[AC_Hacker]

If I may join in...

PEX is High Density PolyEthylene (which is heat weldable) that has been subjected to a process that causes some of the molecule chains to cross-link. The 'PE' is Polyethylene and the 'X' indicates cross-link. Cross-Linking makes is no longer weldable (I tried and failed). It also makes it much stronger. It was designed in Europe specifically for hydronic floors. It is cheap and it is the industry standard. Why risk any other kind of pipe when PEX is so strong and affordable??

Everything BBP said about Oxygen barrier is correct.

With the addition that if you design your system carefully to use no iron (bronze or stainless pumps for instance), you don't need to worry about oxygen barrier.

I don't agree with BBP regarding the hierarchy of quality of PEX, as he does. I think each type has its own advantages.

PEX-AL-PEX is definitely the best O2 barrier. It does have a larger bending radius however, and is more likely to kink if the bend is very tight. If you are after high-performance floors, you will want close spacing which usually require smaller radius bends. Beware PEX-AL-PEX.

PEX-B is good as an O2 barrier. It has a coating on the outside which is designed to prevent O2 from entering the system. The outside coating is a bit 'sticky'. In concrete, this is not a problem. But if you are going under floor or above floor, or in-wall, the sticky surface can 'grab' and 'let go' when hydronic temperatures change. Which will be noisy (ask me how I know this). PEX-A has a normal bending radius.

PEX-A is not coated on the outside, and it is rather slippery by comparison to PEX-B. This might make it cheaper, but that doesn't make it lower quality. If your system has no iron components, it works just fine. PEX-B has the smallest bending radius.

-AC

NOTE: edit per BBC