It's not just the front and back doors that you need to worry about. Cold air can also enter the house through gaps around any door leading to an uninsulated space, such as a garage or basement.
HOW TO DO IT:
Weatherstrip the top and sides, and add an inexpensive door sweep to the bottom. TOH general contractor Tom Silva recommends a wood sweep that you can stain or paint to match your door and that has an attached nylon brush to follow the contours of irregular flooring or carpeting. Sweeps usually need to be cut to size, so be sure to measure the width of the door first. To insulate the top and lock-side jambs, use peel-and-stick high-density foam tape or nail-on vinyl gaskets. For the hinge-side jamb, a premium adhesive-backed strip made of EDPM rubber will retain its shape after years of use.
For $20 and 30 minutes of your time, reduce cold air infiltration by as much as 11 percent.