Hi, I am not an expert at all, but I am having a bathroom renovated at the moment and winters are cold here. All of the plumbing is against an exterior wall, and the bathroom is on the top floor as well. My GC built a shower column on the exterior wall, and all the shower pipes come up into it from beneath the floor. There is also a rain shower, with the piping up through the shower column and then down from the ceiling. To "weatherproof" the rain shower, an additional insulated box was built into the ceiling/roof, not visible from the bathroom. Unless you have a way of putting your rainshower piping away from an exterior wall or ceiling, it's not clear that it is an alternative.

Thanks for your input Northsong. You've given me hope!

Actually I recently had a post asking about the same question. The conclusion was that I can have a shower on an outside wall. My walls are going to be 2 x 6 construction so that will allow plenty of room for insulating the pipes. If I was advised correctly, then it's not necessary to build a box or column or anything special to house the pipes.

Although I raised the question about why it is OK to put toilets, laundry connections and frequently sink plumbing on an outside wall, I never got an answer.