yes, I concur that xps refers to extruded polystyrene (dow blue is a
common brand). architects in these parts use that abbreviation.
as for attaching it, it partly depends on what is going over it. you
do need to cover it because it will burn and release toxic gas if
exposed to flame. I'd recommend using as much foam as you can stand
and can afford. if you really want to do it right, use two layers and
stagger the joints. xps gives you about R 5 per inch. I don't think
glue is necessary.
you can get 2" metal z strips that hold the foam in place and are shot
onto the walls with powder actuated fasteners or short tapcons. I have
also used wood firring strips as wide as the foam is thick, and fit the
foam between the wood strips. i do this as i go --apply strip, then
foam, then another strip, etc so you can get a good fit witht he foam.
attach the wood strips with tapcons. you could even place the foam on
the wall and hold it in place with a 1x2 over the foam, though it would
take a very long tapcon. there is no one right way. if you are
putting up drywall, the metal z strips will work, but if you are using
wood panelling, you will want some wood back there to nail to.
i tape the seams with tyveck tape. caulk would do the same thing, but
the tape is easier and cleaner.