A moister sensor, for measuring moister in a building without leaving visible scars to the building surface, is provided. The moister sensor includes a flexible carrier carrying an antenna for receiving EM-radiation between 9 kHz and 11 MHz and a resonant circuit including a moister reactive element. The moister reactive element includes a hygroscopic electrolyte arranged between a first and a second electrode, wherein the electrolyte in the presence of moister forms mobile ions and provides a complex impedance at least in response to the alternating voltage, which complex impedance varies with the moister content of the electrolyte.