Thanks for the responses. We are now leaning towards PolarGuard GPS from Polar https://polarcentral.com/. It uses Neopor from BASF - graphite infused, which apparently helps with R-value. The 2 1/8" board is rated at R-10 and 13psi. I've been told this will behave similarly to EPS w.r.t. moisture and is more green than XPS.
As for the outside, we have waterproofing membrane, drainage board, leading to a drain pipe at the footing that runs to a sump pump to pump it to Cultecs (we couldn't drain with gravity due to a lot of rock on site).
I drew the attached picture to show some of the options. Higher res link
here:
Option 0 (not drawn) is do nothing. Just put the slab on gravel. This was the original design.
Option 1 just runs the insulation under the slab, butting up against the footing. Since we hadn't planned for insulation at the time the foundation was dug, this approach has the advantage of not losing ceiling height.
Option 2 runs the insulation over the top of the footing and up the wall, then the slab floats on the foam.
Option 3 (not drawn) would be to run the foam on top of the footing, but no insulation at the edge (so the slab would touch the foundation wall).
I'm wondering how much insulation value we'd be losing by going with option 1 vs 2, and whether that is worth keeping the additional 2" ceiling height (we're at ~8ft now, without the insulation)
SailawayRB and newbostonconst, what do you think of the assertion that putting the foam between the foundation wall and concrete (per option 2) would provide a vector for moisture to come up? This is what the contractor and architect are saying, but I'm not convinced - if there is water under the slab, it is just as likely to migrate through the concrete as through a 2" foam at the edge IMO.