Overall heat transfer coefficients (U values) of existing wall systems in Turkey have been measured. Comparative energy consumption analyses for each material have been performed according to ISO 9164 (International Organization for Standardization 1989). As an application, the city of Gebze has been chosen, and annual energy consumptions for various materials are defined. Space heating consumption in Gebze is 1.5-3 limes the consumption in other European countries and the United States. Fifty percent of the total possible savings would be obtained when only exterior wall systems are insulated, and 68% of the total consumption for space heating would be consented by applying the available insulation techniques to the entire enclosure of all buildings in the country.