The Typical Thickness of Rigid Insulation


Rigid insulation refers to foam or fibrous boards. These precut lengths of compressed foam or mineral fibers are used in various parts of the home preventing internal heat escaping or excessive exterior heat entering the building. They are manufactured from a number of materials and are available in a standard range of sizes.

Foam Types

  • Rigid foam-board insulation is most commonly fabricated from polystyrene. The polystyrene is treated with a blowing agent making the beads expand, or are liquefied and then combined with a gas. In either case, the polystyrene is then compressed and molded into boards. Rigid foam boards are also manufactured from polyurethane or polyisocyanthurate. These materials compress more than does polystyrene, so thinner boards create the same insulation level. Foam boards insulate many areas of the house, including attics, basements and crawl spaces.

Fiber Types

  • Rigid fibrous boards are manufactured from fiberglass or mineral wool. Fiberglass is a polymer mixture of plastic and glass. Mineral wool is constituted from industrial processes byproducts, such as ore smelting. The fibers are densely compressed and then mixed with a resin bonding them together. This creates a rigid structure primarily used for insulating areas prone to high temperatures, such as furnace and chimney flues. Both fiberglass and mineral wool are naturally fire resistant.


  • Foam boards typically come in sizes between 2 and 4 inches in depth. Fiberglass and mineral wool rigid insulation boards come in thickness of between 1 and 2-1/2 inches. However, there are no standard sizes, and manufacturers make different depths.


  • The R-value is the measurement unit that indicates a material’s effectiveness as an insulator. It is determined by dividing the material’s thickness by its thermal conductivity. The thermal conductivity is a complex equation that analyzes the flow of a certain amount of energy through a set area of substance in a prescribed time period. The higher the resultant number, the better the material is at insulating against heat penetration. Polystyrene foam boards have an average R-value of between 3.8 and 5.5 per inch. Polyurethane and polyisocyanthurate boards offer between 5.6 and 8 per inch. Fiberglass and mineral wool rigid boards have an average R-value of 4 per inch.

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