Insulation is one of the most effective methods intended for reducing energy consumption in both heating and cooling of buildings. Selecting the right materials and determining the optimum insulation thickness in building insulation application is an important issue. In 2000, the "Thermal Insulation Requirements for Buildings" was enacted in Turkey, energy saving by limiting the energy amount used for heating in buildings being the target. In this study, the effect of the alteration of windows and exterior wall areas on the heating energy requirement of the building and on the optimum insulation thickness has been examined by using P1-P2 method. The study has been carried out for four degree-day regions of Turkey for various insulation materials, glazing areas, and fuel types; the results have been presented in charts. In the rest of this study, effects of different insulation thicknesses and fuel on fuel consumption and thereby on emissions of pollutants such as CO2 and SO2 are evaluated. For example, in the building where XPS (extruded polystyrene foam) insulation material and natural gas are used and where the ratio of glazing area to exterior wall area is 0.2 (glazing area percentage), energy saving for the four regions has been found to be 13.996, 31.680, 46.613, and 63.071Â $/m2, respectively, and the payback period of investment has been found to be 2.023, 1.836, 1.498, and 1.346Â years, respectively. The emissions of CO2 are decreased by 50.91% for the cases in which optimum insulation material (XPS) and natural gas are used. The emissions of CO2 and SO2 are decreased by 54.67% for the cases in which optimum insulation material (XPS) and fuel oil are used.