Doug, you can also use mineral wool, roxul has a product that will work under slab.
http://www.roxul.com/files/RX-NA_EN/...INSULATION.pdf since it is also used for exterior insulation commercially it may be easier to find.
Steve, here is a house built in Asheville, NC similar to how I think you are planning to build.
A PhD and an Architect Build a Net-Zero Home | GreenBuildingAdvisor.com In the comments the owners said if they were to do it again they would likely skip the radiant floor.
Like I said earlier as long as there is conditioned air on each side of the floor it will be the same temperature as the air temp. It's not OMG luxurious warm, but it won't be OMG freezing either.