Abstract: A method for adding insulation or bulk absorbers into high temperature sandwich structures following fabrication is presented. A slurry of ceramic fibers and/or particles, opacified particles, fugitive fibers, organic binders and inorganic binders is prepared as an aqueous solution. The slurry is cast within a prepared sandwich structure, dried, and heated to form a low density ceramic core material to provide insulation or noise absorption. Following incorporation of the ceramic material, aerogels or phase change materials may also be added to provide additional thermal management benefits.