Plastic skylights, generally acrylic or polycarbonate, come molded in flat, bubble, round, pyramid, ridge-shaped, dormer, and barrel-shaped profiles as a rule, and they’re available only in standard sizes. Plastic glazing is chosen where light weight, durability, and economy are important.

Weight is a big factor. A 4-by 4-foot plastic-dome skylight weighs about 65 pounds compared to a glass skylight of the same size at about 120 pounds. Most quality plastic skylights are double-glazed with two plastic panes separated by an air space.

Glass is generally preferred for skylights because it has a more elegant appearance than plastic, offers a clear view, doesn’t scratch, and can be ordered in a wide variety of sizes and types. Of course, the more features you want, the more expensive the glazing will be.

Following is a closer look at typical choices:

• Tempered, double-insulated glass. This is the minimum standard for most quality products: tempered for safety and two layers of glass with an air space between for energy efficiency.

• Tempered, double-insulated glass with low-E (low-emmissivity) coatings. With this glass, the outer pane is tempered, the inner pane has an invisible low-E coating that helps hold heat in during the winter, reduces heat gain in the summer, and cuts ultraviolet (UV) rays, which bleach fabrics and damage furniture and floors.

• Tempered, double-insulated glass with low-E coatings and an argon gas filling between panes. This glass is the same as above but has even greater insulating value. Some companies, such as Marvin and Pella, offer low-E, argon-filled glass as standard; other manufacturers sell this as an option.

• Tempered, low-E, argon-filled glass with a bronze tint. This features the same qualities as above but is designed to cut back on solar heat gain.

• Tempered over laminated, low-E, argon-filled glass, with or without a bronze tint. Designed to eliminate the danger of interior glass breakage in areas that require laminated glass, this type blocks nearly all UV rays. It’s a sensible glazing for regions where the sun’s heat buildup is a problem, such as on south- and west-facing roofs in hot climates.