An increase in the awareness of the influence of building maintenance on the environment affects the search for better engineering solutions when it comes to decreasing energy consumption within a structure. In reference to better thermal insulation, maximum results are impossible to achieve without focusing on the importance of thermal bridging. Not isolated reinforced concrete balcony slab represents a large thermal bridge which can, as has been shown in many studies, to contribute to an increase in energy consumption in a structure by as much as 30%. However, in order to achieve goals for reconstructed buildings as well, as directed by the European council, it is necessary to properly carry out the repair of reinforced concrete slabs of existing structures in order to meet these goals thus decreasing heat loss. This study illustrates three types of sections that are present in existing structures built in the seventies, eighties and nineties of the previous century. Ten varying solutions have been selected for the purpose of repairing reinforced concrete balcony slabs using combinations of 2, 5 and 10 cm of expanded polystyrene thermal insulation for the exterior wall and 2 and 5 cm of extruded polystyrene thermal insulation for the balcony slab. The calculations were computed using the PC Proektant program developed at the Bulgarian University of Architecture, civil engineering and geodesy in Sofia. The program determined the coefficient for thermal transmittance (U), heat quantity (Q) and temperature at key points in the section (interior section points) for all types of sections. The results were thus analysed and an optimal solution was recommended as well as guidelines for further research.