How to Calculate How Many 4x8 Sheets of Insulation I Need

Divide the area to be insulated into rectangles and triangles. Regular shapes are easy to work with and make the math easier. For example, an L-shape floor can be divided into two rectangles, and an end wall consisting of a rectangle and a triangle can be clearly divided into two separate areas.

Measure the length and height, or width, of each separate area, and then record the data. Use the measurements to calculate the surface area of each section. The surface area of a rectangle is found using the equation area = length x height, and the surface area of a triangle is half the area of the rectangle with the same height and width. For example, a rectangular wall 20 feet long and 10 feet high has a surface area of 200 square feet — 20 x 10 = 200 — and a triangular section 15 feet wide and 10 feet high has an area of 75 square feet — 15 x 10 / 2 = 75.

Combine the individual areas to find the total surface area to be insulated. For example, the rectangular wall and triangular area calculated in Step 2 have a combined surface area of 275 square feet — 200 + 75 = 275.

Divide the total surface to be insulated by the surface area of a 4-foot by 8-foot sheet of insulation. A 4-foot by 8-foot sheet has a surface area of 32 square feet — 4 x 8 = 32 — so divide by 32. The result is the number of sheets required to cover the area. For example, nine sheets of 4-foot by 8-foot insulation are required to cover 275 square feet because 275 / 32 = 8.59.