Consider insulating your attic if your existing insulation is less than 6 inches thick or rated at less than R-25. Insulation is rated by R-value and the higher the R-value the better the insulation resists heat flow. Loose-fill insulation is blown into attics, using an insulation-blowing machine. It is inexpensive and easy to install. Adding 6 to 10 inches of insulation to achieve at least R-35 to R-50 is an excellent investment.
Before you insulate your attic, take two important preparatory steps. First, make sure your wiring is modern plastic-sheathed electrical cable. If you have knob-and-tube wiring or older cable with an organic fabric sheath, rewire the attic before installing the insulation. Second, seal the air leaks in the attic, since insulation doesn't seal air leaks. The air leaks are located around the chimney, plumbing vents, light fixtures, and wherever the ceiling has been dropped—in kitchens and bathrooms, for example.
Many lumber yards and rental businesses loan or rent small insulation-blowing machines. If you are handy and don't mind getting dirty, you can install the insulation yourself. However, you may not save much money over professional installation, which ranges from 45¢ to 85¢ per square foot. The professional insulator buys the insulation much cheaper than you can, and will use a high speed blower that gets the job done a lot faster than any machine you’ll use. But in either case, if your home is short on insulation, get it improved soon.
Both The Homeowners Handbook to Energy Efficiency and Saturn Energy Auditor Field Guide contain more information on insulation types and installation.