In the IEA-ECBCS Annex 23 ＇Multizone Air Flow Modelling,＇ a sensitivity analysis procedure, that included both the Monte Carlo and Fractional Factorial analyses, was defined to evaluate COMVEN, a multizone air flow code. This procedure is here applied to evaluate COMVEN, when the simulation of the ventilation of a detached house is performed for the case of ventilation driven mainly by stack effect. The simulated values are compared to the measured values using the uncertainty range for each value; the confidence interval for the simulated values are obtained using the Monte Carlo method while the fractional factorial analysis has been used to identify the relative effects of the input parameters on the output accuracy. It is an interesting case which makes it possible to demonstrate the application of both sensitivity techniques.