The main purpose of this thesis is to perform a numerical thermal simulation over a period of one year on a model of a building. The model represents a family house with three apartments, which is the heated part of the house, and other adjacent auxiliary spaces which are not heated. For the purpose of this simulation, an old building with modest thermal characteristics was chosen because it represents a typical house built in the second half of the twentieth century in costal Croatia. The simulation takes in consideration heat transfer of the object to the surroundings and the usage of various HVAC installations. In order to determine what should be done during the renovation of such house, to reduce the overall energy consumption, several cases were considered. Cases with various thicknesses of the external insulation (from 6 to 10 cm) and with various types of heating, cooling and ventilation systems were compared. Cases with the usage of renewable energy with solar collectors and heat pumps were also considered. Although the initial case had very poor thermal characteristics and high annual energy consumption, the renewed version showed very good results and a low energy demand. The low energy consumption was achieved with the renewed facade with 8 cm of mineral wool and the usage of mechanical ventilation with heat recovery. The best results for house and domestic hot water heating were achieved in the case with a low temperature furnace coupled with a solar system (S4) and also in the case with a heat pump (S7). The analysis also included the consumption of primary energy, for which the case with the heat pump (S7) and the case with hot water solar collectors (S4) showed best performance. The final stage of the analysis considered the investment costs related to supply and installation of all materials required for thermal insulation of the house as well as HVAC equipment for all analysed cases. The calculation considered the total cost and the distribution of costs per year of exploitation. The results showed that cases S4, S5, S6, S7 and S8 have high initial investment cost and therefore in total the cost become less economical. Case S2 has been identified as the economically best solution while case S4 as the least energy consumption solution. The simulation confirmed that even from an old building with high energy demand, with the right insulation and proper HVAC system, a renewed building with very low annual energy consumption can be obtained.