Highly porous mullite-based thermal insulators were fabricated using a gelation freezing technique. In this process, a honeycomb-like arrangement of pores is created by forming ice crystals within a gel containing dispersed mullite particles, followed by sublimation of the ice under vacuum and subsequent sintering. The uniform arrangement of pore channels and cell walls results in a mullite insulator with a very high compressive strength, even for porosities of up to 91.5 vol%, together with low thermal conductivity. The mechanical strength of this insulator was found to be strongly influenced by the amount of antifreeze protein, the sintering additive in the raw mixture, and the sintering temperature. The use of 0.25 wt% antifreeze protein with the additive 8Y-ZrO2 gave a compressive strength of 11.3 MPa, a porosity of 89.1 vol%, and a thermal conductivity of 0.28 W/m K. This approach can be used to produce macrocellular insulators with specific porosity, thermal conductivity and mechanical properties, suitable for a variety of industrial applications. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.