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.
In fact, a 1/8 inch opening around just two doorframes can let in as much cold air as a 12 inch window opened 6 inches all winter long.
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.This one season "pay-back" period means that money for heating fuel is saved equal to or greater than what you spend for caulking and weather stripping materials.
A clean joint is the first and most important step. 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. A good rough estimate is that you will need 1/2 cartridge per window or door, 4 for the foundation sill, and at least 1 more for around faucets,vents, pipes, electrical outlets and so on.
Cut off about 1/2 inch of the cartridge tip on a 45degree angle and puncture the tip seal with a nail. 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. Finish the surface with a moistened finger if you like but that's not necessary. Remember to use a filler, like oakum, for wide joints before you caulk.
Some but not all the places you should look when surveying you home before caulking are around doors and windows, dryer vents, faucet pipes and wires, where porches attach to the house, seams between masonry and siding, chimneys and inside corners
Article source: http://www.411homerepair.com/diy/windows-siding/8528-Caulking-Windows.html