The most important place for insolation on a radiant slab is the perimeter walls, around 80% of heat loss comes from the sides.
Wish I could get my Dad to believe that, he's got it in his head that we can replace just some of the concrete in the shop, add radiant to that and heat the whole shop without insulating the perimeter. My opinion on the whole matter is if your not going to heat the majority of the floor and insulate it properly it ain't worth doing.
I'd like a heated floor no doubt about that, but it's not really a necessity, my feet don't get cold and the majority of the work we do in the shop is either fabrication, tractors or equipment so very little laying under something on a creeper anyways. Most that may happen then is changing oil and greasing the straight trucks, grain trucks and semi which I usually do when it's warmer anyways. I do have one clutch job that needs done on a straight truck but will pay a local truck shop, I don't have a transmission jack, and I'm not going to buy one rated to handle a 9 speed eaton just for a clutch job every 5 years or so.