"Aaron
Do not use liquid nails for anything styrofoam! In fact, do not use any construction adhesive not specifically marked 'foam board adhesive' as it will melt the foam board , leaving you with the board loose and big gooey pits where the glue was. That includes Gorilla Glue, Pathrat..great stuff, but the wrong application.
As far as what to use for the actual insulation, if it is rigid and doesn't bend, then any of the foam insulating products out there will work fine. I would shy away from the foil backed stuff, though as you will have problems with getting anything to stick to the foil."
Good, now I know. Wasn't aware that there was a special adhesive.