In this paper, the thermal efficiency of a solar air collector called unglazed transpired collector (UTC) has been investigated both experimentally and numerically (CFD). Experimental measurements included the following variables: the temperatures of the air entering and leaving the UTC for three air mass fluxes, three distinct irradiations and two plenums thicknesses. A commercial finite volume software (Fluent) was used to model the heat transfer through the UTC. Turbulent effects were modelled by the RNG k–ɛ turbulence model. It was found that a weak heat exchange process mainly took place at the plenum; the maximum efficiency differences expected between the two plenums widths (5 and 15cm) was 2.85%.