The ideal thickness for attic insulation depends on several factors, including the local climate and the type and grade of insulation used. Whether you are planning for new construction or re-evaluating and improving an existing home's insulation, be sure to consider all the pertinent factors for your case. Then add any insulation necessary to meet the recommended thickness. Doing so will improve the structure's energy efficiency and cut down on heating and cooling costs.
Types and Grades of Insulation
Insulation comes in different types, including loose-fill insulation and rolls of fiberglass bats or blankets. When adding insulation to an existing structure, you may layer either type on top of the other. When using a combination, use unfaced fiberglass blankets, without any metallic backing. Laying loose insulation requires a special blower device, which you can usually rent from a hardware shop. When laying fiberglass blankets, roll them across the joists in the attic flooring, not the same direction as the joists. For inset "can" light fixtures, which protrude through lower-story ceilings into the attic floor, be sure to create a wire or metal barrier to leave some distance between them and the insulation material. The grade of insulation is designated by an "R" number, such as R25, a low level of insulation, or R38, a highly insulating grade.
For insulating existing wood-framed buildings, the US Energy Star program recommends insulation thickness depending on location. The United States is divided into eight climate zones, each of which requires different grades of insulation. The southern tip of Florida, Hawaii, Guam, Puerto Rico and the Virgin Islands are all in Zone 1. Zone 2 covers the rest of Florida, coastal areas of the Gulf states plus southern Georgia, south and east-central Texas, southern Arizona and the southernmost tip of California. Zones 3 and 4 extend northward, with Zone 3 covering much of California, Texas and the southeastern-most states and Zone 4 crossing Appalachia, the Delmarva peninsula and coastal areas of the Pacific Northwest. The northeast, midwest and part of the mountain west are largely Zone 5, with higher western areas, the northern Plains states and northern New England in Zones 6 and 7. Zone 8 only applies to parts of Alaska, not found in the continental U.S.
As a general rule of thumb, apply insulation to completely and evenly cover the floor, coming above the tops of floor joists. Besides the insulation thickness, consider the grade of the insulation you're using. To insulate a new attic or an existing one without insulation, apply grade R30 to R49 for attics found in geographical Zone 1, grade R30 to R60 for Zones 2 and 3, grade R38 to R60 for Zone 4 and R40 and higher for Zones 5 through 8. If you are adding to 3 or 4 inches of existing insulation, your new insulation material should have a rating of R25 ro R30 for Zone 1, R25 to R38 for Zones 2 and 3, R38 for Zone 4 and R38 to R49 for Zones 5 through 8.
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