Steel and fiberglass doors typically have more insulating value than wood doors. Models that are Energy Star-qualified must be independently tested and certified, and often boast tighter-fitting frames, energy-efficient cores, and, for models with glass, double- or triple-panel insulating glass to reduce heat transfer. You’ll find more details on the federal EPA’s EnergyStar website. But you may not save as much as you think, since doors are a small part of the surface area of a house and typically don’t allow significant amounts of warm air to escape. What’s more, heat is generally lost through air leaks around the door, not through the door itself.
Entry doors are also known as door systems because they come pre-hung in a frame and are often pre-drilled for a knob and deadbolt. Unless a replacement door is part of a larger remodeling project, you may want the new door to be the same size as the old one. Choosing a larger door or adding sidelights means redoing the door framing around the door—a job best left to a contractor. Home centers generally offer installation or referral services. Unless you’re a skilled carpenter, you may also want to hire a pro to install same-size doors.