The optimum insulation thickness of the external wall for four various cities from four climate zones of Turkey, energy savings over a lifetime of 10 years and payback periods are calculated for the five different energy types and four different insulation materials. Foamboard 3500, Foamboard 1500, extruded polystyrene and fiberglass as insulation material are selected. In this study, it is calculated the value of the amount of the net energy savings using the P1-P2 method. The results show that optimum insulation thicknesses vary between 1.06 and 7.64Â cm, energy savings vary between 19Â $/m2 and 47Â $/m2, and payback periods vary between 1.8 and 3.7 years depending on the city and the type of fuel. The highest value of energy savings is reached in AgrI for LPG fuel type, while the lowest value is obtained in AydIn for natural gas.