Your attic is the most critical area of your house to be adequately insulated in order to keep you toasty warm in winter and cool as a cucumber in the summer. Since heat rises, a poorly insulated attic allows valuable heat from the living space of your house to escape during the winter. During the summer, heat trapped in your attic can reduce your ability to keep your house cool. In colder, Northern climates, an R-value of 49 is recommended for adequate attic insulation. In warmer climates, an R-value of 30 is recommended. Fiberglass insulation has an R-value of roughly 3.0 per inch of thickness. Cellulose is has an R-value of almost 4.0 per inch.
When insulating an attic, you should also consider how your attic is ventilated. Particularly in northern climates, your goal should be to insulate the living space of the house, while allowing the roof to remain the same temperature as the outside. This means you should allow a free flow of outside air from the eaves of your roof (in through soffit vents) all the way to the gable (out through gable vents, louver vents or other types of roof vents). In areas where insulation butts up against rafters or roof sheathing, use baffles to prevent the obstruction of free airflow.
If you currently have a floor in your attic, you need to pull up pieces of the floor to install the insulation. It will be easier to use a blower and loose-fill insulation to effectively fill the spaces between the joists. If you choose to go with blown-in insulation, you can usually get free use of a blower when you purchase a certain amount of insulation. Either way, you won't feel Old Man Winter breathing down your back quite so hard when you properly insulate your attic!