Bad idea in just about any place but SoCal.

How big is your shower? Do you have any room to spare that you could build out the wall by a few inches? Or possibly build a 45 degree corner for a niche?

It's a small shower so there isn't much space to build out the wall. Won't a bunch of corner shelves look funny? I have a ton of shower products!

Maybe you can cantilever your niche out the exterior wall, so you can still take advantage of full insulation. Just kidding ;)

I'm in a similar climate as you and our builder said he won't do it. I didn't really argue. We're just doing corner shelves instead. I just made sure they were in a corner you didn't see when you walked in the room.

Yeah, I wouldn't do an outside wall niche either. I'd be afraid I'd freeze the pipes in the bathroom! Maybe it wouldn't be that extreme but you'd be feeling the coldness radiating through while showering, I bet. Corner shelves might be just the thing, and you could get more shelving right? All up and down the one corner.

I agree, bad idea. Spray foam is very efficient, but that's going to be the coldest spot. At best you'll get condensation, and it'll be cold. Last thing you want in a shower area.

yeah, you are all making sense. i'm not going to do the niche on that wall and just have a ton of shelves. :/