Bottom plate directly on concrete. Alternative?

[Borrall Wonnell]

For basements, the usual for walls seems to be a pressure-treated bottom plate attached directly to the concrete floor.

I'm using a (waterproof) subfloor material that channels water to the floor drain. The perimeter concrete walls will be sheathed in XPS styrofoam (any seepage through the concrete wall should drain to the floor rather than through the XPS). The manufacturer of the subfloor (OVRX Barricade) indicates that walls can be built on top of the floor, but it is a floating floor so the idea of building finished walls on top of it seems counter-intuitive...so I'm going with the assumption that the walls shouldn't be built on top of the floor.

Is there any reason not to place the bottom plate for exterior walls on top of 0.5" XPS styrofoam rather than directly on the concrete? The idea is to cut the foam into 3.5" x 4' lengths (separated by small gaps to allow moisture flow). Without the gaps, moisture may collect behind the wall with nowhere else to go. With the gaps, moisture could flow under the subfloor and to its final destination (the drain).

Makes sense to me...but I haven't seen anyone else doing it. Thoughts?

[Colbyt]

I fear the weight of the wall will compress and distort your styro. Also I am not sure what the life of that product it. You might combine it with my suggestion below.

What I did in a old house with a rock foundation that sometimes had minor water on the floor was to use treated 1x4 scabs with gaps along the sole plate which was also treated. I kept the drywall 3/4" off the floor. I kept the base 1/4" off the floor and did not use shoe molding. This allowed water to follow its natural course.

[clare]

Your "floating" floor is stronger than the XPS - building "partitions" on top of the floating subfloor is standard practice. Better than using PT wood on concrete.

[Borrall Wonnell]

My subfloor *IS* XPS (5/8" OSB laminated to 1/2" XPS)! They're built in 2x2 panels, and the manufacturer recommends 1/2" gap to surrounding walls.

Concerning building partition walls on top of the floating subfloor:

1) Won't the walls move as the subfloor expands/contracts (why the 1/2" gap required around the perimeter?). Seems like it would break lots of plaster joints.

2) Alternately, if I secure the partition walls through the subfloor to the concrete below (creating a 'fixed' wall), wouldn't I be risking that the floor expand and then buckle? I've seen this happen with laminate (finished) floors that were installed too close to a wall.

3) Admittedly, there would be a lot less work installing the floor first. Might be a lot more work if I ever need to remove portions of the subfloor.