Can I Add Rigid Foam Board As Attic Insulation?

Typical Attic Insulation Types

• Most attics are insulated with blown-in cellulose. This type of insulation is cheap and well-suited to a horizontal application like an attic. Many homes have cellulose in the walls as well, but this sometimes causes problems because the cellulose settles, leaving gaps where heat can escape. Rolls of fiberglass insulation are also commonly used in attics, and they work as well as cellulose, although fiberglass can cost more on a square-foot basis.

Rigid Foam Board

• Rigid foam board is usually manufactured in 2-by-8-foot-long sheets and in thicknesses of 1 or 2 inches. The main advantages of rigid foam are its ability to pack a lot of insulating value into a small space and its resistant to water and damp. While rigid foam can be used in attics, this doesn't take best advantage of its particular strengths. In addition, it can be difficult to cover the tight spaces and nooks and crannies that are often found in attics with rigid foam. Something like blown-in cellulose, which assumes the shape of its surroundings, is much more appropriate for these spaces.

Cost

• The value of an insulation method is determined by its R-value. R-value is a measure of how effective an inch of material is at keeping heat inside and cold outside. Rigid foam is more effective per inch than cellulose or fiberglass, but it isn't more effective per dollar. Because there is usually plenty of space to pile up insulation in an attic, there's no particular reason to use rigid foam as opposed to cellulose or fiberglass. You will get more insulating value for your dollar in an attic by investing in cellulose or fiberglass than in rigid foam.

Benefit

• If you do want to install a layer of rigid foam in your attic, there are some benefits. If you tape the rigid foam together, it is very effective at keeping out drafts. It won't decay over time and is less appealing than cellulose to mice and squirrels if they get into your attic. If you happen to develop a roof leak, rigid foam will be less damaged by the leak than other forms of insulation. However, this would not be a reason to avoid fixing the leak in your roof.