Abstract: The region between the outermost firing shaft and the outer wall of a chamber type furnace for baking carbon blocks, in particular anodes for the fused salt electrolytic production of aluminum, comprises essentially at least one layer each of firebricks as the wall of the outermost firing shaft, insulating refractory foamed bricks and moler stone or calcium silicate bricks at the outer wall of the furnace. The layer of insulating, refractory foamed bricks is fixed to the outer wall without room for play. Sealing elements on a projection of the outer wall extending over but at a distance from the insulating layers and on the outermost firing shaft interlock with one another.