How Much Weight Can One Inch Insulation Board Hold?

Compressive Strength

• Deformation and compressive strength are related values in board insulation. Compressive strength measures the point at which a material fails under a load applied to it. Deformation is structural distortion, in which the material loses its shape. Foam boards are measured for compressive strength when deformation reaches a specified percentage of its thickness. There are several types of foam board, and they are manufactured with a range of compressive strength ratings to suit construction needs.

Foam Board in Construction

• Rigid insulation, or foam board, is used for insulation in roofs, walls and foundation walls and slabs in a range of residential and commercial construction. Board insulation used for construction must withstand the weight of building materials and loads of the structure, foot traffic, and stresses during installation. Foundation walls are subject to hydrostatic pressure from the buildup of water in the backfill, and board insulation needs to resist that pressure and lateral pressure from the weight of the soil.

Board Thickness

• Foam board is manufactured in different grades and densities, and the compressive strength is associated with those classes, rather than thickness. Boards of different thickness will be rated with the same minimum compressive strength. Thickness is chosen for the R-values, which are measured per inch of thickness. Foam insulation R-values range from R-4 to R-8 per inch of thickness, or two to three times greater than the R-value of other insulating materials of the same thickness.

What the Board Can Hold

• The American Society for Testing and Materials sets standards for the minimum compressive strength for foam boards. Polyisocyanurate, or polyiso, board, for example, has to meet a minimum stated compressive strength of 16 pounds per square inch (psi). The insulation is often manufactured with a compressive strength that is higher than the minimum. ASTM standards are based on short-term load conditions. Long-term, continuous loads can cause foam products to creep, and this needs to be calculated into the design.