Water vapor control, which is arranged facing the inside of a building, comprising a first layer having a water vapor diffusion resistance (sd-value) of 1-5 meters diffusion-equivalent air space width, measured at a relative humidity of an atmosphere surrounding the layer of 30-50%, and having a sd-value of 0.6 meters diffusion-equivalent air space width, measured at a relative humidity of 80-100%, and wherein the second layer is located at the side of the vapor barrier facing the inside of the building, relative to the position of the first layer.