Use of a composition, comprising at least one first and second binder, as a fungicidal wood preservative for wood fiberboard, is claimed, where at least one binder is a polyimine and the composition is directly applicable on the wood fiberboard during the manufacturing of the wood fiberboard. Independent claims are included for: (1) manufacturing wood fiberboard (preferably wood fiber insulating boards) with fungicidal properties comprising (a) preparing a wood fiber from wood chips, (b) contacting the wood fibers with a composition comprising at least one first binder and at least one polyimine as second binder, where in addition to the above composition a polyimine is directly applied on the fiber mat before pressing, and (c) spraying the mixture of wood fibers and the above composition on a conveyor belt and pressing the formed fiber mat； and (2) the wood fiberboard (preferably wood fiber insulating boards) manufactured by the above method, which exhibits increased resistance against the attack of fungi and other microorganisms at 100% relative humidity over a period of two weeks (preferably at least four weeks).
