I am no expert and there are several that can comment. But, what about cement board on studs, then a topical like hydroban, then tile? Skipping the poly because you don't want a waterproof membrane on both sides of the board.

Agree with enduring.

"Easiest and better performing" would probably be HydroBan over cement board. More expensive, and maybe slightly more difficult due to never having worked with it before, would be the Kerdi Board. KB is fine...but for a one-off project, the things you'd have to buy...sheets of board, Kerdi Band, etc...it might not be economically efficient.

Most economical would indeed be the poly behind cement board. But topical membranes like Hydroban and Kerdi do have a performance advantage over a membrane located behind the tile backer board, especially when it comes to detailing the poly to tub flange.

I think Kerdi Board is an advantage for demanding designs. Most tub surrounds are fairly straight forward.

As a DIYer, my favorite (balancing economy, ease of application and promise of success) is standard kerdi over drywall.

"Mongo, will you tell me how to seal the bottom of the board with a hydroban application? How do you "meet" the hydrobanned cement board with the tub flange? "

Let's say that you installed the cement board right on the studs and held it just above the tub flange.

Mix up a bit of stiff thinset and fill the gap. Make it flush with the face of the cement board.

Then when you Hydroban, just Hydroban the face of the cement board, and down and over the thinset-filled gap. That should give you a continuous coverage of thinset all the way to the tub deck, if that makes sense.

Then when you tile, hold the tile a grout width's gap off of the tub deck. Caulk that gap.

Sort of like this:

Excellent! THANK YOU!