QUESTIONSealant and Backer Rod for Bath tub to wall tile joint transition - I would like to try using polyethylene backer rod to solve my problem of a crack at the joint between the tub and the tile. What size backer rod should I be trying to use? With water in the tub, the crack was opening up to close to 1/4 inch. (Without water it's negligible.)
ANSWERANSWER - The polyethylene backer rod is not likely the cause of your cracking at the tub to shower wall transition, but if there is room for it in that joint, then you should use it to fill the excess space and to gauge the thickness of the sealant you are using.
Generally the transition between the tub and tile on the wall is either wrongly filled with cementitious grout or the installer wrongly fills (caulks) it with a latex sealant sometimes referred to as a caulking. The latex sealants don't perform well, don't have warranties, and do not last long.
Our industry standards requires that an ASTM C920 sealant be used which is either a 100% silicone (not Siliconized latex) or a urethane sealant; there are other types. These types of sealants will last for years and have warranties.
Our industry standards says to use a polyethylene backer rod to fill up excess space in the joint. The backer rod should be larger than the joint and be compressed into it. The backer rod will fill up the excess space plus you can control how thick the sealant will be on top of the backer rod filling the joint. The sealant cannot be applied too thick or too thin for it to perform correctly. Follow the sealant manufacturer's instructions, but normally it should be at least 1/8" thick and not thicker than 1/2". Normally the manufacturer requires the use of a primer prior to apply the sealant.
Based on your description that when the tub is full of water the joint is less than 1/4" wide, I would put in the 1/4" backer rod with the tub full of water, and apply the sealant without water in the tub. This is based on the joint being deep enough so you can apply between a 1/8" to 1/4" thick sealant on top if allowed by the sealant manufacturer. If 1/4" backer rod is too thick then use a smaller one; you can get it as thin as 1/8". If 1/8" backer rod is too thick then don't use a backer rod.