Attics in Europe are more and more used as a living room. In tropical regions, summer comfort in attics becomes critical when the roof system is badly designed. The European standards advise to form an open thermosyphon system into the roof under the tiles for many purposes. In order to evaluate the air channel's efficiency, an experimental study using a 2D-PIV system was carried out. A numerical model representing the natural convection within the thermosyphon was also developed. The predicted velocity distributions and the induced mass flow rate were in good agreement with the experimental results. Correlations for the air flow rate and heat transfer coefficients were proposed. The impact of the inclination, height, opening sections and Rayleigh number on the channel's efficiency was also investigated. (C) 2010 Elsevier B.V. All rights reserved.