The coupling between natural convection and gas and wall radiation is studied numerically in a differentially heated cubical cavity filled with an air/CO2/H2O mixture. In order to solve coupled flow, heat transfer, and radiation equations, we develop a 3D radiative transfer model based on the deterministic ray tracing method, coupled with a pseudo-spectral Chebyshev method for natural convection under Boussinesq approximation. An absorption distribution function (ADF) model is used to describe gas radiative properties. Coupled simulations are performed at Ra = 105, 106, and 3 × 107, considering wall and/or gas radiation. Steady solutions were obtained except at the highest Rayleigh number in the case of radiating walls. Results show a strong influence of radiative transfer on temperature and velocity fields. The global homogenization of the temperature field induced by radiation leads to a decrease of the thermal stratification parameter. Two different mechanisms leading to this behavior, involving either wall/wall or gas radiative exchanges, are identified. In addition, we observe a thickening of the vertical boundary layers and an increase of the global circulation in the cavity. The influence of the Rayleigh number and 3D effects are also discussed.