Windows and doors are usually the weak point of energy efficiency in well insulated homes. Approximately 25%1 of a home’s energy is lost through the windows alone. Double-paned windows can reduce this figure down to as low as 7%. Plus, double-paned reduce noise pollution. Adding reflective coating to and solar screens will improve the energy efficiency even further.
Doors generally lose energy around the perimeter. Air gaps or damaged weatherstripping will allow air to freely flow in and out. Weatherstripping is generally cheap and easy to replace. Sometimes the gap is large and it’ll require the weatherstripping to be stacked.
You will want to inspect the caulking seal on the exterior side of the house each fall and spring. Water moisture present in between panes of glass is an indication of a leak.
The weatherstripping around the perimeter of each door keeps it sealed. It will wear out over time with generally use of the door. Other side effects from use is wear on the hinges and locks seizing up. Keeping hinges and locks well maintained will prevent the door from seizing up or becoming “squeaky.”
References: 1 - http://www.servicemagic.com