Foam backer rod comes in several variations suited for specific projects. Before making a selection, it’s important to fully assess the designed traits for the intended application purpose.

What is a backer rod and what does it do? 

Backer rods are usually round, flexible lengths of foam that are used as a “backing” in joints or cracks to help control the amount of sealant/caulking used and create a back stop. Many sizes/diameters are available for optimal fitting to the size of the joint being sealed. Besides shape, an important attribute is the variety of foam cell structure types: closed cell, open cell, and bi-cellular. It’s important to understand that cell structure makes a critical difference when it comes to irregular joint application, the desired flexibility in the joint, or sealant being used. Armacell’s FillPro™ backer rod material is made from polyethylene or polyurethane foam, making it compatible with the common industry joint sealants or caulking. 

Backer Rod’s four main benefits:

  • Increase elasticity in the joint sealant.
  • Reduce caulk consumption by controlling sealant depth— this prevents excessive use of caulking or sealant.
  • Force adhesive into contact with the sides of the joint creating an hour-glass shape for a superior bond.
  • Create a “bond-breaker” which allows the joint to be easily cleaned out and refilled as needed over time.

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 than an open cell product. FillPro Standard is a good example of a closed cell PE backer rod.

Common uses:

  • Glazing installations
  • Window and door applications
  • Expansion joints
  • Curtain wall joints
  • Partitions
  • Log construction
  • Pavement joints and repairs
  • Pre-cast units and copings

Some closed cell backer rods are crosslinked PE, creating a product that not only resists moisture, but also can withstand sealant temperatures up to 400°F. Armacell’s Hot Rod XL, is an ideal material for hot-pour sealants.

Common Uses:

  • Contraction and expansion joints of concrete highways
  • Runways
  • Driveways
  • Parking Lots

Open Cell

Open cell backer rod is unique; it can easily compress and return 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 is a highly conformable “spool-less” backer rod available in bags.

Common uses: 

  • Expansion and contraction joints
  • Window glazing
  • Curtain wall
  • Construction partitions
  • Precast assemblies and copings
  • Parking decks
  • Bridge construction

Open cell backer rods may not be ideal for sealants that are susceptible to bubbling of air escaping the backer rod material. 


FillPro Soft Type is a unique bi-cellular construction that offers high compressibility, but has a closed cell outer skin to resist moisture.  This is suited for specialty applications where standard backer rods are not appropriate, and is ideal in irregular joint applications 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.

Common uses: 

  • Irregular joint applications

Armacell offers the widest range of backer rod materials available on the market today. For more information, visit our product pages here