Uncertainties in airtightness measured using fan pressurization test should not be defined by the scattering of the points around the line defined using ordinary least square method anymore. Its definition requires first to know the uncertainties in pressure and airflow measurements. This works aims at quantifying one of the component of the envelope pressure uncertainty: the uncertainty in zero-flow pressure approximation due to short-term fluctuation of wind speed and direction. This is done by statistically analysing the approximation quality of 40 zero-flow pressure tests performed on 30 different units on eight different sites in Brussels. First, the analysis showed that this component of uncertainty could be slightly reduced by increasing the period of measurement used to compute zero-flow pressure approximation from 30 to 60 seconds. Second, it allowed to study the impact of multiple variables on the quality of the zero-flow pressure approximation. Third, it allowed to develop three different models to predict approximation quality as a function of different variables. This study suffers from some limitations due to the sample of units tested available. These limitations lead to important further work: the validation of the model on other samples of building tested and the adaptation of these models if needed. Further work should also focus on integrating these results on the uncertainty in envelope pressure measurements and the on uncertainty in airtightness estimation of the building.