Skylights are light transmitting fenestration (elements filling building envelope openings) forming all, or a portion of, the roof of a building’s space for daylighting purposes.Skylighting types include roof windows, unit skylights, tubular daylighting devices (TDDs), sloped glazing, and custom skylights. Uses include:
- Daylighting elements used to allow direct and/or indirect sunlight, via toplighting.
- Providing a visual connection to the outdoor environment to interior occupants.
- Sustainable building — passive solar heating, and with operable units; ventilation for passive cooling and fresh air exchange
Plastic glazing infill is commonly used in many skylights and TDDs. These assemblies typically contain thermally formed domes, but molded shapes are not uncommon. Domed skylights are typically used on low slope roofs. The dome shape allows for shedding of water and burning embers. Plastics used in skylights are UV stabilized and may feature other advances to improve thermal properties. Lack of accepted standards for measuring light transmittance is a disadvantage for comparing and choosing skylights with plastic glazing. Acrylic is the most common plastic glazing used for dome skylights. However, Polycarbonate and Copolyester materials are also used as glazing, where additional properties such as impact resistance may be required
Many recent advances in both glass and plastic infill systems have greatly benefited all skylight types. Some advances increase thermal performance, some are focused on preserving and utilizing daylight potential, and some are designed to enhance strength, durability, fire resistance and other performance measures. Contemporary skylights using glass infill (windows) typically use sealed insulating glass units (IGU) made with two panes of glass. These types of products are NFRC-ratable for visible transmittance. Assemblies with three panes can sometimes be cost-justified in the coldest climate zones, but they lose some light by adding the third layer of glass. Glass units typically include at least one low emissivity (Low-E) coating applied to one or more glass surfaces to reduce the U-factor and especially SHGC by suppressing radiant heat flow. Many varieties of Low-E coatings also reduce daylight potential to different degrees. High purity inert gas is frequently used in the space(s) between panes, and advances in thermally efficient glass spacing and supporting elements can further improve thermal performance of glass-glazed skylight assemblies.
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.
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.
Fixed unit skylight
A fixed skylight consists of a structural perimeter frame supporting 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.