What is a backer rod and what does it do? 

Backer rods are round, flexible lengths of foam that are used as a “back stop” in joints or cracks to help control the amount of sealant/caulking/chinking. Many sizes/diameters are available for optimal fitting to the size of the joint being sealed. Besides shape, an important attribute is the 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. 

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.
  • Provides a stable chinking or caulking surface
  • Compresses to fit a wide variety of log shapes.
  • Can be used on exterior or interior surfaces

Backer rod application in log cabins:

Since logs settle, twist or move around, they need to be sealed, either caulked or chinked, keeping the structure weather tight. Proper use of backer rod and chinking can significantly reduce the heat loss of a log wall as well as in and around window frames and doorways. Filling these gaps properly will end up saving you a lot in utility bills and future repairs. Before chinking a log cabin home, backer rod must be installed in the chink joints. It’s important you don’t skip this step. If you don’t, you can end up getting splits. Even if there is a gap between sections, movement can cause the chink or caulk to stretch and eventually break free from the void. 


For log cabin homes, a wide variety of backer rod materials are offered for chinking and caulking application. There are many log configurations, each one’s unique. Backer rod comes in several types and each have its own niche application. Typically, you will find triangular, trapezoid, and round-shaped rods, offered in a many diameters and lengths for use in log home construction. The backer rod sizes are chosen based on the type and size of gap or void you need to fill. Remember, voids vary in shape and size width; you must select a backer rod most appropriate to control sealant depth.


You have a choice for backing materials, but most of these materials sacrifice one important property or another. Triangular, trapezoid and round-shaped rods must be cut to fit  to ensure proper fit and spray adhesive or staples would be used to secure it within the joint. 

The installation is very simple. One method for speeding up installation is using spray adhesive. Coating the backer rod and let dry until tacky, then carefully place into the joint pressing firmly until it fits snugly between the logs. The other method is stapling the backer rod until all the joints are filled. It’s convenient to have several sizes on hand for filling different sized voids. Typically, you want to leave at least a quarter inch depth from the surface for sealant or caulking to be applied, follow the manufacture’s recommendations. If the backer rod sticks out too faruse a knife to carefully trim the backer to size. Don’t press the backer rod in to tight, as you can end up wrinkling the material which may show in the surface of the chinking.

Other application areas:

A foam backer rod will be especially useful before caulking bathroom tubs, showers, kitchen counters and sinks. It can also be used for cement repairs on patio or sidewalk cracks, foundation cracks, driveway cracks and swimming pool foundations.  

To read more on the different types of backer rod and other applications click here

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