Allowable Thickness of Exterior Rigid Insulation

Basic Types

• Three basic types of rigid foam are used in exterior insulation. Polystyrene or molded expanded polystyrene board is made in varying densities for use on walls and roofs. Extruded polystyrene is similar to molded expanded polystyrene, but is more consistent in density, more resistant to moisture and less subject to compression, making it better for walls and roofs. Polyisocyanurate and polyurethane are similar; they have the highest thermal resistance and resistance to compression or denting.

Varying thicknesses

• All rigid foamboard is made in varying thicknesses from 1 inch to 4 or more, depending on the use. Foamboards on exteriors are usually 1 to 1 1/2 inches thick, on roofs from 1 1/2 to 2 inches thick. Thicknesses vary with the type of board and the climate; very cold regions require thicker board, hot areas less thick. Type of wall construction also affects thickness; most houses are framed with 2-by-4-inch stud walls.

Thermal Resistance

• Foamboard thicknesses vary with the thermal resistance or R value of the material. Expanded polystyrene board values range from 3.8 to 4.4 per inch, Extruded polystyrene is about 5 per inch and polyiso or polyurethane from 5.6 to 8 per inch. A 1-inch polyiso board will insulate better than a 1-inch molded expanded polystyrene board, but is generally more expensive. The International Residential Code provides a manual with guidelines for foamboard thickness by climate zones.

Thickness Guideline

• The basic IRC guideline for typical 2-by-4-inch framed walls calls for R2.5 in marine or warm zones, R5 for more variable temperature zones, R7.5 for cold zones and R8 to 10 for very cold regions. Almost any thickness is acceptable in warm and dry areas, and moisture condensation is less of a problem because the foam will never get cold enough for moisture to condense. Foamboard in other areas requires a vapor or moisture barrier.

SIPs

• All three types of foamboard can be used to make structural insulated panels. These are wall components framed in wood with very thick foamboard in the interior. They may be 8 or more inches thick. They are used mainly in very cold regions requiring maximum insulation.