Looking at replacing the insulation in the walls of a small office. I
pulled down the (wrecked by glued panelling) drywall, to reveal some
*very* old fiberglass insulation (which is now about 1" thick).

Question is, should I just replace with R13 batts, or can I put rigid
foam (like Tuff-R) in its place. I know the Tuff-R gives a better
R-rating, so should I fill the cavity completely? (2x4, so 3 layers of
1" Tuff-R), or just do 2 layers and leave the rest open?

Also, if I use the Tuff-R, do I need to put up a plastic vapor barrier
before putting the drywall back on? Or would the foil facing serve as
a vapor barrier?

Thanks in advance!
-Chris