A ventilation duct for a fire rated ventilation duct wall penetration has one or more metal sheets forming said duct, wherein said metal sheet duct is covered on the outside by a heat insulating material, and said duct includes elongated stiffening members located on the outside of the duct and attached to said metal sheets. The stiffening members each comprises a metal profile and at least one non-combustible bar of inorganic material. The metal profile is fixed to the metal sheet of the duct and retains the non-combustible bar by at least partly encircling the bar.