Highly porous silica thermal insulators were produced by gelation of a gelatin solution with dispersed silica particles, followed by freeze-drying and heating at 800 degrees C. The relationship between the ice-template morphology and the parameters involved in gelation freezing was investigated, in addition to the effect of the microstructure on the thermal conductivity of the silica insulators. The porosity and thermal conductivity of the insulators ranged from 88 to 98vol% and from 0.168 to 0.054W(mK)(-1), respectively, both of which could be controlled by changing the silica content in the gel stage. Gelation-freezing route was proved to be a feasible and promising route for the production of very highly porous, machinable insulators.