Both the ASTM Standard Test Method for Estimating the Long-Term Change in the Thermal Resistance of Unfaced Rigid Closed-Cell Plastic Foams by Slicing and Scaling Under Controlled Laboratory Conditions (C 1303) and the Standard for Determination of Long-Term Thermal Resistance of Closed-Cell Thermal Insulating Foams (CAN/ULC-S 770) are based on accelerating the foam aging process by slicing the foam into thin specimens. This accelerates the diffusion process so that thermal conductivity for foam insulation of varying thickness can be determined in a short period of time, typically less than one year. The C 1303 process calls for a series of measurements to define a relationship between thermal conductivity and a scaled aging time which is then analyzed to calculate the time-average thermal conductivity over any given service life. The S 770 process also uses scaled aging time, but uses the projected thermal conductivity at precisely five years of age to represent the insulation's useful service value. There is also a difference in how the thermal results are reported. The S 770 protocol calls for very careful determination of the initial thermal resistivity of a full thickness board, to which an aging factor is applied to determine the five-year value. The C 1303 protocol calls for reporting both the average thermal resistance for the selected thickness and service life and the aging curve data from the thin specimens. During a round-robin exercise performed in support of the S 770 standard, parallel measurements were made on the same specimens to permit application of the C 1303 procedure. This paper presents the results of that comparison for several types of foam. This paper also gives more explicit instructions on the proper application of the C 1303 methodology than is found in that document.