An air duct system for vehicles, in particular for passenger rail vehicles, has at least one roof venting duct, which has a component venting duct for warm air and a component venting duct for cold-air. A separating means which is arranged inside the roof venting duct is designed so it can be deformed or is held in a movable fashion in such a way that the cross section of each of the two component venting ducts can be enlarged by making the respective other component venting duct smaller. These changes in cross section are brought about solely by the air pressure and volume flow conditions without external energy.