told
EBN that she is seeking funding to carry out further research. She wants to try to replicate these findings in other places, and she wants to examine daylight-induced productivity improvements in other building types.
Natural daylighting makes a lot of sense in almost any non-residential building. Energy savings can be realized through reduced use of electric lighting and—in many cases—from reduced cooling loads. More significant, daylit spaces feel better, and, perhaps as a result, they appear to boost productivity. New studies make the clearest case to date that daylighting can offer retail establishments a strong competitive advantage and that daylighting can be a very worthwhile investment for school buildings. Previous, more anecdotal evidence indicates that improvements in productivity from daylighting may also be occurring in commercial office buildings and factories.
Daylighting design, however, is neither easy nor inexpensive. It necessitates in-depth physical or computer modeling. While some daylighting measures are cost-neutral (such as moving enclosed offices from the perimeter to the core of the building), others will likely increase construction costs, especially if the starting point is a bare-bones box. It will almost always cost more to add special features to enhance daylight penetration, control glare, and control for electric lights so that energy savings will accrue from the daylighting measures. The more enlightened clients will see these features as fundamental to good design—and a better investment than high-end finishes in a lobby. As this awareness spreads, both the building occupants and the global environment will benefit.
For more information:
Windows and Daylighting Group
Lawrence Berkeley National Laboratory
One Cyclotron Road, Building 90-3111
Berkeley, CA 94720
510/486-6845, 510/486-4089 (fax)
www.lbl.govTips for Daylighting with Windows: The Integrated Approach available in PDF format from Web site.
Time-Saver StandardsSee review, page 15 of this issue
Daylighting Performance and Designby Gregg Ander, AIA