I have the same Questions staple pex to foam (won't I lose some heat transfer from NOT being in concrete)
No. Heat follows the path of least resistance. With the PEX on the bottom, the heat can go two ways: down through the insulation, or up through the concrete. The concrete is a much better conductor (that's way insulation is insulation) and the concrete will literally **** up and radiate most of the heat.
Some of your heat will eventually make it's way through the insulation to the ground, but it makes no material difference if the pex is in the middle of the slab or on the bottom. Heat is heat and it doesn't matter where it enters the slab. (middle or bottom)
Zip tie to wire mesh with pex on top and lose reinforcement in concrete.
Zip tying pex over mesh on the bottom works like a less expensive and higher performing (you get the full insulation thickness everywhere) version of those fancy stamped insulation panels. If you want reinforcing in the slab, you'd have to add another layer of mesh or rebar, floating above the pex.
Stapling (IMO) is not a reliable way to fix this stuff. Sections can pop up during the pour or cure and you end up getting the issues Doug talks about above with saw cuts - except worse! (You don't know where the tubes have popped up.)
BTW - You should also be sure to plan your saw cuts before your pour and read your pex/radiant instructions. You should have special sleeves where your pex crosses control joints and the slab is expected to crack, etc.
Or add wire mesh on top also and concrete guys will destroy it all anyways.
The concrete finishers will stomp whatever you put down. Having the pex on the bottom is more resilient for what will come, here.
I'm a big fan of putting reinforcing bar on top (instead of another layer of mesh). The larger squares should be much easier for the crews to work around and get on proper chairs.
