Natural convection in partitioned enclosures is extensively investigated due to its importance in many applications. Real enclosures have diffusive walls and partitions. A method has been developed to evaluate the global natural convection Nusselt number for a differentially heated (heated from sides) partitioned enclosure with walls and vertical partitions of finite thickness. A formula has been elaborated, giving the global Nusselt number as a function of the global Rayleigh number, global aspect ratio of the whole enclosure, and two other dimensionless parameters: the conductive thermal resistance ratio and the porosity. After validation of the method by comparison of our results with those from both the literature studies and the numerical simulation, a parametric study is carried out to show the influence of the dimensionless parameters. The method is a helpful tool for thermal engineers and scientists to design such a common system.