The provision, determined by the “Energy Performance of Buildings Regulations", which requires that the energy performance classification of the new and already existing buildings need to meet a minimum “C” energy class, has increased awareness in the construction sector regarding the heat insulation applications. However, in this process, we are confronted with the thermal insulation applications that are carried out in standard thickness without taking into consideration the climatic region where the buildings are located.The purpose of this study is to compare the impact of the thermal insulation applications which are carried out in standard thickness without taking the climatic region into consideration with the applications carried out by using optimum thermal insulation on the energy loads of the buildings and building energy classes obtained in the consequence of calculations in the Bep-Tr1 software. For this purpose, the energy performances of the sample residential building in the pilot cities in five climatic regions (Antalya, Trabzon, Ankara, Diyarbakır, Erzurum) are determined by using both the standard and the optimum thermal insulation thickness values and in the Bep-Tr1 software, the impact of the head insulation thickness on the building’s energy class has been investigated. The conducted study has shown that by using optimum thermal insulation thickness values, an average of 9% efficiency may be obtained in all regions in the building heating and cooling energy loads. However, the obtained efficiency was not sufficient to change the buildings’ energy class in every region.