Essential Elements
Shading Devices
All windows incorporate some type of solar shading into their design. Shading may be accomplished by the position of the window itself, for example by locating it high up, or in a northern corner of a room. However in most windows, shading is addressed separately from the aperture itself using a shading device. Horizontal elements such as overhangs and light shelves work well for orientations within 45 degrees of south since the sun is high in the sky. Vertical elements work well for elevations within 45 degrees of east or west since the sun is low in the sky. Shading devices serve a dual purpose: To reduce heat and to prevent glare. The most efficient way to manage the solar heat gain absorbed by the shading device is to keep it outside of the building envelope. To control glare, consider the brightness of the room side view of the shading device in comparison to the surrounding room interior, and use surface finish and geometry to keep within an acceptable brightness ratio.
Light Shelves Light shelves are functional for façades oriented within 90 degrees of south. The exterior portion of the shelf shades the lower window from direct sun to reduce the transfer of heat into the building envelope when it is not needed. Extend the shelf into the interior to prevent glare in near-window task areas by redirecting sunlight up on to the ceiling plane. See Figure 13. It is important to note that a standard light shelf does not increase overall daylight in the room; rather it decreases illumination near the window wall where it would otherwise be the highest. By doing so, it fulfills its primary function to even out the illumination gradient from the window to the rear of the room.
Design For Climate Zone The action of the shading device, the ability of the glazing to reject solar heat gain and resist the transfer of heat through the building envelope, are all performance characteristics that can be optimized to suit a particular climate zone. In an area where cooling is a major building expense, shading window apertures throughout the cooling period and using sun-protecting glazing will make a building less dependent on its mechanical systems and save energy. In regions where heating is a major cost, allowing sunlight into the interior building envelope to heat a large mass that in turn releases heat long after the sun has set, is a well established passive solar heating strategy. Using a window system with a high resistance to the transfer of heat, measured by U-value, will work to keep this passive collection of energy inside the building envelope.
Integrate Electric Light In order to maximize your daylighted floor area, daylight first, then supplement with electric light. After this, explore using light for general illumination during the day, which at night can be lowered or completely removed, as a person’s eyes adapt to lower light levels in preparation for sleep periods. Whatever your strategy, make sure to implement a daylighting control system, whether it is a combined daylight and occupancy responsive dimming system, or a person whose sole responsibility is to switch light fixtures and operate shades.
Remember, if the building’s electrical demand for light and heat is not being reduced, it is not daylighting. Keep these basics in mind and you too will master the age-old art of this renewable energy source.
Matthew Tanteri and Chris Meek will present “Advanced Sidelighting,” a 3-hour workshop covering lighting and shading, energy performance, and HVAC integration on Monday, May 7, 2007, from 9a.m. to noon as part of the Lightfair Daylighting Institute. For more information visit www.lightfair.com.