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 to act as a "bond-breaker" and promote two-point adhesion, 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.
Backer rods come in a variety of shapes and sizes, and in open or closed cell configurations.