I did a 50' x 60' steel shed 2 years ago. Pretty much like MScott's garage. 6 mil vapor barrier everywhere with 2" high density foam around entire perimeter walls and beams coming up above cement level which I cut off flush later with top of floor. I have 8" walls which sit on top of slab which brings the inner wall 6" into the slab. 2" HD under slab with 1/2" pex loops on 12" centers stapled to foam and wire mesh on top of that which the contractor then lifted slightly as he poured. At all door openings, two 20' overhead doors and one service door, I used a high density 1/2" rubber matt cut into strips wide enough to get below the pad thickness. This will not break away or absorb moisture and crumble over the years. Then after curing I used self leveling caulk over the rubber thermal break as a final seal. All loops should be close to the same length (I have 10 loops in my shed) and the pex closest to the exterior walls should be about 1' from the inner wall surface. Take pictures in case you have to know where your lines are in the floor in the future. Plan your manifold location and put protective sleeves around the pex coming up out of the floor as well as any sleeves needed for floor penetrations after the pour (i.e. electrical, radiant supply or return, etc.). I also have a center floor drain and two 4'x4' double thick areas (for future vehicle hoist) which I had to work around.
Using a 40 plate exchanger to heat it. My in floor system is pressurized and has glycol in it just in case.
Absolutely love it and has worked excellent, I keep my shed at 55 which for me is perfect.