I am putting pink foamboard insulation on the outside walls that are between the basement I just finished and the floor above. The wall height that is being covered with this is between 2-3 feet high. Part of the wall is under ground and part is above ground. There is R-11 insulation in the floor above and none in the ceiling below. My question is: Should the pink foamboard have a vapor barrier placed over it? If not is it considered as being "exposed" if it is between the two floors and unaccessable? I ask that because of the warnings stated on the board itself that it is combustable and must not be exposed to the buildings interior. Thanks for your help in advance.

My understanding is that foamboard insulation should be covered by sheetrock to provide a fire break and not be left exposed inside the house. You can't fire proof it... but you will delay it enough to get out of the house before it starts to burn.

Why not use standard roll(batt) insulation?

I used R13 batt kraft faced insulation in the walls below the ceiling because I had 2x4 framing to work with, but in the space between the two floors there is nothing to attach the batts to. It is cinder block. I guess it was somewhat uncommen that I had such a high space to work with. There was almost 11' ceiling height from the basement floor to the floor joists above, so I lowered the ceiling in the basement to 9'by framing it, leaving the 2' cavity between floors. Now I am just trying to figure out how to insulate the block on the outside walls in this cavity. In hindsight, I guess I should have used the foam board and insulated the entire basement before framing it in, but to late for that now, so I am trying to figure out the best solution moving forward. Thanks again for any and all advice.

You have framed down the 2 feet,how does the framing end at the walls?

If there is spacing between the framing and the walls install batts to those openings, you can install plastic to the actual walls by spraying contact adhesive to both surfaces and 'gluing' them to each other,then use standard insulation for space left over.

I don't think I am expalining the scenario very well... sorry. The two foot wall space between the new basement ceiling and the floor joists above... is not framed. It is bare block walls. That is why I thought that using adhesive to attach foam board insulation would work best here. Im just not sure if it needs a vapor barrier and/or covered up with some other building material.. i.e. sheetrock. I Wish I could draw a picture on here of what I am talking about... Im not doing very well at explaining it... thanks again.