Domed cavities are found in many applications in buildings, particularly in conventional skylights and tubular daylighting devices. Heat transfer through domed cavities is thus an important factor in evaluating the energy performance rating of such skylight systems and in calculating the heating and cooling loads of buildings. Although there have been many studies on the convective heat transfer in related geometries, there is very limited information on natural convective heat transfer in domed cavities with planar inner surfaces. In this paper, a numerical study is conducted on natural laminar convective heat transfer in horizontal domed cavities with planar inner surfaces. The bounding surfaces are subject to uniform temperature conditions. The numerical model is based on the finite element method. The model predictions are successfully compared with published data in literature for concentric spheres and hemispherical cavities. The results show that for different boundary temperature conditions, the airflow in the cavities is mono-cellular and reaches steady-state conditions for both cold and hot weather conditions. The numerical results are used to develop practical correlations for the Nusselt number.