A defined product moisture content must be maintained for the production of vehicle interiors where different materials are mixed in a specified ratio to each other and processed into a mat. For the subsequent shaping, the product moisture content must be known in order to be able to direct the process without any loss in quality. For this purpose, a humidity sensor is mounted above the material touching the product surface. The equilibrium moisture content in relation to the material moisture of the product is determined. The sensor is installed in a cover which prevents exposure to the influence of external moisture from the surrounding environment.