Cracks, joints, and voids in construction applications vary in shape, width, and form so understanding the design requirements for an application before making a backer rod decision is critical. Backer rods are usually round, flexible lengths of foam that are used as a “backing” in joints or cracks. Backer rods should be used when you need to increase elasticity in sealant, reduce caulk consumption by controlling sealant depth, force adhesive into contact for enhanced bonding, or to create a “bond-breaker” which allows the joint to be easily cleaned out and refilled. Besides shape and size, an important attribute to consider is the variety of foam cell structure: closed-cell, open cell, and bi-cellular. It’s important to understand that cell structure makes a difference when it comes to irregular joint application, the desired flexibility in the joint, or sealant being used.
Closed Cell
Closed cell foam is distinct in that its cells are totally enclosed by its walls and not interconnecting with other cells. This means the material will not wick moisture making it suitable for joints with a high presence of water or dampness. Sealants, caulking or other chinking materials will stick more easily to closed cell backer rods which have a better bonding surface area, and it is compatible with most cold-applied sealants.
FillPro™ Standard Backer Rod is an extruded round, closed cell, low-density polyethylene (PE) foam material with a skin-like outer texture. Some common FillPro Standard backer rod uses are in glazing installations, window and door applications, expansion joints, curtain wall joints, partitions, log construction, pavement joints, and pre-cast units and copings.
FillPro™ Hot Rod XL Backer Rod is an extruded round, closed cell, low-density PE foam material which has been specially crosslinked to withstand temperatures more than 400°F (204°C). It is chemically inert and will resist oil, gasoline, and most other solvents. The material will not stain, absorb moisture, nor adhere to sealant materials and is non-exuding. Hot Rod XL backer rod is a backup material for most hot-pour sealants used to fill contraction and expansion joints of concrete highways, runways, driveways, and parking lots. It’s compatible with most rubber-asphalt and coal tar-rubber polymer thermoplastic compounds.
Open Cell
Open cell backer rod easily compresses and returns to its original state due to unrestricted air movement. This flexibility makes it easily conform to sealing applications and helps limit the depth of the sealant while also adding good sealant airing, which can reduce drying or set up time. 
FillPro™ Open Cell Backer Rod is a pliable open cell polyurethane (PU) foam material used for filling irregular joints. It is chemically inert and will resist oil, gasoline, and most other solvents. The material will not stain, or adversely adhere to sealant or caulking materials. Open cell backer rod is commonly used in expansion and contraction joints, window glazing, curtain wall construction partitions, precast concrete assemblies and copings, parking decks and bridge construction. It’s compatible with virtually all known hot-pour and cold-applied sealant including silicone and rubber asphalt.
Bi-Cellular
Bi-cellular construction offers high compressibility with a closed cell outer skin to resist moisture. Bi-cellular backer rod is suited for specialty applications with irregular joints where self-leveling, flowable sealants are employed. Bi-cellular is also recommended with sealants where bubbling concerns are critical. It’s an excellent all-purpose backer rod that may be used in place of most standard backer rod applications.
FillPro™ Soft Type Backer Rod is a pliable bi-cellular PE foam material used for filling irregular joints. It is chemically inert and will resist oil, gasoline and most other solvents. The material will not stain or adhere to sealant materials and is non-exuding. An excellent all-purpose backer rod, it may be used in place of most standard backer rod applications.
Combined with outstanding service, prompt delivery and competitive pricing, Armacell offers the most extensive collection of backup materials and the largest range of sizes in the market. For more information on our backer rod solutions, please visit our backer rod solutions page.