Natural convection in enclosed spaces has been analyzed and numerically evaluated using a direct buoyancy model which replaces the pseudo-density difference which has been heretofore standard in all previous studies of internal natural convection of gases. It was demonstrated conclusively that the pseudo-density-difference model is totally irrelevant to the physical processes which create the buoyancy. When properly accounted, the presence of the pseudo-density difference does no harm, but it provides a source of confusion for the source of the buoyant forces which create motion. This conclusion was drawn from numerical solutions of three-dimensional natural convection in an oven-like cavity. These solutions were irrefutably supported by experimental data. It was also demonstrated that accounting for the naturally occurring pressure variations within the enclosed space had a negligible effect upon the surface heat transfer coefficients. The commonly used Boussinesq equation of state was found to provide accurate surface heat transfer results provided that the density, thermal conductivity, viscosity, and coefficient of thermal expansion are evaluated at a temperature that is the average of the temperatures of the surfaces which bound the enclosed space.