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 ASTM C1303 and ASTM C518, 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 test procedure is applicable to cellular plastic insulation manufactured to retain a gas or mixture of gases, 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 days period, this test procedure shall be applied. This procedure specifies reference time, sampling and testing requirements to determine LTTR for closed-cell foams such as extruded polystyrene, sprayed polyurethane, and polyisocyanurate.

The changes made to this edition of the Standard include clarifications of the terms "base formulation" and "mean temperature", new Clauses to address the testing of materials thicker than 75 mm and a new section on testing of actual 5 year samples.