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

Although often hidden, insulation sheets are an integral part of most new buildings.

Insulation is an important aspect of new builds and most home alterations, extensions and renovations. Providing warmth in cold weather, and keeping the house cool in hot weather, insulation plays an important role in making homes more comfortable and less costly to run, so correctly fitting the right quantity of insulation is important. Determining the quantity of insulation sheets required for a building project is straightforward, as long as the area requiring insulation is known.

Things You'll Need

• Tape measure

Instructions

1. 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.

2. 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.

3. 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.

4. 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.

Tips & Warnings

• The measuring process is identical for floors, walls and roof areas.

• If more than one layer of insulation is required, multiply the number of sheets by the number of layers. For example, if 10 sheets are required to cover a wall, and two layers of insulation are specified, multiply 10 by 2 to find the required number of sheets.

• Some suppliers will refund you for unused sheets when they are returned in perfect condition.

• It may be necessary to order additional sheets to compensate for on-site damage and incorrect cutting.

• Some suppliers sell insulation in packs, so it may be necessary to order more than you actually require.

References

• Oak Ridge National Laboratory: Why Insulate Your House?
• Flooring Supplies: How to Measure Your Floor
• Math Is Fun: Area Calculation Tool

Resources

• Mathamnesia: Finding Complex Areas
• The Home Depot: Insulation Calculator