An emulsion comprising water as the continuous phase, a wax as the discontinuous phase, an emulsifier and a preservative having the general structure: wherein R.sup.1 can be a heterocycle containing nitrogen and sulfur, such as thiazolyl, isothiazolyl, or thiadiazolyl, which can optionally be substituted with C.sub.1-C.sub.6 alkyl; R.sup.2 can be hydrogen or C.sub.1-C.sub.6 alkyl, specifically hydrogen; n is 0, 1, 2, or 3; each instance of R.sup.3 can independently be hydrogen, C.sub.1-C.sub.6 alkyl, phenoxy, C.sub.1-C.sub.6 alkoxy, halo, amino, C.sub.1-C.sub.6 alkylamino, di C.sub.1-C.sub.6 alkyl amino, imidazolyl, thiazolyl, isothiazolyl, thiadiazolyl, thienyl, furyl, pyrryl, naphthyl, phenyl, halophenyl, C.sub.1-C.sub.6 alkyl phenyl, or C.sub.1-C.sub.6 alkoxyphenyl. The preservative may be added to the emulsion after the emulsion is formed. The emulsion can be incorporated into a gypsum product such as gypsum board or gypsum wood fiber board. The gypsum product may be made by forming a slurry containing gypsum, water, and the emulsion into a solid product. A method for improving the water resistance of a lignocellulosic composite product prepared by mixing lignocellulosic material with a binder to form a mixture and solidifying the mixture in a selected configuration to form the composite product may include adding to the mixture an emulsion as described above.