Porous Al2O3 and Al2O3-mullite ceramics have a potential application as a high-temperature heat insulating materials. The effective thermal conductivity is an important property characterizing the heat insulating materials. The aim of the present work is to investigate the effect of the silica source (micro size SiO2 and SiC nanopowder) on the thermal conductivity of porous Al2O3-mullite ceramics in comparison with unmodified porous Al2O3 ceramics. The added or in situ formed SiO2 reacts with Al2O3, and forms mullite. Investigated materials were produced by chemical foaming of slurry. Pores form in the result of a chemical reaction between aluminium and water in an alkaline medium (pH>9) during hydrogen gas evolution reaction, and solidification of the suspension.