If using simulation tools for predicting the overall building performance, important decisions concerning the input of those tools need to be taken. The input is based on experience but also assumptions. It is obvious that this introduces uncertainties on which the quality of the simulation output is dependent. It is self-explanatory that there is a demand for analyzing and quantifying those uncertainties regarding their impact on the building design. Therefore a case study is described to evaluate the necessity to use uncertainty analysis (UA) and sensitivity analysis (SA) in building performance simulation (BPS) especially during the detailed design. The experimental set-up using an UA and SA performing tool called Simlab coupled with a c...