Radiant PEX Tube In Cement Slab with 2 Manifolds - Typical PEX Tubing Layout with 4 Port Manifolds. You will notice that the PEX Starts and comes back to the same location. You always want the hotest part of the PEX on an exterior wall and use bend supports or something else to protect the PEX when it comes out of the cement. And make sure you use insulation under and around the floor. Vapor barrier is also need under the floor - and our Foil Slab Shiled insulation does this too.