In Turkey, heat loss from buildings is one of the primary sources of energy waste since no or little insulation is used in existing and new buildings. Therefore, considerable energy savings can be obtained by using proper thickness of insulation in buildings. Given the significant climatic variations that exist in different parts of Turkey, 16 cities from four climate zones of Turkey are selected for analysis and optimum insulation thicknesses, energy savings, and payback periods are calculated. The annual heating requirements of buildings in different climates zones were obtained by means of the heating degree-days concept. The optimization is based on life-cycle cost analysis. Five different fuels; coal, natural gas, fuel oil, liquefied petroleum gas (LPG), and electricity, and as an insulation material polystyrene are considered. The results show that optimum insulation thicknesses vary between 2 and 17 cm, energy savings between 22% and 79%, and payback periods between 1.3 and 4.5 years depending on the city and the type of fuel. (C) 2005 Elsevier Ltd. All rights reserved.