In a roof window system, the roof window (1) has a frame with a frame top member (21), and a sash with a sash top member (31), and further a ventilation device (40) for connection to a ventilation assembly by means of transition means provided between the ventilation assembly and the frame top member (21) and the sash top member (31) of the roof window (1) to accommodate a set of flow paths for air to and from the ventilation assembly. The transition means comprise a plurality of apertures (2102) extending through the frame top member (21).