There are two problems that might come up with what you want to do with that exterior wall. First, I'd be bigtime scared to put a sheet of plywood in an exterior wall, between masonry and styrofoam insulation, and a hot shower and vapor barrier. It's like setting the banquet table for mold. One other thing-- you might want to put a straightedge across those firring strips. There's a good chance you'll need to do some work shimming the cement board. It'd almost be EASIER to float that wall. As for the ends of the tub, you stated that you have a problem with the opening being about 1 1/2- 2" too big, but then didn't mention any nitended remedy. What you might do is sister up some 2x4's or 2x6's to the existing framing to bring the wall out. As for the drain pipe, that I have no more expertise than you do. :-)

Bill, thanks for the reply..

Attached is a photo of the exterior wall. The tub lip will be near the top edge of the lower piece of blue insulation. With the window, the additional furring around it, and the cross furring there is not that much rigid insulation. The plywood on the exterior wall was to take the place of furring strips to move the durarock out to fully overhang the tub. This would also give me more to screw the durarock to. As a byproduct, I hoped to gain a bit more insulation.

Is the concern more the plywood (as opposed to furring strips which would allow more air movement), or is it the rigid insulation trapping moisture, or the combination? Would plywood be ok if there was no rigid insulation?

From other posts, it was recommended that if using fiberglass with kraft facing, to cut the kraft paper face every foot. This would allow moisture to escape. Since the rigid insulation is not wider than a foot I thought it would allow sufficient moisture migration.

As an alternative, I could just peel 1 inch of unfaced fiberglass insulation and place it on the exterior wall instead of the rigid insulation. I did this in the paneled basement that has a similar exterior wall (and it helped).

Or if the concern is more the plywood, I could put furring strips in.

Yes the existing furring strips were not very level. I replaced 2 (both sides of window), and added another just above the tub lip (for screws). Then with the plywood, shims and screws planned to make the surface more level for the durarock.

From your post, I interpret that you may also be suggesting durarock directly on the existing furring strips (with vapor barrier). Thus no plywood or additional furring strips to move the durarock out farther and fully over the tub lip.

Floating the wall might be easier, but I have my experience with thinset and tile so hoped to stick with that.

For the 1.5 2 inches on the ends, I was going to again use plywood to furr the wall out. I planned to split the difference and make up some of the gap on each end. However, based on your suggestion I now plan to sister the 2x4 on the back wall and put most of the 1.5-2 inches there.

Attached is a photo of the floor for completeness.

I added another joist (2nd out from wall) between a steel I-beam and a wall that sits on a concrete floor below. The new joist was cross braced to the existing joist along the exterior wall. It was placed to sit under the left set of feet on the tub. The position of the tub feet are marked with white paper in the photo.

I then used joist hangers to support the joist to be cut (3rd out from wall). I placed the cross support to sit almost under the right front tub foot.

I then cross braced to support the tub left back foot. So all the tub feet will sit directly over a structural member.

The tub feet are referenced from someone sitting in the tub facing the drain.

From your post, I interpret that you may also be suggesting durarock directly on the existing furring strips (with vapor barrier). Thus no plywood or additional furring strips to move the durarock out farther and fully over the tub lip.

Not at all. If you're set on going with CBU, You can always attach pieces of the plywood you wanted to use to the firring strips, thereby firring the durock out to where you need it. I have two concerns with that, though-- first, I don't know if you have enough support for the durock, and secondly, like I said above, if those firring strips are not perfectly flush to each other, this wall's going to be a nightmare to tile.

Just an FYI-- floating a wall is not rocket science, by ANY stretch. Matter of fact, you use your back more than your brains! The toughest part of it is getting your spots, or screads to be nice and flush to each other.

Ya know, why I didn't think of this before, I don't know-- but why not, instead of using a vapor barrier, waterproof the surface? Then you COULD use the plywood, because although it would still be between the styrofoam and waterproofing, it would be a little further removed from the waterproofing, as opposed to the vapor barrier, which would be right on top of it. As for the mud, I did a job just this summer that was very similar to yours, but on a much bigger scale. This was a steam room I did in Hartford, Ct. for Travelers Insurance's corporate health club:

Very nice work on the steam room!

I think we will go with your suggestion of waterproofing. This allows for the slightly extra wall thickness and moving the Durock out slightly, the insulation (rigid foam and plywood), and more to secure the Durock too.

The Durock wall installation calls for 1.5 inch screws at 8 inch spacing for 16 inch spaced studs. The existing 1x5 furring plus a sheet of plywood, plus the durock should be sufficient depth for the screws. (sorry I have been mis-spelling Durock in previous posts)

The plywood and Durock would run from the lower furring strip up. I will add an additional horizontal strip on the left of the window to support any seems.

I have one additional question. Is thinset needed between the plywood and Durock? For floor installations, the Durock installation guide calls for thinset to fill voids and prevent cracking. But for the walls, the Durock installation guide example seemed to be directly onto the studs. Since no one should be walking on the walls, it does not seem thinset is needed between the plywood and Durock.

You're right. It's not needed on vertical surfaces.

Thanks immensely. And thanks for the pictures of your steam room job. It looks more back breaking than the 3000 lbs I pulled out of my 8x5 foot bathroom (weighed at dump). My wife just found Mongo's extensive post from Dec on his Kerdi shower. With that additional tutorial and diagrams I feel (reasonable) confident to proceed. And my wife is much more comfortable with the Kerdi.