Abstract: A method and apparatus for impregnating low density fiberboard with asphalt is disclosed, and which includes stacking the sheets, placing the stacks in a sealable chamber, and flooding the chamber with a treating solution composed of a mixture of asphalt and solvent. The chamber is thereafter drained, and the fiberboard is then heated by the injection of live steam into the chamber. A partial vacuum is then drawn in the chamber, while condensing and separating the withdrawn solvent and water vapors. Both the condensed solvent and water of the steam injection system are maintained in closed conduit systems to permit their reuse and to prevent contamination of the environment.