July 15, 2020
The term “glazing” originally referred to the glass in a window. Today, it usually means any transparent material in a window, although it also may refer to a film over that material. Skylight manufacturers use a variety of materials as glazing, primarily for the purpose of controlling its impact resistance. Glazing also affects the properties of heat and light that pass through the skylight. This post discusses some of the most common glazing options for skylights and their effects.
Annealing is the process of slowly cooling hot glass, which relieves internal stresses and makes it stronger. However, it breaks into shards, which poses a safety hazard. Tempering the glass solves this problem by using chemical and thermal treatments that place the outer surface of the glass into compression and its interior into tension. Skylights with tempered glass are much stronger than ordinary glass, and it also crumbles into granular chunks when it breaks, which is safer than shards.
Laminated glass consists of sheets of plastic such as ethylene-vinyl acetate (EVA), polyvinyl butyral (PVB), or thermoplastic polyurethane (TPU) between one or more sheets of glass. This plastic interlayer holds the layers of glass together when they’re broken, causing spider web patterns rather than shards. This safety feature is particularly important for skylights since it prevents the broken glass from falling into your home or building. White laminated glass uses a white interlayer that’s nearly opaque to give skylights an elegant appearance.
Polycarbonates are a class of thermoplastic polymers that contain the carbonate chemical group. They’re generally strong materials that can be easily molded with heat, and some types are transparent. This combination of properties makes polycarbonates highly useful for skylights. In terms of impact resistance, polycarbonates are stronger than commodity plastics but weaker than engineering plastics. However, polycarbonates are also vulnerable to scratching, so they require a protective coating. They degrade quickly due to the elements and cannot compare to the longevity of glass.
Poly (methyl methacrylate) (PMMA) is known by many other generic names like acrylic, acrylic glass, and plexiglass. It’s also known by trade names such as Acrylite, Astariglas, Crylux, Lucite, Perclax, Perspex, and Plexiglas among others. Acrylic glass is a transparent thermoplastic material that manufacturers often use in skylights as a lightweight, shatter-resistant alternative to glass. It’s less impact-resistant than polycarbonate glass, but also less expensive. Skylight manufacturers typically use acrylic glass when tensile strength, transparency, and UV tolerances are the most important properties.
Glass normally emits high levels of heat, which is usually an undesirable property for skylights. Low emissivity, or Low-E, glass is coated with materials that greatly reduce the heat that passes through it. Manufacturers typically make Low-E glass with chemical vapor deposition or magnetron sputtering. Vapor deposition involves the use of high heat to coat the glass with fluorinated tin dioxide during manufacture. Magnetron sputtering uses vacuum chambers to deposit layers of silver on the glass, which reflects heat.
LoĒ³ 366 glass is a proprietary line of Low-E glass made by Cardinal Glass Industries that provides superior thermal insulation. Its defining characteristic is three layers of silver, resulting in a clear coating that reflects heat while allowing visible light in. However, it does block 95 percent of UV radiation, which is a leading cause of fading. LoĒ³ 366 is the only Low-E glass that qualifies under the ENERGY STAR program for all four climate zones in the United States.
Obscure, or frosted glass, is made by creating a pitted surface on the glass, which makes it less translucent. This property blurs images while still allowing light to pass through the glass, producing the frosted effect. Etching with acid and sandblasting are the most common methods of producing obscure glass. The most significant benefits of this glass with respect to skylights are to reduce glare and provide visual privacy. Manufacturers may also use frosting to produce decorative patterns on skylights.
Some skylights contain tinting to reduce the heat and light entering your home. This feature becomes more beneficial in warmer climates, where tinted glass lowers cooling bills and reduces glare from the sun. Tinted skylights also reduce ultraviolet (UV) radiation, which is particularly hazardous. The tint is typically in a film over the glass, although the glass itself can also be tinted.