The purpose of this diploma thesis is the energy balance calculation of an apartment building in Logatec, at the address Sončni log 1 and 1a. This data represents the energy efficiency or inefficiency of the apartment building and can be used to introduce renovation possibilities, with which the criteria of an energy efficient building, according to the Regulations on Energy Efficiency in Buildings (PURES 2010, Ur. l. RS, nr. 52/2010: 7840), can be met. . The calculations were carried out using the KI Energija software that is based on the methodology of the Regulations PURES 2010 and the directive Tehnična smernica - učinkovita raba energije (TSG nr. 1-004, 2010). Also carried out was a comparison of the actual U-factors with those in effect at the time of construction planning, because of the inadequate values of thermal transmittance factors of constructional complexes. The KI Energija computer software recorded energy balance calculations which indicate energy efficiency. The chosen building’s energy balance does not meet the criteria of PURES 2010, because the legislation in effect at the time of construction planning, in 2005, involved criteria that were less strict. The most substantial deviations occur in the split-level constructional complex, which divides the heated zone and the garage, because it is not thermally insulated and the U-factor exceeds the threshold by 1,1 W/(m2K). The most energy-efficient renovation measures include additional thermal insulation of split-level constructional complexes and the non-permeable exterior, while also installing a mechanical ventilation system with heat recovery.