I have a pretty unique situation that I’d appreciate some fresh eyes on. We have a 100 year old fully gutted, masonry block, 2 story house. Based on research, we’ve settled on 3″ of closed cell foam insulation against the masonry block, with studs 2″ off the concrete walls.  (1. Untreated framing can’t touch the concrete and 2. this will leave a convenient empty space for utilities 3. minimized thermal bridging)

We are required to have fireblocking along every single wall. The house is a modified  balloon frame but all the floor joists are embedded in the concrete – instead of a normal wood walls along the exterior.  We’re building the walls, so we can have insulation and, you know, outlets and stuff.

My thought is to use 2×6’s for the top and bottom plates, with 2×4 studs lined up 2″ away from the concrete wall.  Result – Flat surface for drywall; 2.5″ empty space for wires, etc. and  best of all, built in fireblocking everywhere.

Sanity check. Am I overlooking any problems with this? And, since I will have the edge of the 2×6’s touching the concrete, should I use pressure treated? Or staple a thin strip of poly sheathing along the edge of the 2×6 that’s touching the concrete?

Thanks in advance for pointing out any issues with my reasoning. Or any tips relating to this.