Abstract: A moisture-actuated apparatus for controlling the flow of water by expandingly absorbing moisture from and contractingly emitting moisture into a moisture-conducting medium. The actuation is binary, with one state resulting from the expansion of an actuating element when it absorbs moisture from a water-conducting medium having a higher concentration of moisture than the element. The second state results from the contraction of the actuating element when the medium contains a lower concentration of moisture than the element, causing the element to emit moisture into the medium. The actuating element includes a thirotropic hydrophilic expandite material disposed in a matrix of wicking material. In one embodiment the absorptive expansion is used to provide a high-moisture signal in the same embodiment, contraction of the element produces a low moisture signal for opening the valve. In other embodiments, the expansion and contraction of the actuating element is used to directly actuate a valve.