It is safe to say that although you lose heat and energy through gaps in windows and doors, the biggest culprit of energy loss in the home is through the basement or the attic.  Basically, cold air enters in your basement, rises up through the home, causing drafts and exits through the attic, otherwise known as the chimney effect.  When repairing energy loss in the attic or basement, it is sometimes more beneficial to call in the professionals to do the job, rather than attempting to do it yourself.  However, there are some things to look for and fix immediately.

One cause of heat and energy loss in the attic is through the attic pull down stairs, door or access panel.  A quick fix is to attach fire rated insulation to the backside and install a weather stripping around the sides of the door.  Ensuring that the door has a tight fit will prevent air from flowing up into the attic.  Recessed lights are another perpetrator of air leaks.  To right the problem, the lights either need to be replaced, or consider having a professional properly seal the top side of the box with the appropriate insulation.  Chimneys and vent stacks may have gaps around them; these can be filled with foam sealant or caulk.

By taking care of these problems, the chimney effect has been minimized.  However, there are still leaks in the basement, and it is wise to take care of them to prevent any more heat or energy loss.  The most common source of leakage in the basement is through cracks or gaps in the rim joist.  The rim joist is the perimeter framing of the flooring.  These gaps can sufficiently be filled with caulk or spray foam.  In addition, be sure to seal other gaps around dryer vents or ductwork, electrical passages or any other utilities that require access to the outside.  Now that the major causes of leaks have been taken care of, fixing gaps in windows and doors will make more of a difference.

http://www.energycircle.com/learn/air-sealing/plan-your-attack