The coldest part of your house in the winter and the hottest area in the summer is the attic. For this reason, it's important that your attic floor have adequate insulation to prevent the attic from heating or cooling the rest of your home. Having inadequate insulation or, even worse, having no insulation in your attic can be expensive. In fact, the Department of Energy estimates that having no insulation in an attic can add 30% to a home's energy costs, and, with the average cost of heating a home over $1500 per year, that adds up pretty quickly.
Adding insulation to your attic will provide a relatively quick payback (possibly as fast as one year), and it's the kind of job a DIYer can do. Upgrading insulation isn't a hard job, but it is messy, awkward, and uncomfortable. Remember, as well, that anytime you're working with insulation you should wear long sleeves, a hat, gloves, eye protection, and a dust mask to protect your lungs from any loose fibers.
How Much Insulation Do You Have?
Start by figuring out how much insulation you already have in your attic. You can do this by going up into your attic, seeing what kind of insulation is there now, and measuring how thick it is. The most common types of attic insulation are fiberglass or rock-wool batts and blown insulation (usually cellulose fiber—a paper product impregnated with fireproofing chemicals). Both have R-values of approximately 3–4 per inch of thickness—use 3 per inch as a good 'guesstimate' for how much insulation is already present.
How Much Insulation Do You Need?
Determining the amount of insulation you should have in your attic will depend on where you live. The Department of Energy has a Web site that can give you a specific recommendation for your home, by ZIP code: http://www.ornl.gov/~roofs/Zip/ZipHome.html. Once you know how much insulation you should have, you can figure out how much more needs to be added.
Most home stores have computer programs to help you estimate how much insulation you need to add to bring your attic up to recommended standards. Both fiberglass and cellulose insulation are readily available there, plus most stores can rent you the necessary blower to install blown insulation.
Is Your Attic Ready?
Before you add any insulation to your attic, you need to install rafter vents adjacent to the soffit vents. Rafter vents will stop the insulation from blocking and cutting off the necessary airflow through your attic.
What's the Best Way to Add Insulation?
When upgrading insulation, it's often easier to add more insulation to an attic using fiberglass bats or blankets. Start at one side of the attic and simply lay the insulation in between the existing joists until the cavity is full. When the cavity is full, you can lay more insulation at right angles across the insulation already there.
Insulation works best when it isn't compressed, so just lay it down; don't squeeze the fibers together. If you are adding more insulation to what was originally installed, you don't want to add another vapor barrier on top of the existing one. This means if you're using fiberglass batts or blankets, you want to use unfaced products (facing on insulation is designed to act as a vapor barrier).