Closed Cell Backer Rod is a foam material used to fill large checks and gaps between logs. It also provides support for chinking and sealants. Its primary purpose is act as a "bond-breaker" and promote two-point adhesion (see diagram), giving the joint and sealing material the ability to compress and expand as logs move. Sealants and chinking do not adhere to the backer rod - only to the log surfaces of the joint - allowing for movement without damaging the sealing material. Closed cell backer rod is probably the most common form of backer rod. It does not hold moisture and actually repels water. Closed cell backer rod comes in a variety of sizes ranging from 1/4" to 5” wide and can be used on both the exterior and interior of the home. Round backer rod can be used in almost every joint style except flat Appalachian style logs. The round nature of the backer rod allows the caulking / chinking to adhere well to the wood, while the slightly thinner layer in the center allows the material to move with the logs. This also makes it so that any failure due to extreme movement will generally split down the middle of the bead line, allowing for an easy fix.
