Faced rigid board polyisocyanurate insulation is the most popular type of insulation used in low-slope roof systems in the U.S. However, after years of debate, there still is no industry-wide consensus of an appropriate R-value for design purposes.
Since 2004, polyisocyanurate insulation manufacturers in the U.S. have reported R-values using the Polyisocyanurate Insulation Manufacturers Association's (PIMA's) QualityMark certified R-value program. Within this program, R-values are determined and reported using the long-term thermal resistance (LTTR) method. Currently, LTTR values are determined using CAN/ULC-S770-03, "Standard Test Method for Determination of Long-Term Thermal Resistance of Closed-Cell Thermal Insulating Foams."
The U.S. product standard for polyisocyanurate insulation is ASTM C1289, "Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board." Up until the 2011 edition of ASTM C1289, CAN/ULC-S770-03 also was the method used for LTTR determination in ASTM C1289.