Douglas is correct about the cold and water vapour.
If you are going bale around the steel supports then have the supports on the inside of the wall not the outside. The theory being that the walls will insulate the posts and reduce condensation. If render is going directly onto the posts then wrap the posts in hessian / jute to "key" the render to the post.
Rendering a strawbale bathroom is probably one of the most challenging things you could do. Getting it wrong would be disasterous. Make sure you do the research first. Use waterproofing agents and damp course materials in the right places. There are fibreglass shower recesses available and one of these would help minimise water directly onto the wall or use a bath and tiles.
Good luck with it. I recently did a Strawbale course and had a great time.
294 acres (120 ha.) Native Cypress Pine
Yuleba, SW QLD. pop. 250 (80km east Roma)
Sandy Soil; 500mm Rainfall (i hope)