What is effective daylighting? Daylighting design leverages sunlight to enhance a building’s interior illumination through direct, diffused and reflected lighting approaches. It can be used to complement electrical lighting, create more stimulating environments, and significantly reduce energy costs.
Why is it important? Effective use of daylighting can improve one’s mental state, enhance moods, boost focus and mental acuity, lower fatigue, and reduce eyestrain. Other significant benefits have been documented, from hastening healing in healthcare environments to improved student performance in educational environments.
What are the financial implications? In the short term the costs of implementing effective daylighting measures are readily offset by energy efficiencies resulting from reduced electrical bills. In the long-term, the superior qualities of the interior spaces which result will realize tangible increase in the resale value of the home.
Ten tips for effective daylighting:
1. Plan ahead: Implementing effective daylighting in the new home or remodel begins from systematic evaluation and programming of the opportunities presented by the building spaces, building location on the site, solar orientation, and illumination needs of the various rooms. Then, comprehensive schematics should be prepared which answer to the opportunities and strategies identified in your programming exercise.
Design your home around your solar and climatic cycles to take maximum advantage of both (image credit: Real Goods: The Passive Solar House Book).
2. Know your site: Leverage your site’s specific climate, solar path and orientation. What your daily climatic cycle? Is it foggy in the morning with clearing in the afternoon? Where does the sun rise, and where does it set? Your climatic and solar patterns should inform where you place windows, the type of glazing used, and shading strategies.
3. Target your design goals for exterior openings carefully: Because the sun is an extremely powerful light source, providing up to 10,000 foot-candles, but also a source of heat gain of up to 300 Btu/hr/ft2, it is important to balance the lighting benefitts against heat gain liabilities. The desirable illumination levels inside a building are up to 1000 times less than they are outside, so a small amount of sunlight can be distributed over a large area and still provide adequate illumination. Make the design goal to meet minimum illumination levels through as much of the day as possible without significantly exceeding the minimum, because excess light also means excess heat gain.
Windows of equal area will light the space differently depending on their shape and location in the wall.
4. Daylight zones: arrange rooms so that activities that need higher lighting levels are near windows while activities that need less light are farther from daylight sources. Most homes have a range of activities that have varying visual tasks and therefore different illumination needs. If activities are zoned so that those that need light are placed near openings in the skin and those that don’t are placed in the interior, then the amount of relatively expensive skin and glazed openings can be reduced because of a smaller skin/volume ratio.The rate of electric light use, and thus heat gains, are also reduced.
5. Locate windows next to a side wall wherever practical: Placing a window adjacent to a returning wall results in additional reflection of light deeper into the space. But avoid placing windows next to a single wall. A window next to both side walls or a window next to one side wall with an additional supplement vertical element from another orientation provides better distribution.
A shed dormer providing controlled daylighting high in the space (Alta House)
6. Place windows higher: use window shapes that place a high percentage of the glazing above the wall’s mid-point. Windows with higher head height project light deeper into the room. Secondly, since it is tucked close under the eaves light admitted at this location is automatically shaded, thus eliminating undesirable direct light penetration. Thirdly, in this location the direct light of the sky dome is effectively blocked, preventing glare. As a general rule of thumb, for rooms with windows on only one side, place about 30% of the window area in the upper 1/3 of the wall.
7. Use shading devices: Exterior shading devices can be either horizontal, vertical, or a combination of horizontal and vertical called “egg crates.” Horizontal shades provide effective shading on the south facade when the sun altitude is high. Vertical shades are effective when the sun is low and the broad side of the vertical elements faces the sun. Vertical shades perpendicular to the window are most effective on the polar side, where no horizontal element is needed.
Light shelves can be used to shade view glazing, evenly distribute light, increase light levels away from windows, and reduce glare.
8. Light shelves: A window divided into upper and lower sections by a horizontal light shelf performs better than an ordinary window in several ways. If the light shelf extends beyond the exterior surface of the glazing on sunny exposures, it can be used to shade the view glazing. At the same time, it can reflect light off its top surface through the upper glazing to the ceiling, where it is then reflected deeper into the space. The light shelf section that extends into the space also reflects light from the upper part of the window deeper into the space, while decreasing the light levels immediately adjacent to the window, thereby evening the distribution of light throughout the space. The interior portion of the light shelf also blocks the occupant’s view of the sky, which is a potential source of glare.
Used effectively, light wells are a good way to control glare caused by skylights.
9. Use of skylights: think of roof openings (skylights and light tunnels) as substitutes for light fixtures. Among the benefits of skylights are greater illumination on horizontal surfaces than sidelights. When light is admitted from above, the walls are free and the daylight distribution can be more uniform. Finally, because they have horizontal as opposed to vertical orientation, skylight systems admit more light per unit area than windows. But be careful in their utilization, since mitigation of glare is more challenging with skylights in particular.
10. Finishes matter: select interior design finishes to leverage your daylighting approaches. Use light colored surfaces throughout the building. Consider painting ceilings white to promote bounce and diffusion through spaces. Also, gloss or semi-gloss paint schemes reflect more light than eg-shel or satin finishes.
Except as noted, images in this article Sun, Wind, and Light: Architectural Design Strategies/ Wiley & Sons/ New Jersey/ 3rd edition, 2014.
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