Skylight types include roof windows, unit skylights, tubular daylighting devices (TDDs), sloped glazing, and custom skylights. They are used to allow in direct and/or indirect sunlight, via toplighting. They provide a visual connection to the outdoor environment and sustainable building — passive solar heating, and with operable units; ventilation for passive cooling and fresh air exchange.
Fixed unit skylight
A structural perimeter frame supports glazing infill (the light-transmitting portion, which is made primarily of glass or plastic). A fixed skylight is non-operable, meaning there is no ventilation.
An operable (venting) unit skylight uses a hinged sash attached to and supported by the frame. When within reach of the occupants, this type is also called a roof window. A retractable skylight rolls – on a set of tracks – off the frame, so that the interior of the facility is entirely open to the outdoors, i.e., not impeded by a hinged skylight. The terms retractable skylight and retractable roof are often used interchangeably, though skylight implies a degree of transparency.
Tubular daylight device
Active daylighting uses a tubular daylight device—TDD. It is a roof-mounted fixed unit skylight element, condensing sunlight, distributed by a light conveying optic conduit to a light diffusing element. Being small in diameter, they can be used for daylighting smaller spaces such as hallways, and bounce light in darker corners of spaces. TDDs harvest daylight through a roof-mounted dome with diameters ranging from about 10 inches for residential applications to 22 inches for commercial buildings.