A water-resistant orthopedic undercast sleeve that includes a tubular sheet material formed of extruded filaments having water-resistant properties and a thickness sufficient to provide both cushioning and moisture transport in a single layer between a cast and a limb onto which the sleeve is applied. The water resistant properties are achieved by at least one process, including a water-resistant filler, finish or coating applied to the filaments during extrusion, applying a water resistant finish to the finish during extrusion, applying a water resistant finish to the filaments during a subsequent spinning process, or coating the filaments with a water-resistant finish in a separate process. Moisture egress channels are formed in the sheet material for transporting moisture away from the limb, longitudinally along the length of the sleeve, and out of the space between the sleeve and cast.