In the countries provide a major amount of their energy from abroad, using of the energy effectively and so obtaining of energy saving become more and more important. In this study, when the different energy sources (coal and fuel oil) were used for heating in the buildings in Denizli, optimum insulation thicknesses, energy savings and payback periods were calculated. Rock wool was used as the insulation material for the external walls. The optimum insulation thickness was calculated according to interest and inflation rates. The calculations were based on a life-cycle cost analysis (LCCA). When coal was used as an energy source, the optimum insulation thickness, the energy saving, and payback period were obtained 0.048 m, 42%, and 2.4 years, respectively
