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 (click 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.