Incorporation of rice hull ash into investment casting shell molds improves the permeability, shell build thickness, and/or shell build uniformity of the molds. A method to make the molds entails mixing refractory filler, rice hull ash, and preferably a fiber to form a dry blend; mixing the dry blend with a binder sol to form a refractory slurry, and employing the refractory slurry to produce an investment casting shell mold.