Backer rod is a round open- or closed-cell foam rod used to fill joints between building materials. It is used to fill most of the void in a joint so caulking or chinking material, a mortar-like sealant, can be applied to finish filling the void and create an air-tight and water-tight seal. Caulking or chinking material can be used to fill the entire gap, but the cure time for large amounts of these products would be a number of days or weeks, depending on the size of the gap.

There are significant differences in the characteristics between open-cell and closed-cell foam backer rod. The closed-cell version repels moisture, provides a firm finish to the gap, and can be set into place with a spray adhesive. It is commonly used for smaller joints in the 1/4 to 3/8 inch (6.35 to 9.52 mm) range, and it possesses a higher insulative value than open-cell backer rod. Caulking or other chinking materials will not stick to closed-cell backer rod. This allows free and independent movement in the building materials and the chinking material.

Open-cell backer rod products are very easy to install because they are softer and more pliable than closed-cell backer rod. Large diameter pieces can be compressed to fit into gaps that vary in thickness in some areas. Open-cell backer rod is designed to be used only indoors, and does not possess insulative characteristics that are as good as its closed-cell counterpart.

Chinking materials are flexible sealants that will remain flexible throughout their usable life span. They will stick to the building materials, but not to the backer rod. For this reason, backer rod is also called a bond breaker. It allows the building materials to move, bend, and flex based on the environmental factors they are exposed to, such as humidity, heat, cold, wind, and ultraviolet light exposure.

Other sealing materials can be used to fill gaps, including elastomeric and other flexible caulking products. Elastomeric caulks are able to bend, flex, expand, contract, and stretch as needed while keeping a tight bond and an air-tight seal intact. These caulks and sealants are able to stretch to 200% or more of their original shape and size. They paintable with acrylic latex paints, but should be allowed to fully cure before being painted.

Both open-cell and closed-cell backer rod is available in a wide range of diameters, from 1/4 inch (6.35 mm) to 4 inches (101.6 mm) or larger. Some retailers sell backer rod by the foot. Other retailers sell it in small packages. For industrial uses, it can be purchased in 1,650 foot (502.92 meter) rolls.