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