15" OD, corrugated plastic tubing insulated inside with Urethane foam to a 10" ID. Used to insulate the Heat Well below the frost line, preventing the loss of energy into the frozen ground. Won't rust, resists rot & hold up to chemically active soil conditions. Unaffected by ground movement due to freezing & thawing. Corrugated design provides rigidity without bulk when back filled with earth. It will take abuse!
