Abstract: An acoustical and thermal energy insulating liner for gas transfer ducts, primarily in heating, ventilating and air conditioning applications is provided for installation within ducts having curvilinear cross section. The liner is made from a rectangular central block of insulating material such as foam or nonwoven web of glass fibers. A plurality of grooves running the full length of the block second major dimension and parallel thereto are cut through the front face of the block to a depth short of the block thickness. The front face is coated with an elastically deformable substance that forms a barrier layer adjacent the substrate insulating material. The barrier layer bridges the gaps of the grooves and thus contains the insulating material at the floor and side walls of the grooves internal to the central block.The barrier layer can be formed after the grooves are cut by spreading across the front face of the central block a layer of foam.