for at least the second layer 16 is illustrated in detail in fig3 the module 16a is typically a biscuit of substantially aligned end on oriented fibers 30 held compressed tightly together by a netting 32 and or strips 34 or other thermally decomposable enclosing material the modules disclosed in u s pat no 4 194 036 are the type of modules suitable for this use once the modules 16 are in place firing the furnace will cause the thermally decomposable compression means to be destroyed permitting the modules to expand to form a tight seal because of the physical strength of such a structure it is preferred that the same module 16a be used for the first layer 14 as well especially when the second layer 16 is to be secured directly thereto with cement 18 or other suitable adhesive in fig1 the furnace can be fired before applying the second layer of modules which will permit the modules of the first layer to expand before applying and firing the second layer of modules alternatively the second layer of modules can be applied to the first layer without firing the first layer the technique for applying a dual layer of insulation 24a using stud anchors 28 will normally comprise an initial step of laying out and welding the stud anchors 28 to the furnace wall 12 in an array such as is schematically illustrated in fig6 when expanded metal sheet anchors 26 of about 6 inches by 6 inches square are used the centers are normally spaced dimension x in fig6 about 10 34 or 12 34 apart wider spacing about 12 34 is used when the furnace walls 12 are flat vertical surfaces while narrower spacing about 10 34 is.