A method of manufacturing gypsum board includes applying compressed air to an input end of a tube, wherein the tube includes the input end, an output end, and a venturi located between the input end and the output end; admitting a foaming agent to the input end of the tube so as to form a mixture of the compressed air and the diluted foaming agent; passing the mixture through the venturi and out the output end; combining the mixture with gypsum and water to form a gypsum slurry; and casting the gypsum slurry onto a continuous web for forming a gypsum board.