Your local weather . Free classifieds . House plans . Preferred listings showcase . Unfilled gaps and cracks in the foundation, around windows and doors, vents, and so on, may let cold air in the same as leaving a window open.
Caulk is used around outside window and door frames, and to fill outside wall and foundation cracks.
The money you spend on caulking or weather-stripping is usually recovered in one heating season or less. Clean away all old caulk and loose paint or dirt and apply new caulk to dry surfaces. The most common and easiest to use caulking comes in cartridges for which you will need a caulking gun. You can use the nail later to act as a stopper for any unused caulk. With a little practice on a joint that's not visible you'll soon be able to lay a uniform wide bead that overlaps both sides for a good seal.