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 in 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. 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. 
  

Polyiso manufacturers, who belong to the Polyisocyanurate Insulation Manufacturers Association,  have certified LTTR-value products verified by the QualityMark^cm Program. Click here to view the LTTR Fact Sheet.