In this paper, presented is the state of the art of Energy Efficiency (EE) of residential buildings in Serbia. Special attention is paid to energy efficiency in already existing buildings. The average energy consumption in residential buildings in Serbia is over 150 kWh/m2 per year, while in developed European countries it is about 50 kWh/m2 per year. In this paper examined is the contribution of ventilation losses, through the windows of low quality, regardless whether they are poorly made, or made from bad materials, or with no adequate glass. Besides ventilation losses, which are of major importance in our buildings, special attention is paid to transmission losses, which are consequence of the quality and energy efficiency of the facade. All of the above statements are proved by measurements obtained on a representative building of the Block 34 in New Belgrade, built in the eighties of the last century. In addition to measurements performed the calculation of energy consumption for heating during winter has been made. The results of two different methods of calculation of energy consumption for heating are compared with the values obtained by measuring.