The plan I am using has the bathroom in the back corner of the cabin. It is a basic 5 x 8 setup with the vanity, toilet, then a 3 x 5 shower. The normal way would be to have the shower head and fixtures on the same wall as the toilet and vanity. It seems like it could be a problem to have them on the outside wall but I'm not sure what else to do. Ideas or thoughts?
Make the wall thicker at that point so you can have more insulation between the exterior and the plumbing. Otherwise you do risk freezing. Not sure what else to suggest.
There is one possible future problem if it is a shower or tub valve in the exterior wall. It is quite common to replace shower valve mixer units by cutting a hole in the other side of the wall. I've done a couple like that as it beats ripping out tile and re-doing it. If the wall is an exterior wall it is a much more complicated matter.
Those were the 2 problems I was thinking of. Not sure if switching the shower fixtures to the other wall will look right.
Is running the water through the ceiling and down an option?
If the outer wall is not sided yet you could add a few inches of foam on the exterior.
If the design allows, no windows or other openings to interfere, you could add a bump-out to contain all plumbing...including drains and vents. In the end, it could look much like a chimney bump-out, complete with cap, etc. It could be built with removable panels/walls for maintenance actions. The insulation could be made from shaped foam, or loose fill in bags, ... all to be super effective and yet removable if needed.
Not sided yet so something could be planned. Maybe a removable panel or something on the outside wall but that's just another place for water to get in. Maybe I'll just let my kids cut the siding if they ever need in there (when I'm long gone.)
I would rather make it easier to cut if necessary than have a panel already removable. But keep good records on that. Chances are good that it will be a very long time before access is needed.
If overlap construction is used, panels would be water tight and hardly noticeable .. if at all.
Sorry, what is overlap construction?
like lap siding??? Where an outside something overlaps an inside something and directs water to the outside.