A prefabricated modular furnace unit built up of a plurality of longitudinally extending refractory brick elements to form a substantially closed perimetric figure in cross section. The bricks have juxtaposed mitered edges free of any mortar between them to permit them to move relatively to each other in radial directions in response to temperature variations. A spring-loaded sheath of high tensile strength on the exterior of the bricks maintains the unit in integrated compressed condition at all times while permitting expansion in response to thermal variations. The cross sections of the modular units may vary and the units of the same cross section may be combined to any length. The modular units may be closed at the ends thereof with either fixed or movable end closures of sheathed firebrick assemblies. Also, their thermal efficiency may be increased by providing bodies of insulating material between the bricks and sheaths. The modular units lend themselves to continuous car-type muffle furnaces, or ovens, by providing a gap in the perimeter of the firebricks, which are compressed at their juxtaposed edges by suitable spring-loaded sheathing to permit the travel of the furnace conveyor components at said gap relative to the main fixed portion of the furnace housing.