What Is A Backer Rod and What Types Are Available?

When finishing joints or seals, a backer rod is often an invaluable tool.  Generally made of foam, these pieces are designed to be placed inside a joint or seal to limit how much sealant is needed, and how far into the joint the sealant penetrates.  If you were trying to seal a gap that was several inches deep, for instance, you would place a backer rod within the gap so that only an inch or so if sealant was necessary.  Otherwise, you’d have to fill up the entire gap, which is wasteful.

A backer rod brings other benefits as well.  It helps the joint sealant maintain elasticity, as well as creating an hourglass shape within the gap which increases how much surface area contact the sealant makes.  Additionally, this arrangement makes it easier to clean out and re-seal the gap in the future, should it be needed.

There are several different types of backer rods available, with different purposes.

I. Closed-cell rods

Closed-cell rods are basically just thick cords of foam, which can be had in a wide variety of sizes to fit your needs.  They are not highly compressible, and due to their inert design, they do not wick water either.  This makes them useful in situations where high water levels are expected.  In addition, because closed-cell rods are non-porous, they bond more easily to most sealants due to having higher surface area and adhesion properties.

Closed-cell rods are typically used in basic applications, such as sealing windows, partitions, and curtain wall joints.  There are also heat-resistant varieties available that are useful in large-scale applications such as sealing pieces of concrete highways or runway construction.

II. Open-cell rods

Open-cell rods contain a large amount of air, as well as porous surfaces, which make them extremely compressible.  This is good for applications where a large amount of movement is expected, such as in joints that are expected to expand and contract.  They will also capture moisture, which may be useful in some specific applications.

III. Bi-cellular rods

Finally, bi-cellular rods are a combination of the two.  These have an “open cell” internal structure, but with a sealed impermeable outer later.  They can be utilized in self-leveling joints, or in situations where bubbling within the sealant will be a concern.  This is more of a specialty product, and less commonly used.

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