In this case, you might be better off using a surface applied waterproofing instead of a vapor barrier, like Redgard or Laticrete 9235

Hi Bill, I'm a little confused.

My concern is moisture coming in from the exterior through the concrete block and potentially rotting the furring strips that will be holding the tile wall up.

The "warm wall" will be tile on thinset on cement board on vapor barrier on furring strips. But those furring strips will be contacting the outside wall. It is that contact that I am thinking of.

Should I just use PT for the furring strips and be done with it? Or should I install a second moisture barrier on the interior of the block wall before installing the furring strips? I'm inclined to go with the PT directly on the block, but would be grateful for any advice from folks more knowledgeable than me.

One reason I am confused is that I understand that some water vapor will get by my vapor barrier, and when it does I think I would want it to pass to the exterior of the house. But if I apply something to that wall to prevent this, I will get an accumulation. Also, IIRC none of those paintable water barriers actually hold up over time, so I am not sure what I would be accomplishing by using them.

Make any sense?

Thanks again!
Paul

There's no need to use PT strips. it doesn't make a lick of difference that you're doing a shower here. Those firring strips would still see the same kind of moisture from the concrete, and yet, they're used all the time for this purpose.

I don't know what IIRC is, but whoever told you the paintable waterproofings don't hold up over time needs to check his references. I know for a fact that both Redgard and 9235 WILL hold up, and as far as I've seen, that's indefinitely.

Thanks Bill. "IIRC" just means "if I recall correctly."

Just in case you haven't considered this, if I were to seal the interior of the concrete block wall with one of the products you mentioned, would you be concerned about having a moisture barrier on the tub side, and another moisture barrier against the outside wall? Seems to me that this would create an undesirable moisture pocket between the two moisture barriers in the stud/furring cavity.

I am not playing "gotcha" or anything, just looking for the best system to install before I start tiling these walls, I hope you understand.

It seems to me that the best system would be one that minimizes the water vapor leakage from the tub area into the framing cavity to the greatest extent, while still letting that moisture escape to the outside, or back through the tile. And which also allows any moisture that reaches the cavity from the OUTSIDE to return to the outside.

Just thinking out loud I guess, but would appreciate your thoughts,

Best,
Paul

Again, there should be no need for a moisture barrier between the wall and the firring strips. If there's that big of a concern, you need to address that first, and then decide how to do the shower afterward. That said, in your scenario, I would certainly be concerned with using a vapor barrier between the firring strips with a moisture barrier on the block, which is why I said something about forgetting the vapor barrier and using a surface waterproofing instead.

Thank you much Bill, that sets me at ease. Though I think I will go with PT for the furring strips anyway, since that "upgrade" will cost me almost nothing, and so why not.

Thanks again, I appreciate your advice,
Paul

I recently watched an episode of "Holmes on Homes" and he was framing out a basement. On the exterior walls, he said code called for PT lumber if the wood touched the block. Instead, he built it out a bit from the block and put spray foam behind it.

It is a Canadian show, and YMMV, but take it for what it's worth.