The energy requirement of developing countries such as Turkey increase every single year. The energy consumption of this country is divided into four main sectors: industrial, residential, transportation and agriculture. The residential sector accounts for 33% of used energy. Uninsulated old buildings, as well as inadequate insulation in new construction poses massive economic burden to this country, forcing it to over-rely on fuels from abroad. In this study, the optimum insulation thicknesses required in Turkey's 81 provincial centers are determined. The calculations are made on the basis of four different fuels (natural gas, coal, fuel oil and liquefied petroleum gas) and 5 different insulation materials (extruded polystyrene, expanded polystyrene, glass wool, rock wool and polyurethane). Three different cases have been considered: buildings heated but not cooled, cooled but not heated, and both heated and cooled; optimum required insulation thicknesses in accordance with each case are calculated. Optimum insulation thicknesses, total net saving amount, reimbursement period change according to the fuels and insulation materials are calculated on the basis of 5 different provincial centers (Izmir, Istanbul, Ankara, Sivas and Erzurum) selected according to their degree/day values. (C) 2016 Elsevier B.V. All rights reserved.