Abstract: An annular duct insulation structure includes a substantially rigid annular insulation-retaining insert having a substantially cylindrical outer surface. A viscoelastic insulation foam layer is disposed around the substantially cylindrical outer surface of the insert so as to result in a composite insulation structure of the foam layer and the insert. The foam layer is formed of at least one of (A) a compressed open-cell foam layer or (B) a compressed closed-cell foam layer in which at least a portion of the closed cells has been opened, wherein the foam layer has been compressed sufficiently to permit the composite insulation structure to be slipped into an annular duct without substantial friction, and wherein the foam layer decompresses so as to substantially fill a void between an inner surface of the duct and an outer surface of the foam layer of the composite insulation structure.

Abstract: An annular duct insulation structure includes a substantially rigid annular insulation-retaining insert having a substantially cylindrical outer surface. A viscoelastic insulation foam layer is disposed around the substantially cylindrical outer surface of the insert so as to result in a composite insulation structure of the foam layer and the insert. The foam layer is formed of at least one of (A) a compressed open-cell foam layer or (B) a compressed closed-cell foam layer in which at least a portion of the closed cells has been opened, wherein the foam layer has been compressed sufficiently to permit the composite insulation structure to be slipped into an annular duct without substantial friction, and wherein the foam layer decompresses so as to substantially fill a void between an inner surface of the duct and an outer surface of the foam layer of the composite insulation structure.