This research investigates long-term thermal resistance of three different polyisocyanurate (PIR) materials (PIR I, PIR II, and PIR III), and extruded polystyrene (XPS). It includes determining 1. The effect of conditioning at different temperatures on the long-term thermal resistance (LTTR) of insulations as defined by ULC S770 (slicing and scaling method), 2. The temperature dependency of the R-value of insulations calculated by the calculation method in ASTM C1045 when using the ULC S770 test procedure to do the accelerated aging, and 3. The temperature dependent R-value (TDRV) (along with the ASTM C 1045 calculation method) of full thick boards aged for five years (at three temperatures) in the laboratory in comparison to the accelerated aged thin slices. First, the tests were conducted following the methods in ULC S770 and conditioning was carried out at −10 °C, 23 °C, and 50 °C. The LTTR was calculated as per ULC S770. Secondly, in addition to the thermal resistance testing at 23 °C mean temperature, it was tested at 11 mean temperatures. The calculation method in ASTM C1045 was used to determine the temperature dependent R-value of the accelerated aged samples. Finally, the unsliced boards were aged in the laboratory for 5 years at −10 °C, 23 °C, and 50 °C and their TDRVs were compared with that of the samples subjected to the accelerated aging. The LTTR values, measured at 23 °C mean temperature, are in good agreement with the values measured after laboratory aging. When the materials were conditioned at −10, 23, and 50 °C, the TDRVs above mean temperatures of 15 °C for both accelerated aging and for the laboratory aging, showed very close values. Regardless, of the temperature of aging, XPS aged in the same manner in all cases reaching very similar TDRVs at 5 years and 5-year equivalent periods.