Since you have poly behind the Cement Board you technically don't need water proofing on top. But's still not a bad idea. Travertine is pretty porous and will let water through. If you water proof with Redgard none of that water will end up in the cement board.

If your Travertine is white or light creme, use a white thinset. If it's dark you can use gray thinset. If you end up using the redgard, they you need modified thinset. If you go directly on the CB you can use standard.

Is the poly on all the walls or just the exterior wall?

The poly would also have to overlap the shower pan liner to be effective.

Rule of thumb is that if you are doing surface type waterproof membranes, there should be no other moisture barriers behind your backer board. The potential for moisture being trapped in the backer board is not a good thing.

Sounds like your shower is water proofed and you are ready to tape the seams. Use CBU Mesh Tape on all the seams. I'm not sure if Kerdi strips or Kerdi Band will act the same as the mesh in terms of bridging the gaps and helping prevent cracking, but I do know it will cost a whole lot more.

Pete,

You shot those answers like a pistol, lol .. thanx !

Rc,

The Poly is all over the basment. In the shower area, it starts from the ceiling and all the way down and out on the slab for 4-5 inches. The poly is stapled and covering all of the 2x4 frame.
There's extra ply which comes out on to the slab (concrete floor)and covers by approx 5-4 inches. The mud bed sits on top of this poly and ofcourse the pan liner sits on top of the mud and therefore sits on top the poly as well.

Did I answer correctly ?


Mchugh,

I hope I'm ready, I've been at this for 2-3 weeks now. I've already purchased the kerdi band, might as well use it.

Yes

IF your sure theres poly on ALL walls that are covered with cement board then carry on.

I would forget the Kerdi strip because thats going to require you to purchase unmodified thinset just to do that only.

Just buy white modified thinset and use Alkali-resistant mesh tape for the seams.

Yes, there is poly behind the cement board.

Do you still recommend liquid waterproofing the cement boards? if so. Which one would you recommend besides red gaurd.

Thank you

Steven

No, unless your prepared to tear out all the poly.

The generally held reason you don't want to water proof with poly behind there is that you don't want to create a vapor lock. This can lead to condensation getting trapped in the cement board and building up over the years.

I don't necessarily agree with that, but really don't want to start an elaborate debate that would only get confusing.

Go with what RC said.

Pete, here's a little light reading on Vapour Barriers etc.
We won't get into it on this thread as it gets too technical for most people to understand.

http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

Go it thanx