1/2 in. Diameter x 50 ft. Length Backer Rod, $6.99 USD/Box, LL Flooring (Lumber Liquidators)

A 1/2 inch by 50-foot closed-cell PE foam that is perfect for filling gaps and expansion spaces. It is intended for use with silicone sealants to cover areas between walls and flooring, providing a reliable seal around floating floors. Made in the USA.