I’m wondering if anyone has any good rationale for where the vapor barrier should go on a heated slab on grade intallation. It will be a 4″ concrete slab with hydonic tubing and R-10 min. of foam beneath and at the sides. Often I see the plastic go down first and then the foam, but I have also seen the plastic on top of the foam (being the warm side of the insulation).
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With the plastic on top of the foam, I don't see how it would survive the rebar/dobie/tie wire/PEX + concrete pour ordeal without at least a few tears.
Some plans I recently got show the plastic down first, then gravel, then foam, then concrete. I don't see how I could get plastic down and then truck all the gravel in on top of it without some damage.
I know I could get the plastic over the gravel and then foam over that without destroying the plastic.
I started searching through old threads here and at JLC and it seems there is a lot of debate over this but no definative answer. Not just as to the the location relative to the foam and VB but also relative to the slab. The first article swore by a sand layer between the VB and slab and the second one stated how a sand layer between the VB and slab was the worst thing one could do.
A reading of my states energy code sems to permit the VB on either side of the foam. I agree on installations where the VB is on top I have seen multiple penetrations in it made by the dobies, steel and tubing. And I imagine that VB is only as good as it is intact, but it seems pehaps the penetrations help resolve some of the issues of slab curing when the slab is in direct contact with the VB.