The paper highlights the influence of thermal insulation layer applied on exterior walls of the building on its energy efficiency performance. Expanded polystyrene, the most common used material in Romania, at present, for the thermal rehabilitation of buildings, was chosen as thermal insulation material. There are considered five different cases depending on the thickness of polystyrene applied on the exterior walls of the building and the global coefficient of thermal insulation, the thermal resistance of entire envelope and the heat loss through transmission are calculated.