• The "R-value" of an insulation material is a measure of how well it resists heat transfer. The higher the R-value, the better the insulation. All insulation products are marked with a per-inch R-value. Multiply this by the number of applied inches to get the R-value of installed insulation. The Department of Energy recommends attic insulation from R30 to R60, depending on your area of the country. See the link in Resources for the recommended values for your area.
  
  

Fiberglass Batts

• The familiar cotton-candy-like pink or yellow fiberglass pads, or "batts," have a typical value of R3.2 per inch. Fiberglass batting is usually marked with a value for the full thickness. Owens-Corning, for example, sells a 6-inch-thick batt marked as R19 and a 12-inch batt marked as R38. Fiberglass' biggest advantages are its relatively low cost and ease of installation. It's sold in big sheets and paper-faced rolls that can be easily cut to fit any space. It's highly irritating, though, so wear gloves, eye protection, long sleeves and pants when working with it.
  
  

Cellulose

• Loose-fill cellulose insulation has a typical R-value of R3.2 per inch, the same as fiberglass batts. Cellulose is made from recycled paper treated with a binding agent to cut down dust and is applied with a sprayer. The application needs to be even across the attic floor. A low spot will have a low R-value, allowing the attic to suck heat up from lower floors. Cellulose is less common now than it was in the 1970s and 1980s, but it's still an effective insulator. It isn't irritating like fiberglass, and because it's recycled, it's considered "green."
  
  

Mineral Wool

• Mineral wool is similar to fiberglass but is made from waste products of the steel-making process combined with molten rock and chemicals. Also known as rock wool or slag wool, it has a typical R-value of R3.1 per inch. Mineral wool is available in batts or as loose fill, but it must be professionally installed. Its chief advantages include fire resistance and the fact that it acts as its own vapor barrier.
  
  

Foam Panels

• Rigid insulation products, such as polystyrene and rigid fiberglass, are usually used in attics only as supplementary insulation. Their R-value ranges from R4.0 to R6.5 per inch, depending on the product. They're waterproof and easy to cut to fit any shape--but they've shown one key drawback, particularly in Southern areas: damaging insects, including carpenter ants and termites, love to tunnel through the foam.