Natural night ventilation is an energy efficient way to improve thermal summer comfort. Coupled thermal and ventilation simulation tools predict the performances. Nevertheless, the reliability of the simulation results with regard to the assumptions in the input, is still unclear. Uncertainty analysis is chosen to determine the uncertainty on the predicted performances of natural night ventilation. In addition, sensitivity analysis defines the most important input parameters causing this uncertainty. This methodology is used to evaluate one of the possible design schemes of natural night ventilation in a new office building of the Ghent University (Belgium).