The invention is a butterfly valve for a flow duct of an air-conditioning installation. The butterfly valve comprises two butterfly valve faces and is mounted for rotation about an axis. A device with at least two measurement points for determining the volume flow rate of a gaseous medium flowing in the flow duct is provided. In order to specify a butterfly valve, for example for a volume flow rate controller, which can determine a high differential pressure, even at very low volume flow rates, the butterfly valve comprises at least one hollow chamber on each of its two butterfly valve faces. Each hollow chamber has at least one recess, preferably with a multiplicity of recesses formed in the manner of a perforation. One measurement point is associated with one hollow chamber and the other measurement point is associated with the other hollow chamber.