The main objective of this study is to determine the optimum insulation thickness of residential buildings in Turkey, under the influence of cooling and heating loads by life cycle cost analysis (LCCA). Five-storey apartment building with an Area/Volume ratio (A/V) of 0.40 m(-1)and 10-storey apartment building with an A/V ratio of 0.32 m(-1)are taken as reference buildings. The annual energy needs are calculated according to TS 825 standard for heating and TS EN ISO 13790 standard for cooling. Natural gas is used for heating and electricity is used for cooling. LCCA based on the total cost approach is performed for 30 years. Optimum insulation thicknesses (U-values) based on climate regions are calculated between 10.5 cm and 17.3 cm (0.30-0.19 W/m(2)K) for wall, 17.5 cm and 26.8 cm (0.24-0.16 W/m(2)K) for ceiling, 6.0 cm and 9.8 cm (0.49-0.32 W/m(2)K) for floor. When compared with the limit values specified in TS 825, it has been determined that the energy savings provided by an insulated building using optimum insulation thicknesses vary between 12.9% and 21.5% according to the region. As a result, it is concluded that the limit U-values should be revised for all regions specified in the TS 825.