A duct includes a number of wall sections which are at least partly produced from fiber material and a binder agent. Together, the wall sections define a closed, elongate flow space. The wall sections which meet one another are united by the intermediary of at least one longitudinal fold or joint line. In the fiber material and the binder agent of the duct, there are disposed longitudinal rigidifying elements. On the outside of the duct, there are longitudinal bead-liked projections which include the fiber material and the binder agent.