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 connected to a frame member and adapted for providing ventilation of the interior of a building in which the roof window is mounted, and a ventilation assembly comprising an air exhaust device for extracting inside air to the exterior, comprising at least one ventilation unit (261, 361) including at least one ventilator (611a, 612a, 613a, 611b, 612b, 613c), the at least one ventilation unit (261, 361) being adapted to be connected to the ventilation device of the roof window so as to provide fluid connection between the interior of the building and the exterior, in the mounted condition, wherein the air exhaust device comprises a housing having predefined dimensions including a predefined width, a predefined length and a predefined height, and accommodating said at least one ventilation unit, wherein the length of the ventilation assembly corresponds in substance to a top casing of the roof window and the ventilation assembly is arranged in the position of such top casing, characterized in that each ventilation unit (261, 361) includes a flow channel (263, 363) having a first connecting end (263a, 363a) connected, in a mounted condition, to an inside air inlet (262, 362) in connection with the interior, and a second connecting end (263b, 363b) connected, in the mounted condition, to a through channel, and in that the housing further comprises the through channel having an ambient air intake end and an air exhaust end, the through channel further being connected to said inside air inlet (262, 362) via said flow channel (263, 363) so as to provide said fluid connection between the ventilation device of the roof window and the exterior. 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 connected to a frame member and adapted for providing ventilation of the interior of a building in which the roof window is mounted, and a ventilation assembly comprising an air exhaust device for extracting inside air to the exterior, comprising at least one ventilation unit (261, 361) including at least one ventilator (611a, 612a, 613a, 611b, 612b, 613c), the at least one ventilation unit (261, 361) being adapted to be connected to the ventilation device of the roof window so as to provide fluid connection between the interior of the building and the exterior, in the mounted condition, wherein the air exhaust device comprises a housing having predefined dimensions including a predefined width, a predefined length and a predefined height, and accommodating said at least one ventilation unit, wherein the length of the ventilation assembly corresponds in substance to a top casing of the roof window and the ventilation assembly is arranged in the position of such top casing, characterized in that each ventilation unit (261, 361) includes a flow channel (263, 363) having a first connecting end (263a, 363a) connected, in a mounted condition, to an inside air inlet (262, 362) in connection with the interior, and a second connecting end (263b, 363b) connected, in the mounted condition, to a through channel, and in that the housing further comprises the through channel having an ambient air intake end and an air exhaust end, the through channel further being connected to said inside air inlet (262, 362) via said flow channel (263, 363) so as to provide said fluid connection between the ventilation device of the roof window and the exterior.