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posted: 12/7/2013 2:00 AM

More on insulation and R-values

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By Dwight Barnett

Q. I recently read your story about insulating an attic room and you mentioned using materials with a high R-value. Can you tell me what the various R-values are for the different materials available? Should I also insulate the floor joists in my crawl?

A. For those who missed this article, the R-value stands for the resistance of a material to thermal transfer. The higher the R-value of an insulating material, the better it is at conserving energy. The home stores I visit offer a variety of insulating materials, but the most common do-it-yourself insulating materials are: loose-fill fiberglass, Kraft-faced or foil-faced fiberglass batts, loose-fill cellulose; foam panels; and expanding spray foams.

Other insulating materials such as high-density fiberglass batts, mineral wools, cotton, sheep's wool, hemp and straw are not normally used by the DIY homeowner (

Spray foams are tricky to use and should not come into contact with your skin. Low-expansion foam insulating products usually are used around window and door openings or other narrow areas too small to insert fiberglass or cellulose insulation. The R-value of a common cell polyurethane spray foam is R-3.6 per inch.

Adding insulation to an attic is a simple DIY project requiring the help of at least one friend or relative, but the job is easier if three people are involved. Home Depot and Lowe's stores in my area will rent or even loan the equipment necessary to add a blown-in, loose-fill insulating material of fiberglass or cellulose.

Loose-fill fiberglass has an average R-value of 2.5 to 3.7 per inch of added material. Cellulose has an average R-value of 3.0 to 3.8 per inch. You can add either product to the existing insulation in your attic, but I would recommend that you add a similar material to what's there now.

Do not compact the insulation you're working with by walking on the new insulation. Compaction dramatically lowers the R-value and the usefulness of the product.

If you have fiberglass batts with a Kraft paper or similar that is facing the attic (this is incorrect installation), avoid adding insulation over the top until the paper or entire fiberglass batt is removed.

Should you insulate between the floor joists in a crawl space? According to a 2004 research report by Joseph Lstiburek of the Building Science Corp., "Crawl spaces insulated on the perimeter are warmer and drier than crawl spaces insulated between the crawl space and the house."

The perimeter walls of the crawl space and the rim joists at the ends of each floor joist should be insulated, not the floor joists. In most cases, I find the insulation that was installed in the floor joists was upside down, providing a vapor trap that can lead to mold and offensive odors as well as a nesting area for pests.

• Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at

Scripps Howard News Service

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