In recent years, alternative and renewable energy sources have become essential and their use in this direction is increasing. When these energy sources are used, energy saving is also assessed. The main objective of this work is to determine the optimum insulation thickness according to the cooling requirement of a building in a hot climate. During the summer period different wall directions has been evaluated in Bursa. For the optimum insulation thickness in all wall directions, firstly total solar radiation, cooling transmission load and then cost analysis are performed. While solar radiation was calculated, factors such as declination angle, geometric factor, sun clock angle, altitude, ground reflectance and latitude angle were taken into account. The differences in cooling transmission load for insulated and uninsulated walls were observed. The heat transfer coefficient was determined according to inside and outside air-film thermal resistances and total thermal resistance of the wall without the insulation. In addition to these, the volumes, thicknesses, thermal conductivities and costs of the thermal insulation materials are also considered. When cost analysis is performed, PWF is calculated and also interest rate, inflation rate, cost of insulation materials and electrical cost were taken into account. Depending on the location of the building, insulation material which should be used for minimum cost and maximum efficiency and the required properties of this insulation material have been determined.