This paper analyzes the process of sensitivity analysis of energy simulation on the example of Mamutica, an apartment building in the city of Zagreb. To simulate the thermal response of the analyzed building depending on the influencing parameters, a detailed computer program TRNSYS was used, whereby the simulation result analysis was performed primarily on the basis of the required energy for heating and cooling. In the definition of the input building parameters, the project documentation available was collected in the archives of the City Office for General Administration of the City of Zagreb and information obtained from the tenant of the building. The following input parameters were varied for the sensitivity analysis of the required heating and cooling energy: infiltration, internal heat gains, U-value of the external wall, external wall heat capacity and set temperature for heating and cooling. According to the results of the analysis, the change in infiltration and the U-value of the wall represent the most influential inputs to the total required energy for heating and cooling, where the infiltration range of -50% to 100% causes a change in the required heating and cooling energy in the range of -7,9% to 17,3%, while the U-value range of -50% to 100% causes a change in the required heating and cooling energy in the range of -6,7% to 13,7%. Based on the obtained results it is possible to show the sensitivity of the simulation models to uncertainty in the values of the input parameters. The results obtained will be used for the analysis of other buildings of the City of Zagreb in further scientific research as the basis for comparison with the actual data on the counsumption of the building.