The provision, determined by the “Energy Performance of Buildings Regulations", which requires that theenergy 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 instandard thickness without taking into consideration the climatic region where the buildings are located. Thepurpose of this study is to compare the impact of the thermal insulation applications which are carried out instandard thickness without taking the climatic region into consideration with the applications carried out byusing optimum thermal insulation on the energy loads of the buildings and building energy classes obtainedin the consequence of calculations in the Bep-Tr1 software. For this purpose, the energy performances of thesample 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 valuesand in the Bep-Tr1 software, the impact of the head insulation thickness on the building’s energy class hasbeen investigated. The conducted study has shown that by using optimum thermal insulation thicknessvalues, an average of 9% efficiency may be obtained in all regions in the building heating and cooling energyloads. However, the obtained efficiency was not sufficient to change the buildings’ energy class in everyregion.