Styrofoam Insulation Question

No because you would be installing a vaper barrier on the wrong side of the wall.
The vaper barrier needs to be on the inside.

Yes, it's the best way, short version: http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

Long version; http://www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing

Depends on the sheathing (OSB or PLY) and the siding type. Most important= where are you located?

Gary
PS. Welcome to the forums!

Pleas go back and add your location to your profile for ansewers not questions.
Just go to the top left and tap on your name where it says welcome to make the change.

in a cold location its perfectly fine depending on the type of foam your using as open cell foam has to have house wrap installed over it. .closed cell foam can just be put on hte wall by itself but be sure to use shiplap foam and tape the vertical seams horizontal seams are left open so it can still breathe

As long as the poster knows Blue Styrofoam (EXPS) by Dow Chemical from bead board (EPS) The White Cheap Stuff.

Styrofoam is Blue, and is a Registered Trademark of Dow Chemical Co. It is a closed cell extruded polystyrene insulation. It is so resistant to water that it is used in inverted configuration in low slope roof systems where it is placed on top of the roof membrane. (Protected Membrane Roof System).

And Yes, we need to know where you are to know where the Vapor Barrier/ Retarder should go, but Generally Speaking you want the dew point to fall in about the middle of the insulation package.

Thanks for your reply regarding the styrofoam insulation over the OBS, I am having a new home built in Nova Scotia (framing in-process) and was wondering if it may prevent the vapor from excaping.