Air can leak around doors and heat can be lost through the doors because doors generally have insulation values less than half that of the surrounding walls.

Modern doors generally have higher R-values than older doors. The percentage of heat lost through doors, excluding sliding glass doors, is much lower than that of windows. Most heat losses or gains occur when opening and closing the door. Unless your door is damaged or difficult to seal to the door jamb, it is usually not necessary to replace it.

If you decide to replace your door, modern insulated metal or fiberglass doors may be a better investment than wooden doors. The R-values of most steel- and fiberglass-clad entry doors range from R-5 to R-6. A 1+ inch (3.81 cm) thick door offers better than five times the insulating value of the equivalent solid wood door.

The following table sets out R values for standard doors:

Glass doors, especially sliding glass doors, lose heat eight times faster than other types of doors. Even if the glass doors receive plenty of sunlight during the winter, a lot of heat is lost at night. Weatherstripping the door can help reduce air infiltration. The nature of the door's design, however, makes stopping all air leaks (while still being able to use the door) impossible.

When replacing patio doors, keep in mind that French or swinging doors offer a much tighter seal, and are generally more energy efficient than sliding doors. To reduce the possibility of condensation problems, all glass doors should have a "thermal break": an insulator between the layers of glass and the inner and outer parts of the door frame. Doors manufactured with several layers of glazing, low-e coatings, and argon gas between the glass panes are a better investment than plain double glass. Over the long run, the additional cost is paid back many times over in energy savings.

Tight fit and insulation is crucial when it comes to making sure that your door choices perform as best as they possibly can. Regardless of whether you are going for a solid wood door, an aluminum safety door or any of the many other types of exterior doors from which you have to choose from, it has to be insulated. This also includes making sure that the weather stripping between the door and the door jam seals tightly when the door is closed.