Check the insulation in your attic, ceilings, exterior and basement walls, floors and crawl spaces to see if it meets the levels recommended for your area. Insulation is measured in R-values; the higher the R-value, the better your walls and roof will resist the transfer of heat.
The easiest and most cost-effective way to insulate your home is to add insulation to your attic.
To find out if you have enough attic insulation, measure the thickness of insulation. If there is less than R-22 (7 inches of fiberglass or rockwool or 6 inches of cellulose) you could probably benefit by adding more.
Most U.S. homes should have between R-22 and R-49 insulation in the attic.
If your attic has ample insulation and your home still feels drafty and cold in the winter or too warm in the summer, chances are you need to add insulation to the exterior walls as well. This is a more expensive measure that usually requires a contractor, but it may be worth the cost if you live in a very hot or cold climate.
You may also need to add insulation to your crawl space. Either the walls or the floor above the crawl space should be insulated.
A good place to look for insulation values is the internet. Go to your favorite search engine and type in "residential insulation levels." You will get lots of options.
One quick and easy place to look is the Zip-Code Insulation Program by the Department of Energy (see link below)