Hydrophobized granular material comprising from 30 to 95% by weight of a pyrogenic mixed oxide based on silica and at least one oxide of metal M selected from of Al, Ti and Fe with the content of metal M oxide in the mixed oxide being from 01 to 10% by weight, and from 5 to 70% by weight of at least one IR-opacifier selected from the group consisting of silicon carbide, zirconium dioxide, ilmenites, iron titanates, zirconium silicates, manganese oxides, graphites, carbon blacks and mixtures thereof.