Hoping for advice on how best to insulate our home foundation in California zone 4b, (or now called climate zone 2). This is a barndominium type owner build on a budget, but also hopefully our forever home.

Our general foundation Insulation strategy:
Hoping to avoid the need for AC units in with our hot 95-110 deg summer months through a combination of slab/thermal mass heat sink as well as utilizing a whole house fan and the local microclimate that sees a consistent 30 degree summer temp drop at night. Our general insulation plan (Not true R values) is for R 27.6 – 29.6 walls and R 61.2 – 64.4 roof and a very tight ZIP envelope.

Foundation type is perimeter footings and piers with 36″ stem wall (to raise home out of AO flood zone). Then slab cap over 32″ compacted pit run gravel infill (sand gravel mix) from our on site borrow pit. Exterior will have compacted soil fill with some pit run gravel placed immediately against wall for drainage.

Insulation plan is to put 3″x 2′ of  rock wool comfortboard 80 (R12.6) vertically around exterior stem wall and leave under slab uninsulated. Or should we consider even more insulation here? Chose rock wool due to high potential for termites in CA, though not sure if a site specific issue.

But wondering, will the large amount of pit run gravel below the slab behave differently than if it were soil? If so, how might it differ? Good or bad? 

Or should we actually be more concerned that the uninsulated slab will ultimately punish us in the winter? Plan is to have stained concrete floor or possibly LVP. Our primary heat source, an ultra high efficient wood burning stove coupled with interior wall circulation fans to take advantage of our endless firewood supply. Back up option, to add ductless heat pump/AC units if needed.

Any thoughts? Does the exterior insulation plan sound good? Or do you really recommend we shell out for the added cost of foam board insulation under the slab? Thanks!