I am pushing my attic bedroom space out into the unfinished side of my attic. I'm not using the whole space, just pushing out another 8 feet. I am constructing 3 new walls: one full back wall, and two short knee-walls (plus I have to finish the sloped roof interior).

The unfinished attic walls/roof currently has insulation between the studs, but are otherwise exposed. They're just stapled on with paper.

I plan to insulate my new walls, which are currently open on both front and back (only the interior of the new space needs finishing).

1] A regular outside wall has a solid back (such as brick or roofing ply) against which the insulation can be hung, which is then covered in finished panel. My walls have no such solid back yet. Do I put "back" boarding in these walls, so that the insulation will be sandwiched, or can I just put up my finishing interior panelling then and attach the insulation to that *from the back side*?

2] Should I put a vapor barrier in? If so, I might not need that back boarding. I could stuff the insulation in then cover it in the vapor barrier to hold it in place.