Building material is one of the main sources of indoor radon in China. Radon exhaled from building materials enters the indoor atmosphere and transports in the indoor air space driven by buoyancy-driven airflow. This paper established a numerical model to reveal natural-convective effects on radon transport from building materials into the indoor air space. The building wall is approximated by a rectangular porous medium with uniform porosity and isotropic permeability. The buoyancy-driven flow is assumed to be turbulent and incompressible, ignoring viscous dissipation. The model can predict the dynamic coupling of radon in building materials and indoor atmosphere at different temperature gradient. This paper focuses on the analysis of radon transport with average Nusselt number and Sherwood number of interface between the building wall and the indoor atmosphere, which reveal the mechanism of radon exhalation from building materials into indoor atmosphere.