While light shelves help redistribute the sun’s direct visible energy deep into a building, they also reduce the amount of radiant energy at the building’s perimeter (which, in turn, reduces the building’s cooling load and associated energy consumption). The integration of light shelves to create high-performance building design requires a team approach since, not only are the building aesthetics a consideration, the visual and thermal environments are major design factors as well.
The design and application of light shelves requires some common sense and careful analysis. The following rules of thumb are relevant for projects located in the Northern Hemisphere:
- Light shelves are most effective with south-facing glass.
- Light shelves are not very effective with east- and west-facing glass. Limiting the size of windows and using blinds or shades are more effective ways to minimize thermal
- heat gain and control glare from the low sun angles on these exposures.
- Light shelves are not effective with north-facing glass. North-facing skies provide diffuse daylight rather than direct sunlight for much of the year.
- Light shelves are not very effective on any building where the number of overcast days is significantly higher than the number of clear, sunny days at the project location.
- Matte is the recommended finish for the top of the light shelf to diffuse the direct sunlight, which will minimize glare and provide uniform distribution of light inside the room. (Highly specular finishes are currently being studied to maximize daylight penetration; however, diffuse finishes are recommended until more information and design guidelines are defined.)
Now, to the careful analysis: To illustrate the light shelf’s effectiveness, two computer models were prepared – one with a light shelf, the other without. Properties of the model spaces are defined in Properties of the Model Space (see sidebar and graphic).
|Max. Horizontal Illuminance (footcandles)||Min. Horizontal Illuminance (footcandles)||Uniformity|
|No Light Shelf Model||6,546||92||71:1|
|Light Shelf Model||470||53||9:1|
SOURCE: The Lighting Practice Inc.
A number of daylight calculations were performed at different times of the year under varying sky conditions within the room at 2-feet, 6-inches above the finished floor. The calculation was performed for Philadelphia on April 21 at noon under clear-sky conditions. The calculations were performed using Lumen Designer lighting software. The first observation should be the extremely high light levels closest to the window in the office with no light shelf. This begins to tell an even greater story about the light quality and characteristics within the space – the most telling being the uniformity.
It is critical to balance the uniformity of illumination at the work plane and in the immediate visual environment. Uniformity allows us to focus on visual tasks and reduces eye strain. Recent studies (Heschong Mahone Group Inc., 1999, 2001, 2003) have shown that student performance on test scores increased significantly in classrooms with a properly designed daylighting system – one that balances illumination levels and uniformity, and reduces direct glare.
A light shelf is an architectural element which is designed to scatter natural light into a room, reducing the need for artificial lighting. Light shelves also provide shade near the windows, reducing glare and keeping rooms cool when temperatures rise. This design feature can be integrated into a building when it is constructed, or added later to increase energy efficiency. Along with things like skylights and light tubes, light shelves are considered “daylighting” devices, meaning that they are designed to cut down on the use of artificial lighting during the day.
In order for a light shelf to be effective, the ceilings in the room typically need to be high, to provide a lot of room for the light. The shelf is placed above eye level in a window, and it can be located either indoors or outdoors. On the upper surface, the light shelf is coated in reflective material so that when light hits it, the light bounces off and hits the ceiling, pushing light deep into the room. When the room is painted in pale colors, these light colors combined with the light shelf are often enough to illuminate the space without the need for a lamp.
In the northern hemisphere, light shelves are usually deployed along the southern edge of a building, so that they will be able to catch the most sunlight. This is reversed in the southern hemisphere, where light shelves need to be positioned on the north side of a building for maximum exposure. In both cases, the light shelves need to be regularly maintained to ensure that they are as reflective as possible so that they will continue to be effective.
The major drawback to a light shelf is that it only works on sunny days. When the sky is overcast, the light shelf will be able to scatter some light, but not enough to make a significant difference, and it will usually be necessary to supplement with artificial light. These architectural elements can be designed to mesh with the look and feel of a building so that they are not obtrusive, although they will certainly be visible. When the shelves are installed indoors, people must be reminded not to store or display objects on the shelf, as this can interrupt the flow of light.
In addition to increasing energy efficiency by eliminating the need for artificial light during the day, light shelves can also benefit the health and psychological well being of people who use the spaces they illuminate. Natural light appears to be highly beneficial, especially when contrasted with lighting options such as fluorescent lighting, and it can also make a space more pleasant to work in, thereby increasing worker satisfaction.