The exterior of your home is often referred to as the "envelope" or the "shell". Sealing your home against air filtration (air leaking in from the outside) and air ex-filtration (air leaking from inside of the house to the outside) helps to reduce your energy expenditures.
As a homeowner, you can greatly improve your home's energy efficiency by sealing as many entry points as possible. Check seams between the top of the foundation wall and the wood framing, doors and windows, along baseboards, through electrical receptacles and switches mounted on exterior walls, fireplaces, laundry chutes, attic hatchway doors and pull-down stairs, whole-house fan installations and pipe and wire penetrations.
To test for air leaks using a "blower-door test" to accurately measure air leakage of your house. The test is done by temporarily de-pressurizing the house to get an accurate measurement of how much air is entering the house through cracks, gaps and holes.