I am building a 3000 sf ICF ranch with a geothermal/radiant heating system throughout. The first floor will have a wood subfloor with a thin slab of lightweight concrete to hold the PEX. The basement will have PEX in the slab and will be finished. My question involves the insulation needed beneath the wood subfloor. Should I use fiberglass batts between the floor trusses? Foam board? What R-value do you recommend? Thank you for your help.

A conservative rule of thumb is to provide a minimum of 10 times the finishing floor R-value as the underside insulation R-value for a thin-slab floor. Typically the minimum R-values for floors over heated living space, partially heated basement space, and exposed/vented crawlspace are R11, R19, and R30 respectively. Fiberglass batts are not a bad choice.  Really, anything that is easy to install, will retain its R-value, will stay in place, and will meet your "green" goals will get the job done.

Elvis - as a rule we put an extra wall plate down to make up the height of the thin slab. I wonder how it would work to put down two extra and put down 1.5" of eps to staple the pex to and then pour the slab. No messing with insulation under the floor and it is not in the way of wiring etc.. Just a thought. As I said, I have not done it myself YET.

I don’t know if thin-slab floor over EPS would provide sufficient rigidity to prevent cracking. I think NOT. However, can’t say that I tried it either.

Elvis - did you price out any of the icf floor systems to compare them to standard subfloor and thin slab?

Borst - I'll try to find the article I read on this but I believe with fibre it should not be a problem. Maybe adding air will help.
An alternative, if using 3.5" flanges on the tjs would be to place the eps first and then lay the sheathing. The problem with that is you might have to buy your framers ballet slippers.

Thanks for your suggestions. 1.5" of EPS over the subfloor would make a much easier installation, but I was worried about cracking as well. That's why I elected to just add 6 mil of plastic and pour right over the subfloor. Any insulation will have to go between the floor trusses under the subfloor. The good news is the trusses are 24" deep, so there is plenty of room for insulation, wiring, plumbing and ductwork to all fit there.

Please look at http://barrett-inc.com/creteheat.php as only one inch of CreteHeat would work fine and with shiplock joints, no cracking

I went with 3 inchs of closed cell sprayfoam then r-19 batts between the joists. I had the foam sprayed directly to plywood subfloor. Heard someone mention leaving 1/2 inch airspace. Not sure how to accomplish that though:-)  On a 20 degree day its still about 40 degrees in my crawlspace. Not sure how much of that heat is from my radiant system.
I used lightweiht concrete 1.5 inchs deep over my pex. If you plan on keeping that as your finished floor. I recommend having exspansion joints every four feet or so. to try to control the cracking. Any contractor tells you it wont crack have them put it in writing.
I would consider going 1 size over after you calculate your joist size. 1.5 inch of "lightweight" concrete adds alot of weight. any movement will induce cracks over time. Be onsite when your concrete guys pour your floor. Make sure they have a good method for screeting it flat. My contractors came well recommended and hand finished the floor well. The floor is not flat as it should be in places.

If you are installing a geo heatpump system be sure to Isolate the unit and plumbing to your ground loops from the framing of your house. I mounted mine in the garage slab and even placed it on spring loaded feet to minimize noise. It runs quite while standing next to it, the conduction noise in the room next to it is bad.

Be sure to install a backup/auxillary way to heat your boiler if your power goes out or extended cold snap. Lose your power for a week its good to have a propane/nat gas on demand system that can take over. Or invest in a large generator that can handle your geo sytem.

The heat is wonderful. Warm on the feet, no temp swings. No dust flying around.

Good luck with your install.