Each Structural Foam Panel is made from non corrosive structural polymer. The top and bottom rail of the panel is 4" wide and each panel has a series of box style ribs for strength. In addition to these ribs, each 6' panel has 4 vertical ribs running the height of the panel ensuring the panel will not bow when back filled. The area around the Return Fitting is ribbed and marked for drilling the return fitting hole. This reinforces this area and ensures that all return fittings are at the same level.