Skylights are light transmitting fenestration (products filling openings in a building envelope which also includes windows, doors, etc.) forming all, or a portion of, the roof of a building space. Skylights (roof windows, unit skylights, tubular daylighting devices (TDDs), sloped glazing) are used to convey abundant daylight or toplighting, provide a connection to the outdoor environment to occupants, and often to help fresh outside air enter the space below.
Skylight Basics A basic fixed unit skylight consists of a structural perimeter frame supporting one panel of glazing infill (the light-transmitting portion, which is made primarily of glass or plastic). An operable (venting) unit skylight uses a glazed sash attached to and supported by the frame. When within reach of the occupants, this type is also called a roof window. Unit skylights are typically shipped fully assembled to the jobsite.
A TDD has a roof-mounted fixed unit skylight element connected by a light conveying conduit to a light diffusing element, shipped in an unassembled, complete kit to accommodate infinite variations in site configurations.
Sloped glazing differs from other “skylights” in that one assembly contains multiple infill panels in a framing system, usually designed for a specific project and installed in sections on site.
Benefits of daylighting with skylights are numerous. Skylights are widely used in daylighting design in residential and commercial buildings, mainly because they are the most effective source of daylight on a unit area basis. The concept is simple; more daylighting means less artificial light and fewer square feet of necessary glazing, thus saving significant energy and resulting in lower financial and environmental costs.
Skylights are good for people. An independent study analyzed test score results for over 21,000 students from three districts located in Orange County, California; Seattle, Washington; and Fort Collins, Colorado, and concluded that students have significantly higher test scores in classrooms that optimize daylighting design than classrooms that do not. It is becoming clear through several other recent studies that daylight positively affects physiological and psychological well-being and has been shown to increase sales and productivity when properly designed.
Exploring the energy equation in more depth. Savings from daylighting can cut lighting energy use in some buildings by up to 80%, according to the U.S. Department of Energy's (DOE) Federal Energy Management Program. In terms of cost savings, the DOE reported that many commercial buildings can reduce total energy costs by up to one-third through the optimal use of daylighting. Efficiencies achievable in residential construction are not yet quantified, but should be sizable as well. The majority of commercial warehouses and box stores built in recent years have used skylights extensively in daylighting design.
Toplighting (skylights) works well with sidelighting (windows) to maximize daylighting in that 1) toplighting is able to bring light into centralized areas of a building, 2) daylight is available throughout the day from both ambient lighting from the sky and direct exposure to the sun, 3) modern transparent and/or translucent glazing can be utilized to avoid glare, aid in capturing sunlight at low angles and diffuse light to wider areas of floor space. Even on a cloudy day, toplighting is three to ten times more efficient than sidelighting. (source – AAMA Daylighting Fact Sheet).
Many recent advances in both glass and plastic infill systems have greatly benefited end users of all skylight types. Some are mainly intended to 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.
U-factor* expresses the heat loss performance of any building assembly. Solar heat gain coefficient (SHGC) measures the assembly’s transfer of heat from outside to inside that is caused by sunlight. These properties are labeled in the U.S. as a decimal between zero and one, with lower numbers indicating lower heat transfer rates. Depending on the geographic region, optimal U-factor and SHGC performance will vary. In the sunny southern climate zones, a lower SHGC is more important than lower U-factor. In the cooler northern climate zones, lower U-factor is more important, and higher SHGC can be justified.
- It is important to note that U-factor for National Fenestration Rating Council (NFRC)-certified skylights are based on a 20°-from-horizontal orientation and assume a minimum four-inch projection from the mounting surface, whereas windows and doors are tested vertically and are assumed to be inset mounted. Skylight thermal performance is, therefore, often perceived to be inferior to windows, but this is not the whole energy story as indicated above. Window U-factor and SHGC specification criteria are, therefore, NEVER applicable to skylight products.
- It is also worth noting that sloped glazing suppliers often provide a different set of performance values as a means of comparison for selection. Center of Glazing (COG) U-factors will always be significantly better than full product NFRC-rated values. COG values are measured from a single (optimal) center point whereas NFRC-rated values measure the entire assembly performance.
Careful selection of skylights is an important step in good daylight design. A balance must be struck between low U-factor and optimal SHGC values while preserving enough daylight supply to allow for artificial lights to be used only when absolutely necessary. Use of automatic electric lighting controls should be used to maximize energy savings. Unfortunately, NFRC has not yet decided how to rate many popular skylight products for their daylighting potential, so specifiers have to rely on manufacturer-derived claims.
Modern skylights using glass infill, like 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.
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. Acrylic is the most common plastic glazing used for dome skylights today; however, polycarbonate and copolyester materials are also often used as glazing, where additional properties such as impact resistance may be required to meet specific demands. See AAMA Skylight Council’s Plastic Glazing 101 article for more information on this subject. Plastics used in skylights are UV stabilized and may feature other advances to improve thermal properties. Lack of an accepted procedure for measuring light transmittance is one disadvantage when specifying this type of skylight glazing infill.
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