One of the long term issues with foam slabs is that they shrink. If you want better long term coverage for the walls and floor, use sheets half as thick as you want and overlap the joints as well as tape them. That will give you a good moisture barrier. Dow makes a Wallmate board with channels cut for 1x4 furring strips if you want them flush, or you can put a 2/4 in there if you want them to protrude. Leave a gap at the bottom for water and drywall away to your heart's content. The Barricade floating subfloor system works well, you can also install it on top of the existing floor, or on top of other foam boards if you want more insulation. Dead simple to work with, and give you a base to put something else on top. It also has some channels to allow water flow underneath them, but it doesnt' hurt to keep easy access to a drain. If you get a small basement pipe burst type flood event, you can just pop them up and dry/replace them. http://www.ovrx.com/basement-flooring.html The above are what I did in my PNW basement, it's made a noticeable difference. Be prepared to invest in a dehumidifier no matter what.