What is Backer Rod Foam?

If you’ve ever walked across a smooth sidewalk or been in a log home that stayed toasty during the winter, you owe a debt of gratitude to a product you’ve probably never heard of. That product is foam backer rod, and it is an integral building material for masonry and construction that provides durability, flexibility and longevity, all while remaining out of sight.

Backer rod, also known as expansion joint filler, is a waterproof foam that is inserted into the joints between concrete, logs in cabin construction and cracks in other building materials. It functions as a bond breaker between the surfaces and the waterproof sealant that is applied over the foam, because if caulk completely fills the joint or crack, a three point bond would be formed. This means during normal material expansion, contraction and flexing, the adhesive would pull on the two materials it is sealing and the substrate, causing damage or failure. The reason joints in cement and spaces between materials are filled at all is because if moisture is allowed entry it can quickly damage and degrade the building materials. For instance, in a sidewalk, if the joint fills with water and freezes, it will expand and crack the freshly poured cement.

Caulk and adhesive does not bond to the backer rod pieces, which are like tiny cylinder cushions packed into the crack that expand against its sides. By only allowing a two point bond, the masonry or building material can expand and contract with the elements, while the sealant maintains a bond, stretching and compressing like a rubber band. Filling the majority of the crack with foam is also much more affordable than doing the same with costly sealant.