Seems radiant is really hot up North and a lot of guys are using 2" of foam.  I am in Kansas and per some data and research I have found, it seems R5-7 is what is recommended here.  I am looking at 1" foam.  Building is 12,000sf so costs will be high for the foam.  I do NOT want to over insulate.  The R value of the foam would be 4.5.  I am curious to get some opinions here.  If a little extra insulation would then take 30 yrs to payback, I doubt I would want the expense.  Probably much smarter spent on doors/windows. 

Probably much smarter spent on doors/windows.

If you calculate your expected heat losses with the different insulating methods, you will be able to tell if that is true or not.

My first guess would be that with a radiant slab you do not want to UNDERinsulate from recommended. That could be reinventing the wheel, and is potential for a LOT of unnecessary heat usage.

Kansas is zone 4/5 on this map:

http://www.architecture.uwaterloo.ca/faculty_projects/terri/carbon-aia/images/climate_zones.jpg

...which gives you a basis for reading the right-column of table 2, p 10 in this document:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

There's a long-term/lifecycle economic rationale for 1.5" XPS (R7.5) rather than the 1" of high-density R4.5 EPS you're pondering even without a radiant slab, WITH a radiant slab, adding R5 to that table isn't an unreasonable long-term bet.

For shorter term horizons, ih a radiant slab going with 2" of Type-I (low density, ~ R7.2) or type Type-II EPS (~R7.8) would still make financial sense unless you think energy prices are going to crash over the next decade.. EPS genrally costs about 10cents/R/square-foot so for 12000' x R8 you're looking at something getting on to $10KUSD.

If cost is critical, use reclaimed roofing EPS/XPS and go for 2.5-3" (R10-12-ish). If you can't find a local source (try asking commercial roofing contractors), you can probably get a truckload shipped from Insulationdepot.com Reclaimed EPS is usually 3-4cents/R/square-foot, so you'd be at nearly twice the R value for ~$5KUSD. For all but the most extreme-duty use the compressive strenght of Type-I EPS is adequate for under slab, but you may have to argue it with the inspectors. Under no circumstances should you use polyiso under a slab, any density EPS or XPS works for most under slab apps.

I am still working on the compressive data to decide target foam strengths. We will be in the building for the next 50+ yrs. It needs to last and hold up. I will say that we will have some serious weight on the concrete and will also need good stiffness from the floor. CNC machines, trac loader, fork lifts, etc. Probably one of the biggest demands will be moving equipments. 25K lb fork lift carrying a 20k lb machine. That focusing all that weight on the front tires of the fork lift. The 6" floor will hold that no problem as long as the foam agrees. still trying to work deflection data.

The weight of the slab itself it negligible relative to the compressive strength of the material- it's the static & dynamic loading of what's atop the slab, and the stiffness of the slab distributing the weight across more area, and keeping the deflection within the elastic limits of the EPS. A 6" slab is pretty stiff- when you do the math you'll probably find that it works even with Type-I EPS. The high-density stuff is usually only necessary for very high-vibe near expansion joints, such as on roadways.

For reference, a simplified model of the problem can be found here: http://www.insulfoam.com/images/stories/docs/6065_Below_SlabInstalGde.pdf

Note that the thickness of the foam, not just that of the slab affects the density requirements.

Defenetly go with the high density foam board. As far as costs EPS high denity will be the most cost effecient and is not that far off regarding R`s.
If you dont go high dens. there is a probability that a heavy machine in the same spot for a prolonged period of time, will leave commpression marks. Seen it happen!!