Thermal insulation is one of the most effective techniques to save energy in terms of heating and cooling in buildings. In terms of energy savings and providing thermal comfort conditions, the insulation material chosen, and the optimum insulation thickness determined are critical. Many studies have been conducted to establish the ideal insulating thickness. The thermal conductivity coefficient (k) of the insulating material is acquired directly from standardized tables in this research, and the best insulation thickness is calculated. The k value of an insulation material varies in real-world applications based on production conditions, density, and temperature. As a result, the density of the insulation material as well as the operating temperature must be considered. As a result, when selecting the ideal insulation thickness, the density of the insulating material and the operating temperature should be considered. The insulation material employed in this investigation was expanded polystyrene foam (EPS) of various densities, and the fuel was coal and natural gas. For the province of Isparta, the optimum insulation thickness as a function of density and temperature was established using the degree-day technique based on energy costs. As a result of the calculations for the k, dry value of the insulating material for the province of Isparta, which is in the third climate zone, the optimum insulation thickness was established as 0.0428 m for EPS with a density of 30 kg.m(-3). It has been determined that the annual savings are 52.36 % and the payback period is 2.7 years.