How to Calculate Blown Insulation

Blown insulation is an excellent product to help keep your energy costs in check. Improperly insulated attics are responsible for some of the largest energy losses in homes. Calculating the amount of insulation that you need requires measuring the space, reviewing the specifications of the material you will be using, and knowing how thick the insulation must be to properly insulate the area.

Step 1

Measure the space that you intend to insulate. Use a tape measure to determine the length and width of the area. Multiply these dimensions to come up with the square footage. For example, a space that measures 10 x 25 feet will amount to 250 square feet.

Step 2

Determine the amount of insulation that you want in the space by reviewing local building codes. Different regions specify different R values (the insulating factor) to be used. The Department of Energy lists R value suggestions for regions in the U.S., and most local codes list the R value required in their municipalities. Call your local building code enforcement department to verify the requirements.

Step 3

Measure the existing insulation in the space if you do not intend to remove it. Most insulation carries an R value, by thickness, similar to other insulation products. You can leave the existing insulation in place if it is not damaged and blow new insulation over the top of it. Note whether the existing insulation is cellulose or fiberglass, and use the same type of product if you are covering it.

Step 4

Check the specifications of the insulation product you will be using. Blown insulation comes in bags, and the bags list the R value as it is equal to the thickness of the insulation. AttiCat, by OwensCorning, lists an R value of 49 at 18 1/4 inches of thickness. A 49 R value is a typical value requirement in the northern U.S. The specifications will also tell you how much coverage you can expect per bag of insulation. In some cases you may need to do a little math to determine coverage. AttiCat states that 26 bags will cover 1,000 feet to an R value of 49. By dividing 1,000 by 26 you can see that each bag covers 38 1/2 square feet to a thickness of 18 1/4 inches, or approximately 58 cubic feet (38.5 x 1.5 feet).

Step 5

Subtract the amount of existing insulation from the amount you need to achieve your goal. For example, if you have 6 inches of existing insulation and you are trying to achieve an R value of 49, you know that you need to cover 12 1/2 inches more over a space of 250 square feet (using the previous example). Determine the cubic feet of insulation needed by multiplying the length of the area by the width by the depth. In this example, that works out to about 250 cubic feet if 6 inches of insulation is existing and 375 cubic feet if you are starting fresh.

Step 6

Divide the cubic feet requirement by the amount of cubic feet that each bag of insulation covers. In this example, with no insulation in place, you would divide 375 x 58 to see that you will need approximately 6.5 bags of insulation. It is rarely an issue to overinsulate, and you can't buy half a bag of insulation, so you would use 7 bags of insulation for this application.


  • When removing or disturbing existing insulation, it is important to verify that it is not an asbestos-containing material. If you are uncertain, check with an expert.