A number of simple models are available for the analysis of airing by window opening, but its transient behaviour is not yet well known, in terms of air change rates, air temperature and ventilation efficiency. In this paper, a synthesis of numerical modeling activity will be presented. A critical analysis of calculation procedures will be developed, comparing both detailed (CFD) analysis results and simplified (zonal) models predictions. A comprehensive discussion of models limitations, features and applications is presented. Moreover, the performance of theoretical models will be compared with the experimental results obtained during a full-scale measurement campaign. Finally, a simplified procedure, based on charts and non-dimensional groups, is proposed in order to make available to the designers a quick calculation method to evaluate the ventilation rate through an open vertical window.