The best depth for blown insulation in the attic depends on the type of insulation and the climate in your area. Blown insulation is made from three different types of material that are rated using the r-value system. The r-value of insulation defines the effectiveness of different types of insulation in relation to their depth. Maintaining an energy efficient home in a colder climate requires insulation with greater r-values. Increasing the depth of the insulation in your attic is one way to achieve a higher r-value.
The Department of Energy recommends using enough insulation in the attic to provide an r-value between 30 and 38 for most of the central and southern United States. In colder areas east of the Rocky Mountains and north of California, most homes will require insulation with a minimum r-value of 49 in the attic.
Blown cellulose insulation is made from recycled paper and provides an r-value of up to 3.7 per inch. Check the manufacturer's specifications to determine the exact r-value of your insulation. In most areas of the United States, 10 inches of cellulose insulation will provide adequate insulation for your attic. However, cellulose insulation loses up to 20 percent of its r-value over time, due to settling. Installing an additional 2 inches of insulation to account for settling will ensure that your home has adequate insulation for years to come.
Rock wool insulation is made from recycled industrial waste and provides r-values up to 3.2 per inch. This type of insulation is less dense than cellulose but only loses 2 to 4 percent of its r-value to settling. Most homes only require between 9 and 12 inches of blown-in rock wool insulation.
Blown fiberglass insulation has the lowest r-value per inch of all three types. This insulation has an r-value rated between 2.3 and 2.8 per inch. Fiberglass insulation only settles a little over time and is made from partially recycled glass. Depending on the r-value of your insulation, most homes will require between 10 and 16 inches of insulation in the attic.
If your existing insulation is not deep enough, you can add insulation in layers to increase the r-value of your attic. If you add more insulation of another type, make sure that it does not compress the blown insulation below it. Blown insulation is also susceptible to moisture damage and can harbor mold if it is exposed to moisture too often.
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