Foam board must be protected with a fireproofing material, in residential use, typically 1/2" wallboard.

Thermax--polyisocyanurate foam covered with aluminum--is an alternative widely used in industrial settings.

Another alternative is the use of anintumescent coating rolled or sprayed onto the boards.

that slotted xps worked great in areas for me. Its slotted on the ends to fit a 1 x 3 which you may screw into the walls. The 1x3 then can hold the drywall. I did this in areas of my basement where I knew electricity would not be needed...bottom of steps, closets. I hope there is nothing in code about doing this. I was concerned about R value too but just having the xps up had made a huge difference....basement temps used to hit 56 in the winter....I do not think I have seen them below 60 since and usually its 62+. I did stud in front of it some areas and prepurchased fg insulation to put in (have not got to that yet!)...sort of wonder if that was even necesary. Eventually I'll open a couple of heat vents from our new heat pump down there too and maybe I will not need aux heat.