I need some advice with the load bearing stud walls in my basement.

I am a DIY-er working on a 1200 sq. foot addition to my 1600 sq. foot house. The ICF foundation, excavated first week of September is in, the wall are up, the roof is on and the week before Christmas shingled. The windows are in and we are watertight!!! I am currently hanging mansards off the second floor ceiling joists (to match the existing house).

The full basement is about 20 by 40 feet and is 9 feet from the top of granulars to the bottom of the TJIs. There are two load bearing stud walls in the basement, both on poured footings. One, made of 2×6 runs across the basement, the other made of 2x4s tees into the first. The bottom plates of these walls are made of pressure treated 2 bys, the studs are not pt. There is 6″ of granular on the floor, which comes to just below the top of the footings. I have not yet poured the 4″ concrete slab called for in the plans.

I am considering having radiant heating pipes installed in the concrete slab. The plan is to put down 2 ” rigid foam, have a local heating contractor install the mesh, tubing and manifolds and then have another contractor pour the slab.

My concern is over the stud walls. With the present depth of granular, the concrete slab may be higher than the bottom plates of the stud walls. My plan, as agreed to by the architect was to install a 4″ high screed wall made from 1/2″ PT plywood along both sides of the base of the walls. I would paint the bottom foot or so of the studs with wood preservative as further protection against rot. I understand this technique is used in wooden foundation construction.

If I install the insulation and have the radiant heating piping installed, the concrete slab would be about 6″ inches above the footings. Is it reasonable to install 8″ screed walls along the stud walls to keep the studs out of the slab? Is there anything else I should do to protect the studs?

I would appreciate some help.

Bob B.