Building Performance Simulation (BPS) has the potential to provide relevant design information by indicating directions for design solutions or uncertainty and sensitivity analysis. A major challenge in simulation tools is how to deal with difficulties through large variety of parameters and complexity of factors such as non-linearity, discreteness, and uncertainty. It is hypothesized that conducting an uncertainty and sensitivity analysis throughout key stages of the design process would be of great importance. The purpose of uncertainty and sensitivity analysis can be described as identifying uncertainties in input and output of a system or simulation tool [Lomas, 1992; Fuerbringer, 1994; MacDonald, 2002]. In practice uncertainty and sensitivity analysis have many additional benefits including: (1) With the help of parameter screening it enables the simplification of a model [de Wit, 1997]. (2) It allows the analysis of the robustness of a model [Litko, 2005]. (3) It makes aware of unexpected sensitivities that may lead to errors and/ or wrong specifications (quality assurance) [Lewandowska et al., 2004; Hopfe et al., 2006; Hopfe et al., 2007] (4) By changing the input of the parameters and showing the effect on the outcome of a model, it provides a "what-if analysis". It is for instance used in multiple decision support tools [Gokhale, 2009]. In this paper a case study is performed based on an office building with respect to various building performance parameters. Uncertainty analysis (UA) is carried out and implications for the results considering energy consumption (annual heating and cooling) and thermal comfort (weighted over- and underheating hours) are demonstrated and elaborated. The added value and usefulness of the integration of UA in BPS is shown.