Most of the energy resources are used for heating purposes in Turkey. For this reason significant energy can be saved by insulation applications on buildings. However, if the thickness of the chosen insulation material is above optimum thickness, additional costs will incur. In this paper the optimum insulation thicknesses, energy saving and payback periods for four provinces chosen from different climatic regions of Turkey have been calculated. Optimization calculation was made according to life-cycle cost (LCC) analysis. As a result of the calculations made based on five different fuel types (coal, natural gas, fuel-oil, LPG and electricity) and four different insulation materials (rock wool, extruded polystyrene, expanded polystyrene, foamed polyurethane), it was seen that energy saving was 21% and 80.18% and payback period was between 1.247 and 4.6 years at the optimum insulation thicknesses of 0.02 m and 0.202 m.