benefits of daylighting

by Betsy Hooper, CSI, CDT


the rate of learning increased with the amount of daylight...
students with the most daylight performed 20% to 26% better than those with only artificial light


When seeking ways to improve the energy efficiency of our buildings, one place to look is at the way we illuminate them. Daylight is free to all, and available for roughly half of every twenty-four hour day. In commercial buildings in the United States, roughly 34% of all electricity consumption goes into illumination. As a rule of thumb, another half unit of electricity is used in cooling for every unit of electricity used in commercial lighting. Given that the lion's share of commercial activity occurs during daylight hours, there is potential for tremendous savings in electrical consumption by the better use of daylight to illuminate our buildings.

While daylighting cannot be expected to meet all the illumination needs of most businesses, its use in conjunction with daylight sensing dimmers on the lights, can produce substantial energy savings. ESource, Inc. indicates potential savings of 40% to 60% of lighting energy consumption. The savings in cooling will be somewhat less, depending on both the original lighting scheme, and the daylighting strategy pursued. The particular daylighting strategy, materials used, and geographic location of the building all play a part in the amount of cooling load that is saved with daylighting.

In addition to the energy savings, studies indicate that there are across the board improvements in productivity and reduced absenteeism in daylit facilities. Early studies lacked the scientific rigor demanded by skeptics, but more recent studies commissioned by Pacific Gas and Electric in retail stores and schools have confirmed the earlier results. Factoring out other influences on sales volume, skylit stores had on the average 40% higher sales than their counterparts. In a separate study by the Rocky Mountain Institute (1994) productivity improvements of up to 15% were measured. Given that the typical company spends on labor about 100 times what they spend on energy, and ten times what they spend on rent, even a small improvement in productivity can have a major impact on the bottom line.

In PG&E's second study, Daylighting in Schools, students in three different school districts were studied. Test scores were studied for students in the second through the fifth grades. The rate of learning increased with the amount of daylight in the classrooms, with students having the most daylight performing 20% to 26% better than their counterparts in classrooms with only artificial light.

The two primary daylighting strategies are perimeter lighting (old-fashioned windows), and skylights or roof monitors with varying degrees of light diffusion. There is not enough space here to go into the complex issues involved in adequately illuminating the workplace, let alone controlling daylight. However extensive work has been done in the field and there are several good sources of information available (see below). Evaluating a design for daylighting performance has historically meant building a model and testing it with a movable light source simulating incident sunlight at various times of the year. Advanced computer modeling tools are now available to simplify the process of analysis.

While daylighting can reduce the building operating costs and increase productivity, the proper design of a daylit building is challenging for the designer. It can also mean a higher first cost for the owner. However an enlightened client, teamed with a dedicated architect can produce a daylit building that looks to the future, both for the client, and for the planet.

Much of the information in this article comes from Environmental Building News, Volume 8, Number 9. Other sources of information to assist in daylight design are:

  • Tips for Daylighting with Windows: The Integrated Approach. Published by Lawrence Berkeley National Laboratory and available in PDF format on their web site
  • Time Saver Standards. 7th Edition.
  • Skylighting Guidelines available from
  • Daylighting Performance and Design, by Gregg Ander, AIA. Van Nostrand Reinhold, 1995.

Betsy Hooper is an architect with the Martella Association, Knoxville, Tennessee. This article appeared in "The Speck", newsletter of the Knoxville Chapter, CSI. Reprinted with permission.

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