Heating and cooling a house is one of the most expensive and energy-intensive tasks for a modern homeowner. While there are many things that you can do to lower your heating and cooling bills, the quality of your home's insulation is the most important factor. The easiest type of insulation to reach (and the most vital to reinforce) is the insulation in your attic.
Inside Your Attic
Your attic should be fairly easy to access with a stepladder and a flashlight. Make sure to take a look at the access panel once you've opened it; if it isn't insulated, you've already found your first energy leak! Any opening to the attic space should be insulated and weather-stripped to stop drafts. This includes shower vents, electrical cables, and light fixtures.
If this is your first time in your attic, take a few moments to look around and familiarize yourself. There should be a handful of ventilation ducts; make sure that they aren't blocked. Also be sure to check for any signs of water or moisture, as they could indicate serious problems with your roof.
Stepping carefully from joist to joist, you should take a look to see what sort of insulation your home has. There are three likely possibilities: batts, blown insulation, or sprayed insulation.
Batts are large mats of woven fibers, either fiberglass or recycled cotton. Both types have similar insulation qualities, and they both suffer from the same drawback: they do not fill odd spaces very well. For insulation to function, it should fill the ceiling space exactly with no gaps or squeezing. Batts can insulate simple spaces with some success, but they usually leak a large amount of energy in corners and around ceiling fixtures.
Blown insulation is easy to spot, because you'll probably have to clean up the material that falls out onto the floor as soon as you open your attic's access panel. It consists of lots of fluffy material pumped into your attic via hose, covering the joists with a thick blanket of either fiberglass or recycled newspaper. It insulates well, fitting into all the nooks and crannies, but it's a pain to deal with if you do any work in the attic.
If you have a newer home, you may have sprayed insulation. This is a layer of foam that is sprayed either into the usual spaces between the joists or up onto the inside surface of the roof. This is most common when HVAC systems or ducts are in your home's attic instead of in a basement or closet.
How Much Is Enough?
The answer, of course, depends on the type of insulation and where you live. The degree to which your insulation reduces heat transfer is measured by its R-value. The higher the value, the better your house is insulated. Most governmental guides recommend insulating to an R-value of at least 38.
While calculating your home's specific R-value can be useful, it's much easier to estimate using a common rule of thumb. If your insulation fills the area between the tops of the joists and the ceiling of the room below, it's probably not worthwhile to spend much more on reinforcing it (although it's still important to patch any holes you might find). If it doesn't fill that area, or if there are significant gaps and openings between your attic and the rooms below, you should consider reinforcing your home's insulation.
The Next Step
If you do decide to reinforce your home's insulation, you should probably hire a HVAC professional. While batts can be laid down pretty easily, blown and sprayed insulation are much more difficult to lay down well. Remember, too, that you can mix types of insulation; it can be particularly effective to use blown insulation to plug the gaps left by batts.
A self-audit is often the first step towards a more energy-efficient home. Don't be afraid to get up in the attic with a flashlight to see what you're working with!