Rear lining of chimney. An acceptable chimney back lining can be achieved by plastering the interior surfaces of the chimney. A better alternative, however, is a manufactured lining of extruded fireclay. Prefabricated fireplaces often use multiwalled metal chimneys. In these chimneys, cool air from the room is introduced in the space between the flue walls at the bottom of the chimney and serves to insulate the outer shell by absorbing heat from the inner wall and discharging into the outside air above the roofline or into the room below the ceiling. Local building codes specify acceptable minimum construction requirements for masonry chimneys as well as prefabricated fireplaces.