Basement insulation -- XPS on just exterior walls?
Greetings all! Well the time has come for us to finish our basement "main room" and I have a quick question about the insulation of the walls.
First the layout: The room is essentially a square with 2 foundation walls (which obviously are against the earth/underground) and 2 interior walls (both made of concrete block -- separating us from other unfinished but heated basement rooms).
So, my insulation approach for the exterior walls is going to be to attach 1.5" Extruded Polystyrene Foam panels to the walls with the proper adhesive and then frame out the walls and if the 7.5R value of the XPS doesn't make the temp come up a little [it's about 65 degrees in that room in its current unfinished state -- and it's 9 degrees outside] do some fiberglass batting in the stud cavaties.
SO, my question is this!: This XPS isn't the cheapest stuff in the world and I am wondering if that is needed on the INterior walls? (the walls that separate this room we are finishing from the other basement rooms)? The walls of this room are like 27' long so that could save me a lot of money. And it will also save me the 1.5" of wall space tha I wouldn't have to come in without the XPS board. But of course I'm willing to buy the stuff if there's a compelling reason.
Thanks in advance!