A poorly insulated attic squanders energy in both the winter and summer -- sending heat right through the roof in cold weather and serving as an enormous solar collector on hot, sunny days.
How you approach an attic insulating project depends on how you're planning to use the area. If the space will be used only for storage, insulate the floor, as shown in this article. If it is finished or you intend to finish it later, insulate the ceiling and walls, as shown in Insulating a Finished Attic (Related Links, left).
Either way, look first for leaks that might damage insulation, check to be sure you have adequate ventilation, and decide if you want to cut summer heat buildup with an attic or whole-house fan.
If your attic has a floor, you have two installation choices: pull up sections and work the insulation under or hire a contractor to blow in loose-fill through holes bored in the floor. With unfinished floors, be sure to bring up some planks or pieces of plywood so you can get around on the joists without stepping through the ceiling below. Ceiling materials alone won't bear your weight.
Use batts, blankets, or loose-fill. If there already is a vapor barrier, use unfaced materials or slash the facing so moisture doesn't get trapped between the two barriers. Also, it's important you don't cover recessed light fixtures or exhaust fans; doing so could cause a fire. Instead, install baffles that keep the insulation about 3 inches away on all sides.
Caution: Some materials, such as fiberglass and mineral wool, are harmful to lungs and skin, so be sure to wear a painter's mask, gloves, and long sleeves if you'll be working with hazardous materials.