A composite board is produced by the method of blending a cellulosic filler having sufficient extractable organic content and an expandable thermoplastic polymer, adding a tacifier agent during blending for producing a pre-blown mixture, heating the mixture in a mold to a temperature above the glass transition temperature of the polymer for a period of time sufficient to permit expansion of the beads and bonding of the expanded beads with the cellulosic filler to thereby form a composite board. The board may be laminated by a solvent based adhesive or a thermal insult coating so as to form rigid macro-voids between the laminate and the composite board surface.