Incorporation of rice hull ash into investment casting shell molds (10) improves the permeability, shell build thickness, and/or shell build uniformity of the molds (10). A method to make the molds (10) 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 (10). Figure 1