The heat transfer by natural convection and surface radiation in a two-dimensional vented enclosure in contact with a cold external ambient and a hot internal ambient was studied numerically. Inlet and outlet openings were located at a vertical conducting side wall in contact with hot air. Special attention was given on the modeling of the flow and thermal boundary conditions applied at the side wall. The temperature difference between the two vertical facing sides of the enclosure induces downward buoyancy-driven flow of air within the enclosure and boundary layer flow along the surface immersed into the hot internal ambient. The first part of the study focuses on the conjugate problem of natural convection and wall conduction for various thermal resistances of the cold wall, considered as the main floating parameter through the present study. The effects of surface radiation are discussed in the second part of the paper. The radiative exchanges between the facing walls serve to decrease the difference in the averaged temperatures of the vertical walls. Radiation tends to significantly decrease the temperature of the hot wall while the increase in temperature of the cold wall is less important. The interplay between convection and radiation is discussed and it is shown that radiation contribution in heat transfer dominates for all of the cases investigated. (C) 2005 Elsevier SAS. All rights reserved.