"This procedure defines the long-term thermal resistance (LTTR) of a foam product as the value measured after 5-year storage in a laboratory environment and provides means for its prediction based on an accelerated laboratory test.
This procedure, based on ASTM standard test methods C 1303 and C 518, can be applied to a wide range of preformed as well as field manufactured insulating foams. This procedure estimates the change in the thermal resistivity of insulating foam products by means of slicing and scaling.
This procedure addresses faced and unfaced products without consideration to the effect of facers on the LTTR of the product. This method is not intended for products with impermeable membranes such as those with sheet metal facers which will retain most of their initial R-values.
This test procedure is applicable to cellular plastic insulation manufactured to retain a blowing agent, other than air, for a period longer than 180 days. If the thermal resistivity of a product changes by more than 3 % over this 180 day period, this test procedure shall be applied. This procedure specifies reference time, sampling and testing
requirements and is based on ASTM standard test method C 1303 to determine LTTR for closed-cell foams such
as extruded polystyrene, sprayed polyurethane, and polyisocyanurate."