A microwave insulating material includes a dimensionally stable support, a patterned adhesive layer overlying at least a portion of the support, a polymer film layer overlying the patterned adhesive layer, and a plurality of expandable cells disposed between the support and the polymer film layer and defined by the patterned adhesive layer, wherein the expandable cells vary in size. A self-sealing microwave package includes a sheet of insulating material including a first surface, and a thermally activatable adhesive applied to at least a portion of the first surface.