Abstract: Collapsible radiant barrier void filler, comprising: first and second scored sheets having interior and exterior surfaces and fold lines to attain collapsed and expanded configurations; a cell core being collapsible to a closed cell configuration and expandable to an open cell configuration, the cell core having end portions secured to oppositely disposed portions of the interior surfaces of the scored sheets, and the cell core having oppositely disposed cell side portions; and a radiant barrier having a reflective first side and an opposing second side wherein the second side is disposed upon at least one of the interior surfaces of one of the scored sheets and wherein the first reflective side is juxtaposed adjacent at least one of the oppositely disposed cell side portions in the expanded open cell configuration so as to maintain an air space adjacent the first reflective side that extends into the cell core.