Control of the thermal properties of investment casting molds can be used to improve mold filing and control directional casting solidification.  Experiments were performed to design a low thermal diffusivity investment casting mold through the use of hollow spherical refractory particles (cenospheres).  The number of stucco layers in which the Cenospheres were substituted for silica grain was an experimental variable in this study.  The resulting shells were sintered and tests were performed to measure thermal and mechanical properties. Thermal diffusivity of cenosphere-stuccoed shell was determined by the inverse method mold to be 70% lower than silica-based shell molds.  Decreases in thermal properties were achieved without sacrifice of strength in fired condition.  Design applications of molds made using this material are discussed.