The main objective of this study is to determine optimum insulation thickness according to cooling requirements of buildings in a hot climate. The investigation is carried out using an implicit finite difference method under steady periodic conditions for different wall orientations during the summer period in Antalya, Turkey. For this purpose, a computer program developed in Matlab is utilized. Firstly, thermal characteristics such as cooling transmission load, time lag, and decrement factor are calculated. Then, the optimum insulation thicknesses for all wall orientations are determined by using a cost analysis over lifetime of 20 years of the building. It is seen that for cooling season, the lowest value of optimum insulation thickness is obtained for the north-facing wall which has minimum cooling load while the highest thickness is obtained for the east and west walls providing the maximum cooling load. The results show that for cooling season, the most economical orientation is north with an optimum insulation thickness of 3.1 cm. Results obtained are also compared with the degree-days and degree-hours methods. (C) 2012 Elsevier Ltd. All rights reserved.