When selecting insulation, judge it by its R-value, not its thickness. You'll find both the R-value and thickness marked on the wrapper. Having both lets you compare prices -- but remember, buy Rs, not inches.
You'll need to pick an insulation type that fits the application. Types of home insulation are listed on the following pages.
Batts generally consist of fluffy fiberglass or rock wool in sections 15 to 23 inches wide, to fit joist and stud spacings, and 4 to 8 feet long. They're moisture- and fire-resistant and come with or without a vapor barrier. Although batts are easy to handle and install, you'll end up with more waste pieces than with blankets.
Blankets -- also made of fiberglass or rock wool and sized to fit between framing -- come in continuous rolls so there is less waste.
Loose-fill material can be poured or blown into cavities. The most common are fire-retardant cellulose fiber, vermiculite, perlite, fiberglass, and rock wool. This type is the easiest to install in walls and floor cavities that are already finished.
Foam also can be used to insulate finished walls, floors, and ceilings from the outside. Foam has a higher R-value than blown-in insulation, but it's much more expensive. Rigid insulation consists of boards of molded or extruded polystyrene, polyurethane or polyisocyanurate. Polyurethane has a high R-value, but, along with polystyrene, it is combustible and must be covered with a 1/2 inch drywall for fire safety.