[Por_sha911]

I've had heavy vinyl splash proof wallpaper in the bathroom for years. No problem but ymmv.

Outside walls really? I guess if the home is un-insulated and has bad siding maybe.

I am not an expert but have read up on this and nothing says the above are a problem except where the cheap plain wallpaper is in an area where water will be splashed on it or, if you steam up the bathroom so badly that there is moisture dripping from the walls (which is why God invented bathroom exhaust fans).