This paper presents a numerical investigation of airflow in an open geometry. The case under consideration is room with two opposite and decentred openings which create a strong potential for ventilation. The building characteristics dimensions are the followings: H=2.50 m height and W=6.50 m width. A temperature difference between the walls and the outside air is fixed, resulting in a characteristic Rayleigh number (Ra) ranging from 105 to 1.49 108. This room model proceeds from a benchmark exercise "ADNBATI" (http://adnbati.limsi.fr) coordinated by the by the "Centre National de la Recherche Française -CNRS-". This paper presents and discusses the results of this numerical study. Velocity, temperature fields, as well as heat transfer at the walls are analyzed. Values of the Nusselt number and of the mass flow rate according to the Rayleigh number are established from these first results.