How Skylights Work

­Skylights typically are constructed with either plastic or glass, and the choice you make depends on several factors, including climate, location within the home and pricing. Manufacturers refer to types of skylights by their glazing, a general term used to describe how a skylight or window is constructed.

Plastic glazing for skylights is less expensive than glass glazing and yet it's still fairly durable. Plastic is more susceptible to wear, however, and will scratch, discolor or warp easily. And unless the plastic is coated with a special film, these kinds of glazes let in dangerous ultraviolet (UV) rays from the sun, which can harm people's skin and fade furniture, too. If you've ever seen dome- or bubble-shaped skylights in someone's home, chances are those skylights are ­plastic.Generally that design is considered out of style now.

Glass glazing for skylights is more expensive than plastic, but it's generally preferred because it's more durable than plastic, it doesn't discolor and many people simply think it looks nicer. Glass made for skylights must be made of safety glazing material, which means it has either been tempered or laminated. Tempered glass is glass that has undergone a process of heating and rapid cooling to make the material much harder than normal glass. As a result, when it breaks it shatters into small pebble-like pieces with no sharp edges. Laminated glass is made with a thin layer of plastic sandwiched between two pieces of glass. Although there are many different combinations, most manufacturers build skylights using a pane of tempered glass on the outside and a pane of laminated glass on the inside.

Various glazing techniques can increase a home's energy efficiency, too. Tinted glass can absorb heat and reduce the solar heat gain coefficient (SHGC), a measure of the amount of heat from sunlight a window or skylight blocks. How high or low you want your windows' SHGC depends on the climate in which you live -- the lower the number, the less heat is transmitted into your house. A skylight with a high SHGC, for instance, would be best to have for colder winters, while a skylight with a low SHGC is better at blocking heat during hot summers.

There are also low-emissivity (low-E) coatings on skylights, microscopically thin layers of metal spread over one or more of a skylight's panes. A low-E coating reduces the U-factor of a skylight -- the amount of non-solar heat allowed in. The lower the U-factor, the more efficient the window.