All showers and steam showers will fail.
That title line is just to get your attention. I'm hope it's possible to build a shower that will not fail. But a lot of building science needs to be implemented. Having watched, and read, multiple articles, white papers, and YouTube posts, it's hard to find an install that won't fail. One great example, is the 7-day water test Isaac did with a Schluter design. Even following Schluter's instructions and protocols, there was a slight leak.
I live in a 1927 Tudor in Dallas TX. This house is obviously a pier and beam, although I have closed the crawl space vents to move the envelope to the outer wall. However, without extensive weatherproofing and sealing, it will never be tight. And I can live with that.
But building steam shower bathroom in a pier and beam house in Texas is challenging. Proper design of a steam shower requires a vapor barrier as well as a moisture barrier. Typical vapor barrier will be a poly sheeting. Although there are other options. One of which I'd like to ask the community about later.
After reviewing ASHRAE's psychrometric chart, my concern is that any moist, warm air that hits the vapor barrier during winter, will cause condensation on the opposite side of the vapor barrier within the wall. That is unless the crawl space is fully conditioned.
Now comes my question about vapor barrier membranes. Has anyone tried roofing TPO or EPDM 60 mil sheeting? Some of these materials come with adhesive backing, fleece backing, smooth backing, The fact that these membranes are about 1.5 mm thick, and rubberized. They should have good insulation properties as well. When these coverings are attached to roofs, they are often glued directly to the substrate. Frequently plywood. I'd like to hear any thoughts on this?
I think that although we can rarely fully vapor proof a shower, kerdiDS, laticrete sheet membrane, and RSS membrane are all thick enough to slow down vapor penetration such that there will never be an issue. Also, residential steam showers are not that much different than a regular shower as far as vapor permeability go. Continual use steam showers (day spas,etc) are a different story.