Abstract: A non-supercritical method for impregnating a refractive timber substrate with an active agent, including preparing an impregnation fluid including a low boiling solvent (methane, ethane, propane, butane, isobutane, fluorinated hydrocarbons such as chlorodifluoromethane (R22), 1,1,1,2-tetrafluoroethane (R134a), 1,1,1-trifluoroethane (R143a), pentafluoroethane (R125), 1,1-difluoroethane (R152a) and difluoromethane (R32).) and an active agent; contacting a refractive timber substrate (eg heartwood of radiata pine, spruce species; Douglas fir, fir, poplar, willow, sapwood of cypress pine and certain eucalypt and pine species such as heartwood of messmate, larch, western red cedar, European oak and American white oak) with said impregnation fluid in a liquid state in an impregnation chamber for a time sufficient to allow said fluid to penetrate the pores of said refractive timber substrate; and removing said impregnation fluid in a liquid state from said impregnation chamber and returning to a reservoir.