Abstract: A sub-flooring panel with improved impact and/or sound deadening (ISD) properties that is adapted to adjoin other sub-flooring panels to form a sub-flooring panel assembly, includes a base body having an upper surface formed with a groove for receiving a heating/cooling element, such as hydronic piping. A sub-flooring system with significantly improved ISD properties includes the panel assembly and an underlayment assembly which comprises a plurality of stabilizing supports, each support being adjustable in height, to support at least a generally rigid board and a fiberboard on which finishing flooring or laminate may be installed.