Real enclosures have diffusive walls. A procedure is developed to evaluate the natural convection effective global Nusselt number for rectangular enclosures with vertical diffusive walls. The effective Nusselt number is evaluated using the temperature difference imposed over the exterior faces of the enclosure, the usual correlations for rectangular enclosures without diffusive walls, and the diffusive properties of the solid vertical walls, which are concentrated on a single dimensionless parameter. The proposed procedure is tested by comparing the obtained results with those achieved from the complete two-dimensional numerical simulation of the conjugated heat transfer problem occurring in the complete enclosure, with diffusive walls. The result is a helpful toot that promptly helps the thermal engineer when dealing with enclosures with diffusive walls.