The outdoor temperature is one of the most important parameters affecting the calculation of energy requirements of buildings. Determination of the thermal insulation details and heating energy requirement of the buildings in Turkey, where energy consumption is dominated by space heating, is made according to TS 825 "Thermal Insulation Rules in Buildings" standard. In TS 825, the provinces are clustered in degree-day regions (DDR) according to their climatic conditions. The main climatic parameter that separates these DDRs from each other is the outdoor temperature. In this study, it is aimed to discuss the effect of outdoor temperature on the building heating energy requirement and validity of the DDRs in the standard according to outdoor temperature data. In this context; the heating energy requirement of the case building is calculated according to different outdoor temperature data of the provinces and the outdoor temperature data of the DDRs in the current and recommended version of the standard. The effectiveness of the DDRs and the updates in the standard are examined by comparing them separately for 81 provinces. The results show that in some DDR clusters, Qyear values of the provinces from different DGBs are very close to each other: even, comparative Qyear values of some of them contradict with their DDR. This situation suggests that the DDR approach should be re-examined. Besides, the new DDRs in the recommended version of the standard is rather compatible with the comparative results. In addition, the selection of the heating threshold temperature has been shown to have a limited effect on the calculations, while the up-to-dateness of the outdoor temperature data to be used in the analyses has been shown to significantly affect the heating energy calculation of the building.