Exterior doors can contribute to heat loss and solar heat gain in the same manner as windows – if the door contains windows then it is subject to heat loss through conduction, convection and radiation, and is also subject to air infiltration and leakage through gaps and cracks in the door framing.

The NFRC rates doors for U-Factor and Solar Heat Gain Coefficient, just as it does for windows. Doors may also be rated by the manufacturer with an R-Value.

Guidelines for your Doors Project

Working with your existing exterior doors. With a brownstone you may be working with the original wood front entrance single or double doors and the interior vestibule door. These doors, being antiques, are usually much thicker than modern doors and made from old growth wood with very good thermal properties, and being architecturally desirable it is well worth the effort to restore them and to work on the framing to mitigate air leakage. If your doors have lites, consider adding a low-e coating or an additional pane on the inside. Weatherstripping and caulking is essential to eliminate gaps through which air leaks. See our Article Insulation and the Building Envelope: Controlling Heat Loss and Gain for a full discussion of caulking and weatherstripping.

Replacing exterior doors. If you completely replace an existing door, right down to the building rough opening, then you would likely buy a pre-hung door that comes already hanging in a frame and is installed within the rough opening. New doors are made with thermal cores such as polyurethane foam insulation. Be sure to fill all gaps between the new pre-framed door and the building with insulation. Add weatherstripping, and check it annually to see if it needs to be replaced. link to insulation article

Glass sliding doors pose the biggest challenge to energy efficiency. If you must have sliding doors over swinging glass doors, which typically make a much better seal, be sure the door you choose has a thermal break between the inner and outer frame, low-e coating and gas fills between the panes.

Storm doors, like windows, come with all types of framing (wood, steel, fiberglass, aluminum), and glass options (low-e, tinted) with an array of options for operating the windows and insect screens. The Department of Energy recommends that a glass storm door not be used if the door receives more than a few hours of direct sunlight a day because the heat build-up between the storm door and the entry door could damage the entry door. Storm doors are only relevant for the back of the  building when dealing with brownstones in a landmarked zone.