In the countries which supply big amount of their energy needs by import like Turkey using the energy economically is important. Thermal insulation technologies in buildings are the main method for using energy economically. However, choosing the thickness of the insulation material redundant causes high insulation costs. For this reason, an optimum point provides the highest gain in insulation applications is the subject. In this study, for Antalya, Manisa, Ankara and Sivas in the four climate regions in Turkey and optimum insulation thickness, energy saving and payback period in using two different insulation materials (extruded polystrene and rock wool) and five different kinds of fuel (coal, natural gas, fuel-oil, LPG, electricity) were calculated. In addition to this, an analysis of insulation thickness of the harmful emissions of the gas like CO2 and SO2 was made. The results of the study showed that the optimum insulation thickness vary between 0.016 and 0.145 m, energy savings vary between 1.79 $/m(2) and 103.44 $/m(2) and the payback period vary between 1.268 years and 6.022 years depending on the city and type of fuel. Environmental impacts of high used coal fuels were observed during the combustion, and emissions of CO2 and SO2 revealed decreases between 67%-75%.