Backer Rod: Backer Rod is sold in a variety of diameters and is a round, flexible, continuous length of closed-cell, extruded polyethylene. Backer Rod is used with silicone, elastomeric and other cold-applied sealants and caulks. It is used around doors, windows and other construction joints to either reduce the amount of caulking required or to produce an ideal joint shape. Backer Rod is sold by the case.