Experiments were performed to determine heat transfer data for the natural convective flow of air in vertical channels with one surface roughened by transverse square ribs and the opposite surface smooth. Uniform wall temperature conditions were imposed on the ribbed side, while the smooth side remained unheated. Additional experiments were carried out in vertical channels without ribs, under the same channel geometry and thermal conditions. A schlieren optical technique was used to reconstruct the thermal field and to obtain distributions of heat transfer coefficients. The presence of ribs was found to alter heat transfer considerably, causing thermally inactive regions just upstream and downstream of each protrusion. Consequently, the heal transfer performance of the ribbed channel turned out to be lower than that of the corresponding smooth channel.