Adding a storm door can be a good investment if your existing door is old but still in good condition. However, adding a storm door to a newer, insulated door is not generally worth the expense, because you won’t save much more energy.
If you plan to purchase a storm door, consider features that improve the energy efficiency.
Storm door frames are usually made of aluminum, steel, fiberglass, or wood (painted or not). Wooden storm doors require more maintenance than the other types. Metal-framed storm doors might have foam insulation inside their frames.
High-quality storm doors use low-emissivity (low-e) glass or glazing. Other features may include screens with self-storing pockets, full-length screens with removable glass panels, and screens and glass that slide past each other. All of these features add convenience and cost.
A glass storm door could trap heat against an entry door and cause damage if the exterior door gets more than a few hours of direct sun each day. Low-e glass will reduce the heat gained. Check the door manufacturer’s recommendations if this is a concern.
Storm doors for patio doors are hard to find, but they are available. Adding one to a new, multi-glazed low-e door is seldom economic. Cellular shades, when closed for the night in winter or on sunny days in summer, are also a good idea.