Tech Today

Insulation thickness tolerances

During the past several months, NRCA's Technical Services Section has received a number of inquiries from contractors who have noted some variability in the thicknesses of rigid board insulation products. In this month's column, I explore the thickness tolerances commonly associated with rigid board insulation products, explain a concern that may result in the field because of such allowable tolerances and offer a suggestion for addressing the potential problem.

Allowable tolerances

ASTM International's product standards for rigid board insulation products typically contain dimensional tolerances governing variability in products' allowable lengths, widths and thicknesses. These dimensional tolerances vary based on specific product types. The figure shows ASTM's allowable thickness tolerances for rigid board insulation products commonly used in roof systems.

For example, for polyisocyanurate insulation, ASTM C1289, "Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation," provides for an allowable tolerance of ±1/8 of an inch (±3.2 mm) from the product's nominal thickness. Therefore, a polyisocyanurate insulation board with a nominal thickness of 1 1/2 inches (38 mm) is allowed to range in average thickness from 1 3/8 inches (35 mm) to 1 5/8 inches (41 mm) and still comply with ASTM C1289.