This paper presents an analytical model for predicting the air flow and velocity in an open vertical air channel due to natural convection. It can be used in the study of ventilated windows and double-faade systems, which are arousing interest as an energy-efficient means of providing fresh air, daylight and solar radiation to rooms. Unlike most previous work in this field, it proceeds from known surface temperatures instead of known surface heat flux. Model predictions are compared with experimental data from the SOLVENT window and the effect of the choices for the heat convection coefficient at the channel surfaces is discussed. A new approach to obtain a general expression for heat convection is presented and calibrated for a 4 cm width channel.