We use 2" 60psi. foamular  8" 6 bag with fiber  6x6x6 mesh on 2" porches  We zip tie the pex on top of the mesh  Square it to the walls  Take lots of pictures incase you have to drill floor. Also important tight compacted clay base with 8" of 3" compacted limestone topped with 6" of 1" to fines (Gr.8 referred to around here) again watered and compacted. Will have lots of opinions  on here. However we did our first heated floor in 1985. No pex back then. We used plan old black poly pipe. Still running to day with the original boilers.

One thing I would strongly suggest. Get all pipe laid, even length runs not exceed 250'. Install both manifolds. Plug one and install a temporary valve with a air chuck and gauge. Put 25 psi or so charge on the system so if something gets cut or punctured you will know. Have repair parts handy. Had it happen twice.

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