The present invention discloses an extruder for producing a thermoplastic material having a first and second portion comprising an axially extending barrel and a screw conveyer axially disposed within said barrel so as to define a passage for conveying thermoplastic material from an input through a melt zone to an output. Furthermore the present invention discloses an insulation member comprising an elongated insulation member formed of a thermoplastic insulating material, said insulating member having a top edge, a bottom edge, and first and second side edges; the top edge comprising a protrusion, said top edge extending beyond a corresponding top edge of a corresponding panel of siding; and the bottom edge comprising a ledge, said ledge operable to overlap the protrusion.