Daylighting strategies must carefully balance a design with sufficient window area against the potential for glare and unwanted solar heat gain through the glass. A light shelf is one method of controlling glare and direct heat gain.
A light shelf is generally a horizontal element positioned above eye level. It divides a window into a view area on the bottom and a clerestory area on the top. In a detailed model, a light shelf can be external, internal, or combined. The light shelf can be integral to the building or mounted on the building. Glazing should be divided into daylighting glass and vision glass. A light shelf of 2.0 to 2.5 feet shades the lower vision glass and distributes light from the daylighting glass above.
Critical design factors include the orientation, position in the facade (internal, external, or both), and depth of the light shelf. For instance, an internal light shelf redirects the light but may also reduce the amount of light received on the interior.
In the northern hemisphere, light shelves are most effective on south orientations. They can be effective on north orientations for controlling glare but will not bounce light further back into the space. Light shelves on east and west orientations may not bounce light that much further into the spaces, but they are an effective means of reducing direct heat gain and glare. For south-facing rooms, the depth of an internal light shelf should be approximately equal to the height of the clerestory window head above the shelf.
Exterior light shelves reduce daylight near the window but improve the light uniformity. The recommended depth of an external light shelf is roughly equal to its height above the work plane. To reduce cooling loads and solar gain, an exterior light shelf is the best compromise between requirements for shading and distribution of daylight.