The Benefits of Daylight through Windows

Overview

This 88-page literature review was written by Peter Boyce, Claudia Hunter and Owen Howlett of the Lighting Research Center at Rensselaer Polytechnic Institute and dated September 12, 2003. It was produced for the Capturing the Daylight Dividend program, a multi-sponsor effort funded by the U.S. Department of Energy, the California Energy Commission, NYSERDA and several utilities and regional energy organizations. The authors start from a blunt premise: energy savings alone rarely justify extensive daylighting, so the case has to rest on what daylight does for productivity, health and the building's financial value. They review the evidence for offices, schools, hospitals and retail stores and end with a research agenda.

Key Findings

• Physically, daylight is simply visible radiation. Electric sources can copy its spectrum, but none reproduce the way daylight shifts through the day, the seasons and the weather.
• Daylight is a strong stimulus for both the visual system and the circadian system. Bright light by day and darkness at night keep the body clock entrained, and daylight is a convenient way to supply the daytime half of that pattern.
• Daylight is not inherently better than electric light for seeing tasks, but it usually arrives in large quantities with excellent color rendering, so it tends to maximize visual performance. Delivered badly, it causes glare, veiling reflections and shadows, and people will pull blinds or otherwise shut it out as soon as it becomes uncomfortable.
• Windows are strongly preferred for daylight and view, but mood effects depend on expectations and fade as people adapt, and the link from mood to productivity is weak.
• Health effects run both ways: views reduce stress, daylight avoids the flicker problems of some electric lighting, but UV and overheating remain concerns for sensitive people.
• Real-estate professionals told the authors that a near-total absence of windows cuts office rent by $2 to $4 per square foot against typical urban rents of $15 to $20, roughly a 20% penalty. No data existed on how rent varies with the amount or type of daylighting.
• The strongest retail evidence comes from a 108-store chain in which two-thirds of the stores had diffusing skylights. Skylights were one of five statistically significant predictors of a sales index, explaining 4% of its variance (opening hours explained 16%, years since remodel 9%).
• Four research priorities are proposed: reducing discomfort from windows, quantifying the financial return, testing daylight's effect on productivity through the circadian system, and testing the biophilia hypothesis.

Why It Matters for Daylighting

Although the title says windows, the review's most concrete commercial evidence comes from skylights in big-box stores, and its recurring warning is that occupants defeat any daylighting that causes discomfort. Both points favor diffusing toplighting: prismatic skylights spread light across a deep floor without the direct-sun glare and hot spots that make people reach for a blind, and there is no blind to pull on a warehouse roof. The circadian conclusion is the other half of the argument for industrial buildings, where shift workers may see little daylight outside the plant for months at a time in northern latitudes, a scenario the authors explicitly call out as the most promising place to demonstrate a productivity effect.

About the Source

The Lighting Research Center, founded at Rensselaer in 1988, was for decades the largest university lighting research group in the United States, and Peter Boyce is the author of the standard reference Human Factors in Lighting. The review runs 88 pages including an extensive reference list, and was peer reviewed by Kit Cuttle, Judith Heerwagen, Vivian Loftness and Arnold Wilkins. The DOE and other sponsors funded the work but the conclusions are the authors' own. The document circulates as a PDF and through academic indexes; no publisher web page for it could be located.