A roof window system comprising: a roof window having at least one frame (2, 3) defining a frame plane and including a pane, the roof window further comprising a ventilation device adapted for providing ventilation of a building in which the roof window is mounted, a ventilation assembly comprising a housing accommodating at least one ventilation unit (110; 120), each including at least one ventilator (131; 132) and at least one regenerator (171; 172), and a set of flow channel sections (1502, 1504, 1508; 1503, 1507), each ventilation unit (110; 120) being connected to an aperture for air intake and exhaust, and to a transition (t) to the ventilation device of the roof window, to provide a predefined flow path from via the set of flow channel sections (1502, 1504, 1508; 1503, 1507) in each ventilation unit (110; 120), a transition element (161, 163; 162, 164) provided at the transition (t) between the ventilation assembly and the ventilation device of the roof window, and at least one splitter (1521; 1522) to form a pair of transition flow channel sections (1507, 1509; 1508, 1510), wherein the splitter (1521; 1522) divides a transition portion of the flow path at the transition (t) to the ventilation device of the roof window in at least one ventilation unit (110; 120) to form a pair of transition flow channel sections (1507, 1509; 1508, 1510) with a respective width dimension (d1, d2) and in connection with one respective adjacent flow channel section (1503; 1504) with a predefined width dimension, and characterised in that the transition element (161, 163; 162, 164) has a width dimension (dt) substantially corresponding to the respective width dimension (d1, d2) of the respective transition flow channel section (1507, 1509; 1508, 1510). a roof window having at least one frame (2, 3) defining a frame plane and including a pane, the roof window further comprising a ventilation device adapted for providing ventilation of a building in which the roof window is mounted, a ventilation assembly comprising a housing accommodating at least one ventilation unit (110; 120), each including at least one ventilator (131; 132) and at least one regenerator (171; 172), and a set of flow channel sections (1502, 1504, 1508; 1503, 1507), each ventilation unit (110; 120) being connected to an aperture for air intake and exhaust, and to a transition (t) to the ventilation device of the roof window, to provide a predefined flow path from via the set of flow channel sections (1502, 1504, 1508; 1503, 1507) in each ventilation unit (110; 120), a transition element (161, 163; 162, 164) provided at the transition (t) between the ventilation assembly and the ventilation device of the roof window, and at least one splitter (1521; 1522) to form a pair of transition flow channel sections (1507, 1509; 1508, 1510), wherein the splitter (1521; 1522) divides a transition portion of the flow path at the transition (t) to the ventilation device of the roof window in at least one ventilation unit (110; 120) to form a pair of transition flow channel sections (1507, 1509; 1508, 1510) with a respective width dimension (d1, d2) and in connection with one respective adjacent flow channel section (1503; 1504) with a predefined width dimension, and characterised in that the transition element (161, 163; 162, 164) has a width dimension (dt) substantially corresponding to the respective width dimension (d1, d2) of the respective transition flow channel section (1507, 1509; 1508, 1510).