Nice floor. How many loops did you use, and how long per? I had three at 160+ ft per and the return was quite cool. Since I was using a conventional water heater it was pretty hard on the system. A boiler would have handled it easily.
I will be building a 30x60 or close in a year and will be heating the floor. It is such a great way to go.
I will consider your thermal barrier too. I heated the whole pad, glued 2" foam around the perimeter, but I could tell I was wasting heat. It was nise that the garage door never froze to the pad, but that extra bit of concrete outside was probably costing me a lot in wasted heat.