arw01 said:

Radiant heat shop floor:

Everyone in the businss has a different opinion if it's below the foam
or on the foam.

Concrete engineer says on the foam
radiant installer says below the foam.

Opinions are like assholes, so here is mine:

The barrier does not perform the same purpose as the Kraft paper on
insulation, it exists to prevent water migrating into the slab from
the ground rather than preventing humidity from the heated space from
condensing in the insulation. But you DO want a barrier! Can't
decide where? If it bothers you, how about in both locations. I can
guarantee you it will get punctured here and there during the install
of the tubing and during the pour anyway. Redundancy couldn't hurt.

Also, a layer of lawyers under your pour is good luck... for everyone.

FWIW