Abstract: A furnace for high temperature, for example a glass furnace, is insulated thermally by arranging layers of material on the interior wall of the furnace shell. On the layers ceramic fibre material and a heat reflecting surface layer are arranged. According to the invention, the heat reflecting surface layer is arranged by a substance containing silicon. By subjecting the furnace to great heat and cooling it rapidly thereafter, a heat reflection is obtained in the micro-structure of the ceramic fibre. The substance may for example be water-glass. According to the invention, the fibre material is arranged in vertical blocks of trapezium form, then pressed and strapped up. After all blocks have been placed, one removes the strapping so that the blocks expand.