Most existing numerical studies of convective heat transfer between a window-blind system and a room are based on the assumption that the flow remains laminar. However with larger windows transition to turbulent flow will occur in the flow over the window. The aim of the present study was to numerically determine the effect of a Venetian blind on laminar-to-turbulent transition in the flow over a simple recessed window and on the natural convective heat transfer from the window to the room. An approximate model of a recessed window that is covered by a Venetian blind has been considered, the window being treated as a plane isothermal vertical surface. The study is based on the use of the steady, two-dimensional governing equations, these having been solved using the commercial finite-volume based CFD code FLUENT©. The k-epsilon turbulence model with the full effects of the buoyancy forces being accounted for has been used. Results have been obtained for various blind slat angles and for various distances of the slat pivot point from the window. The results show that over a wide range of window height based Rayleigh numbers, the distance of the blind to the window has a stronger effect on the convective heat transfer from the window and also on the laminar-to-turbulent transition in the flow over the window than does the blind slat angle.