The insulation thickness of the building is an important parameter in the design of the building envelope. Life-cycle cost analysis is often used as an optimization method in determining the thickness of the insulation in buildings. In literature, the insulation thickness which the total cost is minimum is defined as the optimum insulation thickness. In this study, the optimum insulation thicknesses of residential buildings in Turkey were determined by life-cycle cost analysis (LCCA). The annual heating energy requirement of the buildings are calculated according to the Turkish Standard 825, Thermal Insulation Requirements for Buildings. Glass wool, extruded polystyrene (XPS) and expanded polystyrene (EPS) are used for ceiling, floor and wall as insulation material respectively and natural gas is used as fuel. Life-cycle cost analysis based on the total cost approach is used as the optimization method. Optimum insulation thicknesses (U-values) based on climate zones for a period of 30 years are calculated between 14,2 cm and 26,8 cm (0,29-0,16 W/m(2)K) for ceiling, 5,0 cm and 9,8 cm (0,57-0,32 W/m(2) K) for floor, 8,1 cm and 16,4 cm (0,38-0,20 W/m(2)K) for wall. As a result, when compared with the limited insulation thicknesses specified in TS 825 standard, it has been determined that the energy savings to be provided by an insulation building using optimum insulation thicknesses vary between 10,08% and 18,88% according to the region. These results have reached the conclusion that optimum insulation thicknesses (U-values) should be revised for all regions specified in the TS 825 standard, as the increase in payback period is considered not to exceed 10% even in the worst case.