By Craig DiLouie
Daylighting, or the use of daylight as a primary source of general illumination in a space, has been demonstrated in research to improve user satisfaction/performance and retail sales, making daylighted buildings more valuable and marketable. Daylighting is closely connected with providing views to space users via glazing, also demonstrated to increase user satisfaction/performance. Use of daylight creates opportunities for daylight harvesting controls, which can generate 35 to 60 percent or more in energy savings. Daylighting is encouraged by LEED and required in some spaces by California's Title 24 energy code.
Good daylighting design makes diffuse daylight the primary illumination, provides gentle uniform light throughout the space, and enables occupants to control the daylight. Getting daylight into a space is not difficult - controlling it is the challenge. Good design avoids glare, direct sunlight penetration, and too much daylight. An excess of direct sunlight, as opposed to diffuse daylight spread uniformly throughout the space, can cause glare and contrast problems, heat gain, and lost energy savings opportunities.
To integrate daylighting into the building and its electric lighting system, try these tips:
- Avoiding direct sunlight: Direct sunlight should not be allowed in the building except in circulation areas.
- Penetration: Daylight can be captured through sidelighting or toplighting, enabling the architect to control the quantity and quality of light entering the space. Generally, daylight should enter the space from as high a point as possible. Sidelighting can be enhanced by using sloped ceilings and lighting a space from two sides. Glazing and/or shading can be used to diffuse the light as broadly and uniformly as possible throughout the space. Perimeter zones can be increased to maximize usable daylighting area. Consider separating windows used for daylight (higher windows, with 70- to 80-percent visual light transmission) and view (lower windows, with lower visual light transmission capability of 30 to 40 percent) to reduce thermal impacts and glare.
- Glare: Blinds and louvers can reduce direct glare from windows. Consider integrating automatic lighting controls with automatic window shades, blinds, or other devices that can reduce direct glare and heat gain. Using the same control station, users can control both daylight and electric light levels.
For windows on all sides of the building used for daylighting except the north side, consider shading with fixed architectural elements such as light shelves, overhangs, shades, or vegetation. Look for other possible solar glare situations and control them as well. Continuous windows in horizontal bands can provide a higher level of visual comfort (reduced brightness contrast) than "punched" windows.
- Color quality: When warm color temperature fluorescent sources (less than 3500 kelvins) are used in conjunction with very cool daylight (greater than 5000 kelvins), the lights may appear yellow. To mitigate this effect consider lamps with a high color temperature (5000 to 6000 kelvins) if the space is used primarily during the day, neutral-white color temperature (3500 to 4100 kelvins plus) if also used at night. Because daylight has a desirable continuous and full color spectrum, the color of the window glass should be as neutral as possible.
- Surface colors: Consider light-colored surfaces in the space, with brighter surfaces kept out of the line of sight of direct sunlight.
- Distribution: The electric lighting system should place light on the same surfaces as the daylight. While studies suggest that occupants prefer light levels to change over the course of the day - preferring higher light levels during the day than at night - significant changes in surface brightness should not be allowed. If the daylight architecture distributes light on walls and ceilings, the electric lighting should also do so.
In deeper daylighted spaces, supplemental wall washing at the rear wall and adjacent ceiling softens the gradation of light from the window to the interior (reduces the brightness contrasts).
Diffuse sources such as fluorescent are considered a good match for the diffuse distribution characteristic of daylight.
- Saving energy: Daylight harvesting strategies entail switching or dimming loads in response to the variable availability of daylight in the space, maximizing energy savings while minimizing intrusiveness of the control effect. Strategies include dimmable ballasts, dimmers, photosensors, time clocks, and other controls. According to Heschong Mahone Group - a professional building energy efficiency consulting firm with a specialty in daylighting research - energy savings from daylight harvesting controls can range from about 50 to 75 cents per square foot, depending on the characteristics of the application.
Craig DiLouie (firstname.lastname@example.org) is principal at ZING Communications Inc.