|Long-term thermal resistance (LTTR)||
A method for determining and reporting estimated long-term R-values of unfaced or permeably faced rigid gas-filled closed-cell plastic foam insulations by reducing the specimen thickness to accelerate aging under controlled conditions. This method is reported to account for the diffusion of the blowing agent from the foam insulation‚Äôs cells over time, resulting in a recognized reduction of thermal resistance of some plastic foam insulations as they age. The R-values produced by the LTTR method correspond to the thermal resistances measured for insulation materials stored in controlled laboratory conditions until they have aged five years. LTTR values are reported for polyisocyanurate rigid board insulation in accordance with ASTM C1289, ‚"Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.‚"