The effect of thermal boundary conditions on numerical heat transfer predictions in rib-roughened passages is investigated. Results obtained using constant heat flux at the walls and conjugate heat transfer are compared to illustrate how the recirculation bubbles upstream and downstream of the rib have different effects on the local heat transfer. Comparison between numerical predictions, experimental measurements and data correlations show that the predicted heat transfer is very sensitive to the type of boundary conditions used in the numerical model. It is illustrated that some of the discrepancies observed between experimental and numerical data can be eliminated if conduction heat transfer in the rib is taken into account.