One of the heat transfer mechanisms in the cavity of the hollow concrete block is the natural convection. The cavity formed by concrete blocks placed in walls, has a geometric ratio of 20:1 for common blocks of 0.1 m width. The thermal gradients applied are representatives of the boundary conditions in a wall exposed to weather conditions in a desert climate. Experimental and numerical convective heat transfer coefficients are presented and compared. The numerical flow and temperature pattern, obtained with CFD software Fluent 6.3 are discussed.