Abstract: A device (20) for reducing evaporation and energy transfer between fluids, the surface (12) of which is exposed to environmental elements. A plurality of buoyant, homogenous evaporation devices (20) are randomly deployed into a body of fluid (10) whereby the devices (20) aggregate to provide a substantially contiguous, gapless blanket barrier. Devices (20) have predetermined number of substantially planar, polygonal, vertical side walls (22) of substantial height dimension. During undulatory conditions of the fluid surface (12), surface-to-surface engagement around the periphery of device (20) with the adjacent vertical walls (22) of similarly formed devices (20) is slidably maintained along the substantial height dimension of side walls (22). The aggregate of devices (20) is kept intact, requiring only minimal lateral pressure, to maintain a substantial evaporation retardation barrier regardless of whether the fluid surface (12) is subject to undulatory conditions or is uniform.