A multiwall sheet can comprise walls, wherein the walls include: a first wall; a second wall; and a transverse wall, wherein the first wall, the second wall, and the transverse wall extend longitudinally; and a rib extending between adjacent walls, wherein a layer is formed by two adjacent walls; wherein the layer is filled with a nanoporous foam material; and wherein the multiwall sheet comprises a normalized thermal conductivity value of less than or equal to 1.00 W·m/kg·K. A method of making a multiwall sheet can comprise coextruding the multiwall sheet described above with a nanoporous foam material; wherein the layer is filled with a nanoporous foam material during coextrusion.