A ventilation device for ventilating a room in a building, the building having a window with at least two essentially parallel panes of glass, the at least two panes of glass being arranged in a common frame, casement or sash, thus forming the window, e.g., a double window, where the at least two panes of glass are arranged with a distance, and thus appear with an air space between the panes of glass, the window further having an intake opening communicating with the air space and with either the interior or the exterior of the building, where the ventilation device further has a preheating unit with a first chassis and a first throttle, where the first chassis has a first opening communicating with the interior of the building, a second opening communicating with the exterior of the building and a third opening communicating with the air space.