How to Measure for Blown Insulation on Outside Walls

Measure the length and height of each external wall to be insulated. Multiply the length by the height to determine the surface area. For example, a wall 30 feet long and 20 feet high has a surface area of 600 square feet: 30 x 20 = 600. The area of the wall is a good indication of the area of the hidden wall cavity.

Select an accessible section of external wall and check for cables and pipes with a pipe and cable detector or a metal detector. Locate an area where you can drill safely. Drill a 1 1/2-inch wide hole through the external wall, passing through the outer layer of bricks and stopping when you reach the cavity.

Insert a metal tape measure into the hole until the tip hits the bricks that form the inner skin of the wall. Use a flashlight to peer into the hole and read the tape measure. Note the width of the gap, the distance between the inner brick and the outer one through which you have drilled.

Convert the width of the gap in inches to a decimal fraction of a foot. Multiply the inches by 0.8333 to do this. For example, a 6-inch cavity is a 0.5 foot cavity because 6 x 0.08333 = 0.5.

Calculate the volume of the air gap in each wall by multiplying the surface area by the depth of the gap. For example, a wall with a surface area of 600 square feet and a gap depth of 0.5 feet has a gap volume of 300 square feet: 600 x 0.5 = 300.

Complete the calculation by multiplying the gap volume by the density of insulation required. For example, a gap volume of 300 square feet filled with insulation at a density of 3.5 pounds per cubic foot requires 1,050 pounds of insulation.