Natural convection in a spherical geometry is considered for prediction of the buoyancy of single- and doublewalled balloons in a cryogenic environment such as Titan's atmosphere. The steady-state flow characteristics obtained by solving the Reynolds-averaged Navier-Stokes equations with a standard turbulence model are used to determine the net buoyancy as a function of heat input. Thermal radiation effects are shown to have a minor impact on the buoyancy, as would be expected at cryogenic conditions. The predicted buoyancy and temperature fields compare favorably with experiments preformed on a 1-m-diameter Montgolfiere prototype in a cryogenic facility. In addition, both numerical and experimental results were compared with correlations for the heat transfer coefficients for free convection internal and external to the balloon as well as in the concentric gap of the double-walled balloons. Finally, scaling issues related to inferring the performance of the full-scale Montgolfiere from the model-scale results are examined.