Long Term Thermal Resistance (LTTR) is a relatively new value used to measure thermal resistance of polyiso insulation.
On January 1, 2014, the ASTM C1289 Standard (Standard Specification for Faced Rigid Polyisocyanurate Thermal Insulation Board) was changed. The change provides a more descriptive measurement of the foam’s thermal performance with a 15-year weighted average.
Reduced thermal performance happens over time as the foam ages along with air and blowing agents diffusing in and out of the foam structure.
It is recommended that design professionals use the new LTTR values as a “design specification”. The new values are compared to the old as follows:
Thermal Values Old and New