The letter “R” in the term R-value means resistance, referring to a products resistance to heat flow. The higher the R-value, the more resistant a product is to heat flow and therefore the more energy that product will help a building save. Since 1981, Polyiso manufacturers have determined R-values utilizing PIMA 101 (RIC/TIMA 281-1), a six month conditioning procedure.  Unlike other insulating materials, the R-value-per-inch of polyiso increases with thickness because of the unique closed-cell structure of the rigid foam boards. Because of its superior R-value and durability, buildings that use polyiso have lower heating and cooling costs.

At the 2002 NRCA Convention, PIMA introduced a NEW advanced procedure for determining R-values for foam insulation products: The Advanced Method for Determining Long Term Thermal Resistance (LTTR).

Based on consensus standards in both the United States and Canada, Long Term Thermal Resistance (LTTR) is a scientifically supported method to calculate the 15-year, time-weighted average R-value of roof insulation. Most polyiso manufacturers have adopted the LTTR method as the exclusive means to measure thermal performance of permeable-faced polyiso roof insulation.

Using techniques from ASTM C1303, CAN/ULC S770 predicts a foam’s R-value that has been shown to be equivalent to the average performance of a permeably faced foam insulation product over 15 years. In Canada this method is used as the Design R-value. This applies to ALL foam insulation products with blowing agents other than air, such as polyiso, extruded polystyrene and polyurethane.

The LTTR method provides users, specifiers and designers with many advantages including:

• A technically supported, more descriptive measure of the long-term thermal resistance of polyiso insulation—a 15-year time—weighted average;
• An advanced test method, based on consensus standards in the United States and Canada;
• A method that applies to all closed cell foam insulation with blowing agents other than air, such as polyiso, extruded polystyrene and polyurethane; and
• A better understanding of thermal performance of foam.

Support materials and technical bulletins regarding Long Term Thermal Resistance values are available at www.pima.org.

Atlas ACFoam® and insulating sheathing products provide the highest LTTR values based on thickness over any other insulation material in the industry. For more information about Atlas insulation materials, please visit www.AtlasRoofing.com.