An air-conditioning duct comprises a regulating membrane attached to a peripheral wall of the duct and a shifting element. The membrane has a first end that is attached to the shifting element and faces an inlet for supplying air. An area of the membrane comprising said first end is adapted for being selectively shifted to a first or second portion of the peripheral wall. The shifting element is for shifting the first end of the membrane to the first or second portion of the peripheral wall. The membrane has a second end, which faces away from the inlet of the duct, secured inside the duct to ensure that shifting of the area of the first end of the membrane to the second portion of the peripheral wall prevents the air flow from passing through that portion of the duct situated downstream of the attached second end of the membrane.