Abstract: An insulating structure or matrix with polygonal cells is formed with flexible partition sheets, each sheet being a laminate including a layer of paper and a layer of metallic foil, the optically reflecting surfaces of adjacent sheets facing in the same direction. Segments of adjacent sheets are joined by adhesive seams and cooperate to define polygonal spaces in a honeycomb-like structure upon expansion of the matrix structure. The matrix is bounded by flanges at its outer surfaces, parallel with the matrix in contracted configuration, for securement of the insulating structure in place upon installation. An alternative embodiment has optically reflective layers at both sides of a paper layer.