A successful sustainable building design must consider the window area of the project carefully. Most heating and cooling energy is transmitted in and out of a building through its windows, but with intelligent design and careful selection of components, windows can be used to provide a comfortable and energy-efficient indoor environment.
Natural light and heat flow through a window can be controlled to some extent through appropriate size, window characteristics (Solar Heat Gain Coefficient, U-value, and Visible Light Transmittance), and solar orientation. Larger windows have more potential to lose or gain heat than smaller windows, and south-facing windows (in the Northern hemisphere, and north-facing windows in the Southern hemisphere) transmit more heat and light than windows on the other orientations. Overhangs and light shelves on south-facing windows can be designed to allow sunlight in winter and provide shade in the summer, but it is difficult to control direct sunlight on the east and west facades, due to the lower sun angles. Residential projects may want to take advantage of passive solar heating, but typically commercial projects use very little heating energy. Non-residential projects need to be more concerned with controlling unwanted solar gain through the windows and providing a means of daylighting the interior spaces (typically the largest single energy usage in a commercial building is lighting).
The daylighting zone may be considered to be a depth of about twice the window head height. For example if a space has windows with a head height of six feet, it may be feasible to daylight up to about 12 feet deep into the space (assuming no internal partitions would block the light).
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 gain.
A light shelf is generally a horizontal element positioned above eye level and divides a window into a view area on the bottom and a clerestory area on the top. It can be external, internal, or combined and can either be integral to the building, or mounted upon the building.
The orientation, position in the façade (internal, external, or both), and depth of the light shelf are critical factors in the design. For instance, an internal light shelf redirects the light but may also reduce the amount of light received on the interior.
Light shelves are most effective on the south orientations in the northern hemisphere and can be effective on north orientation for controlling glare (but will not function to 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 are an effective means of reducing direct heat gain and glare. For south-facing rooms it is recommended that the depth of the internal light shelf be approximately equal to the height of the clerestory window head above the shelf.
Exterior light shelves reduce daylight near the window but improves 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.
Light shelves may be constructed of many materials, such as wood, metal panels, plastic, fabric, or acoustic ceiling materials. Considerations that affect the choice of material are variables such as, structural strength, ease of maintenance, cost, and aesthetics.