when to put down the moisture barrier?

Radiant heat shop floor:
Top soil has been removed very hard base ground (use a breaking bar to be able to dig in it) 3-7" of 3/4" crush gravel
moisture barrier HERE?
2" polystyryene extruded foam insulation
moisture barrier HERE?
concrete reinforcement mesh 1/2" pex heat tubing 4" concrete with plasticizers and fiber reinforcement
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.
Alan
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you want it on top of the foam. Just like your walls, the moisture barrier goes on the warm side of the insulation

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It is always easier to understand and answer questions when they are posted in real english. Cryptic attempts to express bits and pieces lend themselves to misunderstanding and assumption. It's not that hard to form complete sentences that communicate.
Given a conflict between a concrete engineer and a radiant installer, I'd go with the one who knows the product - the radiant installer.
--

-Mike-
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arw01 wrote:

I'm not sure what the correct answer is. Under the insulation I suppose there is a risk of getting holes in the barrier. Above the insulation and you now allow moisture up around the insulation closer to the floor. I do know this though.... I have radiant floor heat in my basement and you are going to LOVE it!!!! My wife and I were in Atlanta when the floor was done so I'm not sure where the plastic is or even if there is any at all. (monkey boy contractor, as my wife likes to call him) We've never had any moisture problems.
Bryan
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DamnYankee wrote:

the heat tubing to the foam, and you'd have to punch the poly full of holes. more importantly, you would tear the hell out of it during the pour.
the moisture barrier under a slab does not function the same as a moisture barrier on a wall. the under slab barrier's role is to block moisture movement by capillary action from the ground into the concrete. a vapor barrier on a wall is blocking moisture from moving from inside the heated space into the insulated wall cavity.
i believe the IRC was recently changed to allow NO vapor barrier if foam is used under the slab (since the foam blocks capillary action anyway).
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Well now, all these posts really cleared things up, eh Allan?!!
Gotta go with the guy doing the heating install, his product.
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wrote:

Go with the method that conforms to the warranty on the system.
Mike O.
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wrote:

I'm not a concrete guy, but I've always seen it below the foam when others are doing it- when it's used at all, that is. They don't do it everywhere. Makes more sense to me to have it below the foam- it's bound to help the foam hold up longer when it's isolated from the dirt.
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