The building code for 1 & 2 family dwellings is mostly prescriptive which means it actually sets minimum and maximum values and sizes structural members for a given location in the house. This is not intended to be used as a design guide but to place limits on what you might design yourself. Using the code as a design guide can result in deficiencies but that is not the fault of the code writers. The joist size tables do not consider long span bounce. The wall systems described are the ones most commonly used but the code does not intend to forbid the use of others. The waterproofing requirements are not extensive or adequate although the addition of the requirement to install materials according to the manufacturer's instructions was intended to fill that gap. Concrete foundations and slabs should be designed to ACI 332 since the IRC does not consider many common detailing problems. Placing pipes in an outside wall is a design problem that may be addressed by some codes but there is no reason to prohibit that condition. If there were to be a restriction it would have to be based on a map or list of locations similar to the requirements for wall vapor retarders. You can base your decision about the location of pipes on anecdotal information and other people's opinions or you can design the wall to avoid freezing of the pipe. If you choose the latter, you can use the formula normally used to locate the dew point in a wall in order to determine where the temperature would be 32 degrees or lower. Ignoring air infiltration for a moment, a water supply pipe 1 1/2" deep in a 2x4 stud wall and insulated with 3 1/2" of fiberglass insulation would probably freeze at about -10 degrees F. Ironically, removing 1 1/2" of the insulation on the warm side would lower the outside temperature needed to freeze the pipe to about -50 degrees but for fiberglass insulation air-infiltrations makes that prediction unreliable. So the design solution is to air seal the stud cavity and place R-10 or greater insulation on the cold side of the pipe. This is best done with spray foam (Icynene, etc.) and if the studs are 2x6's, the R value would be as much as R-14 which would put the temperature of the pipe at about 55 degrees when the outside air temperature is -30 degrees F. At that outside temperature the house temperature would need to drop below 45 degrees for the pipe to freeze. This design is roughly equivalent to a double 2x4 stud wall with no insulation in the interior wall and 3 1/2" foam or dense-pack cellulose insulation in the outer wall. The way it was done in my house back in 1903, was to put a "double wall" of wood lath and rough plaster at the middle of the full 2x4 stud cavity throughout the house and at the shower pipe the space behind the pipe was filled with vermiculite plaster. This pipe has never frozen but because of the risk of air infiltration I don't count on protection below -20 degrees. Remember that these calculations assume that the interior temperature is 68 degrees or higher, so if you set the thermostat at 55 degrees or the bathroom is under heated or you close the shower door you should adjust the calculation.

Mongoct/Live Wire Oak, Thanks for the replies. Here's a photo I took right after the hot mop was done. You see it terminates at the outer half wall surface with a 'lip' maybe 1" tall (I didn't measure it so just guessing). So that centered tile in the entry sits above that and during a normal shower the water line (water that splashes over the drain to the door) doesn't go past about halfway the thickness of those half walls or doesn't go past the glass door that's centered between those half walls. So if I understand what you're saying about the capillary break, the tiler should've physically separated the thinset/mud under the entry area tile by creating a physical divider barrier to separate it from the thinset/mud under the rest of the tiles on the bathroom floor? I'm picturing a thin vertical layer of sealant (or membrane) from the bottom of the tile surface through thinset all the way through the mud down to meet the hot mop 'lip'? That would then form a barrier and force the water that gets under the tile to stay in the pan and go back towards the drain. I'm still trying to get a feel for how much water could be sitting under my bathroom floor (worst case) or if it's limited to a few areas of damp thinset due to wicking that are showing up as dark grout spots (best case?). Relative to the amount of water that seems to be making it down the drain, how much water are we talking about that could be wicking out through the thinset/mud/grout? 10%? 20%? 50%? How long does it take for the water that eventually sits on the concrete slab to evaporate or absorb into the slab itself? And what damage would this cause long term assuming the water never makes it to any wood?

Like someone up thread we have beadboard wainscot outside our primary bathroom. It is small, and the vent fan is not ideal with a few kinks along the run. We get a little mildew on our ceiling every couple of years, our tiles get a little mildewy if I don't keep up on the tilex. With that said, our beadboard that we painted with a gloss exterior paint looks just fine! No mildew, no rot. We even had an area where it swelled a little bit before we had the shower glass door up, and that has stayed fine since it stopped getting pelted with water. We didn't use any fancy product, it was the ready to assemble strips from menards. they were some kind of engineered material, but not anything special water resistant composite.

I don't know the risks, but my friend has a vent in her steam shower, and I'm planning the same. I want to be able to shut the shower door when I'm done, so I'm not warming up the bathroom when trying to get ready. If you aren't worried about the heat escaping into the bathroom for comfort, I would think it's better to put the vent outside of the shower.