The residential sector was responsible for 25% of the Final Energy Demand (FED) in the European Union in 2015. Countries in Central and Eastern Europe such as the Czech Republic are particularly concerned with ensuring security of supply at the lowest possible cost in recent years. FED for space heating still corresponds to the largest fraction of the total FED in the residential sector in the Czech Republic. The vast majority of buildings constructed in 1990- 2000 do not comply with current standards for thermal protection and owners often rely on their own financial means when attempting to improve the overall Energy Performance (EP) of buildings. The goal(s) associated with renovation- or refurbishment measures must be defined to clarify the extent of work and consequently minimize investments. The aim of this thesis, a case study, was to use a whole building energy simulation program (IDA-ICE) to get insight into the potential of different retrofitting measures (represented by scenarios) to improve EP of a single-family summer house located in the South Bohemian Region of the Czech Republic. One of the simulated scenarios was the owner ́s suggestion to replace windows and entrance doors. The goal was to reduce FED for space heating to ≤50 kWh.m-2 floor area and year and by doing so attaining the status of a low-energy building. Simultaneously, reduced FED for space heating was supposed to be achieved without compromising air quality and should involve only the most efficient refurbishment measures to minimize the overall work. The scope was strictly limited to EP improving measures without consideration of mechanical ventilation or modification of the currently used space heating system. It was concluded that the owner's suggestion to merely replace windows and entrance doors would be an insufficient solution. SC-4 (i.e. the combined effect of windows and entrance doors replacement and the ground-floor insulation) and SC-5 (i.e. the combined effect of ground-floor insulation and the insulation of external walls and the roof) were deemed to represent the most optimal solutions from the simulated EP improving measures. The goal was achieved in both, i.e. 46.8 kWh.m-2 floor area and year in SC-4 and 44.3 kWh.m- 2 floor area and year in SC-5. Averages of zone air temperatures in selected zones were found to be more stable in SC-4, however, SC-5 performed better when comparing averages of CO2 concentration-values in selected zones on the first floor. Nevertheless, averages of relative humidity and CO2 concentration-values in all simulated scenarios were within the acceptable range of 35-60% and about 520 to 1000 ppm respectively (except for Bedroom 2 zone).