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In order to study the effect of convection-radiation coupling occurring in the air gap of a solar thermal collector, numerical simulations were conducted for various thicknesses of the air gap with and without radiation. The studied geometry is a closed cavity which represents the confined space between the absorber and the glass. The cavity is inclined at an angle equal to 45 ° and is uniformly heated from below. The flow is three-dimensional and in unsteady state. First, the simulations were conducted considering only convection in the air gap, in this case the radiation is neglected and in a second time, the coupling between convection and radiation was taken onto account. In the first case the results show that the increase of the air-gap thickness leads to an intensification of the natural convection which develops from laminar, chaotic to turbulent regime. When the radiation is taken into account, the results show that the flow regimes are substantially modified, the convection-radiation coupling reduces the temperature of the hot wall, which contributes to the reduction of the intensity of natural convection in the cavity. This observation is verified by the evolution of the temperature field at the absorber and the heat exchange coefficient. So in conclusion, this study allowed us to see the evolution of heat transfer in the air layer between the glass and the absorber, in the absence and in the presence of radiation. Taking into account the radiation in the cavity is essential for the modeling of flows in a cavity (which is often neglected).