Why do I have to use backer rod?
Backer rod is a bond breaker, which means caulk and chink does not stick to it. When put behind caulking, it allows the caulking to only stick at the top and bottom of the joint where it touches the log (2 point adhesion). This allows the material to be pulled only in two directions (up and down) and react to movement like a rubber band.
If no bond breaker is in place, the caulking will stick to the back wall, top and bottom of the joint (3 point adhesion). This holds the caulking in place and does not allow it to move, causing failure.
Backer rod also greatly helps control the depth of the sealant as it is being installed to insure that the optimum amount of material is put into place. If too little material is installed, premature “cohesive failure” can occur. If too much is installed, the sealant is wasted and your costs are greatly increased.