Abstract: The present invention involves the preparation and manufacture of phenolic insulation boards for use in double-walled ductwork systems. The preparation of the phenolic insulation board, having a first and second surface each having a foil coating, includes the cutting or grooving of V-grooves into the first surface of the phenolic insulation and first foil coating. This grooving process creates a continuous piece of phenolic insulation having trapezoidal shapes along the second surface and second foil coating. The prepared phenolic insulation board can then be wrapped around an inner sheet metal duct and then covered with an outer shell. Alternatively, the prepared phenolic insulation board can be inserted between an inner duct and outer shell. Both result in a double-walled phenolic insulation ductwork systems.