Daylighting is the controlled admission of natural lightmdash;direct sunlight and diffuse skylight—into a building to reduce electric lighting and saving energy. By providing a direct link to the dynamic and perpetually evolving patterns of outdoor illumination, daylighting helps create a visually stimulating and productive environment for building occupants, while reducing as much as one-third of total building energy costs.
A daylighting system is comprised not just of daylight apertures, such as skylights and windows, but is coupled with a daylight-responsive lighting control system. When there is adequate ambient lighting provided from daylight alone, this system has the capability to reduce electric lighting power. Further, the fenestration, or location of windows in a building, must be designed in such a way as to avoid the admittance of direct sun on task surfaces or into occupants’ eyes. Alternatively, suitable glare remediation devices such as blinds or shades must be made available.
A daylighting system consists of systems, technologies, and architecture. While not all of these components are required for every daylighting system or design, one or more of the following are typically present:
- Daylight-optimized building footprint
- Climate-responsive window-to-wall area ratio
- High-performance glazing
- Daylighting-optimized fenestration design
- Skylights (passive or active)
- Tubular daylight devices
- Daylight redirection devices
- Solar shading devices
- Daylight-responsive electric lighting controls
- Daylight-optimized interior design (such as furniture design, space planning, and room surface finishes).
A high-performance glazing system will generally admit more light and less heat than a typical window, allowing for daylighting without negatively impacting the building cooling load in the summer. This is typically achieved through spectrally-selective films. These glazings are typically configured as a double pane insulated glazing unit, with two 0.25 in. (6 mm) thick panes of glass that are separated by a 0.50 in. (12 mm) air gap. This construction gives the insulated glazing unit a relatively high insulation rating, or R-value, as compared to single pane glass. A low-emissivity coating is also often part of these high-performance glazing units, which further improves the R-value of the unit.
Many daylighting designs will employ skylights for toplighting, or admitting daylight from above. While skylights can be either passive or active, the majority of skylights are passive because they have a clear or diffusing medium (usually acrylic) that simply allows daylight to penetrate an opening in the roof. They are often comprised of a double layer of material, for increased insulation. Active skylights, by contrast, have a mirror system within the skylight that tracks the sun and are designed to increase the performance of the skylight by channeling the sunlight down into the skylight well. Some of these systems also attempt to reduce the daylight ingress in the summer months, balancing daylighting with cooling loads.
Tubular daylight devices are another type of toplighting device. These devices employ a highly reflective film on the interior of a tube to channel light from a lens at the roof, to a lens at the ceiling plane. Tubular daylight devices tend to be much smaller than a typical skylight, yet still deliver sufficient daylight for the purpose of dimming the electric lighting.
Daylight redirection devices take incoming direct beam sunlight and redirect it, generally onto the ceiling of a space. These devices serve two functions: glare control, where direct sun is redirected away from the eyes of occupants, and daylight penetration, where sunlight is distributed deeper into a space that would not be allowed otherwise. Daylight redirection devices generally take one of two forms: a large horizontal element, or louvered systems. Horizontal daylight redirection devices are often called lightshelves.