A gypsum board comprises a cover sheet and a gypsum layer disposed on the cover sheet. The gypsum layer comprises the reaction product of an isocyanate, water, and stucco. The isocyanate increases the moisture resistance of the gypsum board. A method of manufacturing the gypsum board comprises the steps of combining the isocyanate, the water, and the stucco to form a slurry, and applying the slurry to a cover sheet to form the gypsum layer on the cover sheet.