Polyisocyanurate insulation manufacturers' faced, high-density polyisocyanurate insulation cover board products have gained popularity and acceptance in the U.S. roofing industry. Differences among some high-density products mean some products are not necessarily interchangeable or, using common specification language, they are not "or equal."
The U.S. product standard for faced rigid cellular polyisocyanurate is ASTM C1289, "Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board." ASTM C1289 describes 18 types of polyisocyanurate products, including polyisocyanurate insulation wall sheathing, roof insulation and composite boards. The products are differentiated with ASTM C1289 by type and, in some cases, also class and grade. For example, the proper ASTM International designation for fiberglass-reinforced, cellulose-faced polyisocyanurate thermal insulation with a minimum 20 pounds per square inch (psi) compressive strength is ASTM C1289, Type II, Class 1, Grade 2. In this designation, Class 1 specifically designates the facer sheets and Grade 2 designates the board has a minimum 20-psi compressive strength.
Faced, high-density polyisocyanurate insulation cover board is designated as ASTM C1289, Type II, Class 4 followed by Grade 1, 2 or 3. Grade 1 designates a minimum 80-psi compressive strength; Grade 2 a minimum 110-psi compressive strength; and Grade 3 a minimum 140-psi compressive strength.