A six different cities in hot climate region were studied in this study. Heating and cooling loads were calculated primarily according to absorption rate values of various solar radiation at exterior surfaces of the buildings. When calculating the optimum thickness of insulation, the P1-P2 method is used which is an economical method and includes heating and cooling loads. Accordingly, the optimum insulation thicknesses, energy savings and payback periods for all the cities were determined by using economic data such as interest rate, inflation rate, life-cycle and compared with the non-radiation situation. The total heat transfer coefficient (U) values were calculated for exterior walls according to optimum insulation thickness by considering solar radiation and non radiation condition. The study showed that the U values given at cities of first degree-day region at TS 825 (Turkish Standard) which is known as rules of heat insulation in buildings were insufficient. In addition, condensation controls were conducted according to the obtained optimum insulation thicknesses.