Sean - If the structural requirements of the pan deck don't require concrete in the flutes, the easiest why to do the s.slab would be to lay eps on top of the steel and then staple the pex to the foam. If the engineering requires the concrete to be captured by the flutes, then you would have to glue the eps to the bottom. In some areas you can get eps harp cut to match the flutes.

I would still put foam under the rat slab as well.

Yes, you should be able to get EPS block to precisely fit your situation. We place clips in the concrete to allow the EPS to be strapped in place with a tensioner (i.e., the same way you would strap equipment to a shipping pallet).

Interesting... I imagine that is probably cheaper than ICF flooring designs... where would you source something like that?

Sailawayb, I'm not visualizing how you place the clips in the concrete. Is that sticking through the foam and pan deck? If the EPS nests into the pan deck why would you need straping?

EPS is manufactured all over for ICF and other products. Your builder should have a handle on a good local source so as to minimize shipping cost.

For the fasteners, we use a product manufactured by Simpson. It is "T" with a slot near the bottom of the "T" that the strap can slide thru. A slot is cut into the pan to allow placement of these "T" fasteners prior to the pour. The EPS can be secured and strapped either before of after the pour depending on the access that the crawl "corridor" design provides. Often times we will hot knife the EPS to create the required corridor for any plumbing (e.g., waste drain, etc.) and place the plumbing and EPS before the pour. This works well for a crawl "corridor" because the EPS can be easily removed and replaced if access to lower floor or plumbing is ever required, which is normally the reason we would design/build a crawl "corridor" in the first place.

We originally adopted this approach because we fabricate and ship our water work products (e.g., hydroelectric cross flow turbines, hydraulic ram pumps, water wheels, etc) using this approach and we already have the straps and tensioners. BTW, here is what a hydraulic ram pump looks like in case you were wondering:

Hydraulic Ram Pumps in Pakistan

It is a device for pumping water without using electricity. You can see videos of operating hydraulic ram pumps (including one at our future OR headquarters) here:

Operating Hydraulic Ram Pump Videos

For a full floor, you will certainly want to look at and consider Insul-Deck.