Final answer:
The most energy efficient exterior doors are those with good insulation properties, such as solid wood doors with a polyurethane foam core and fiberglass doors with a polyurethane foam core.
Step-by-step explanation:
In terms of energy efficiency, the most important factor for an exterior door is its insulation properties. Doors with good insulation can prevent heat loss or gain, reducing the need for heating or cooling inside the house. One example of an energy-efficient exterior door is a solid wood door with a polyurethane foam core, which has a high insulating value.
Another energy-efficient option is a fiberglass door with a polyurethane foam core, which offers good insulation and is resistant to warping and cracking.
When choosing an energy-efficient exterior door, it is also important to consider factors such as weatherstripping, air infiltration, and the door's alignment within its frame.