For most building insulation materials, dependency of the thermal conductivity on temperature is close to linear, and it increases with rising materials temperature, but the thermal conductivity of the polyisocyanurate foam (PIR) increases in the lower temperature range, so the heat transfer through the partition insulated with this material is increased in the cold season. The thermal conductivity of PIR also increases during the aging process, especially during the later operating period, which is not covered by the standard aging procedure used to determine the declared thermal conductivity value. Also, there is a lack of information how PIR reacts to the certain temperatures and other climate conditions, and how fast it loses its thermal properties. The aim of this experimental research was to analyse the change of the thermal conductivity depending on aging temperature. 30, 50mm thick PIR samples were prepared for the test, and the thermal conductivity of all specimens was measured at the standard conditions prior to the start of the test using the heat flow meter FOX 314, and again after each stage of aging, when the samples were kept at the certain temperature for 21 days. The results showed that the thermal conductivity of PIR increases more (up to 10 %) when kept at higher temperatures (+50 °C, and +70 °C), and after specimens undergo negative temperatures (-18 °C) it has a minor influence to the change of thermal conductivity of PIR.