The expansion gap around the perimeter of the room must be sealed with 100% flexible silicone sealant following the installation instructions in order to create a watertight seal. Foam backer rod helps to fill the empty space in the room perimeter expansion gap and reduces the amount of 100% flexible silicone sealant needed during installation. It also helps to ensure the silicone sealant stays towards the surface to properly span the gap between the top of the HydroShield floor and the wall. Refer to the installation instructions and warranty for additional details.