The present paper shows the results of a researching work focused on hybrid ventilation of residential buildings. The objective is to identify and assess the ventilation air flows in residential buildings, without taking in consideration the user behavior, this is, considering window airings, and exhaust fans of humid spaces. Different existing calculation methodologies have been studied, and finally, a calculation model based on EN-15242:2007 standard, has been developed for an unique zone, by considering only inlets and outlets of the building as a whole. This model has been implemented in a software tool that uses an iterative process to solve the equations. The use of this software tool has been focused on a sensitivity analysis for which the selected variables are: the façade leakage level, the window permeability, the total area of windows, and the air velocity. These variables have been studied by separated and grouped, and between reasonable ranges. In this way, air flow rates have been represented graphically, and the air velocity for which a backward air flow can be expected depending on the air flow direction, and the percentage of the total air flow rate corresponding to leakages, windows, etc. As a final, and important conclusion of the study, we can conclude the optimal air velocity for which a certain ventilation level can be assured only with natural means.