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Will radiant heat system PEX tubing melt polyethylene vapor barrier?

Mightorta | Posted in GBA Pro Help on

The heating contractor we hired to install a hydronic radiant heat system for our backyard cottage project did not put a polyethylene vapor barrier on top of the gravel, under the 2″ rigid foam insulation. The rigid foam insulation panels he installed have a mylar-like film on the under side, which he thinks is sufficient. We live in VERY wet Western Washington so I’m wondering if this will in fact be enough.

I’ve read a lot of differing views on the best placement of polyethylene sheeting for vapor barriers (UNDER/OVER the rigid foam insulation panels) —- at this point it’s got to go on top or nowhere, so if answers can address only on top of insulation placement that would be great.

The PEX tubing is tied to wire mesh, not suspended above the rigid foam insulation on rebar, so the tubing will come in contact with polyethylene vapor barrier in some places. My question is:

Will the tubing heat sufficiently to melt the polyethylene vapor barrier? The melting point of polyethylene is above 200F so it doesn’t seem like a problem, but wondering if I should look for anything special when buying the polyethylene sheeting. 6 mil, 10 mil, not recycled etc.?


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  1. Expert Member
    Dana Dorsett | | #1

    Polyethylene sheeting won't melt at anywhere near the legitimate operating temperatures for PEX. (PEX is polyethylene too, after all, and would have completely melted away before the temperature of the vapor barrier gotten that warm!)

    Even if you ran the water at the maximum operating temperature for the PEX the polyethylene sheeting would still be below that temp, but it would probably be north of 130F indoors with water that hot in a radiant slab.

    Bottom line, you have huge operating temperature safety margins here.

  2. ohioandy | | #2

    Hydronic will NOT melt poly, and poly above foam is best practice. Drainage-wise, if your contractor has taken pains to insure your footers are well drained, both inside and out, so that the aggregate base under the floor will ALWAYS stay dry, then you shouldn't have a problem with only the foam as a "vapor barrier." However, now before pouring concrete is a GREAT time to call a homeowner's timeout and insist that poly be installed above the foam, because it is best practice by far and way easier than retrofitting a solution if moisture proves troublesome. That's what I would do.

    If you've got wire mesh and hydronic already in place, at the risk of stating the obvious, don't lay poly on top--the mesh needs to float up into the concrete during the pour. Roll out the poly underneath the mesh.

  3. Expert Member

    "The PEX tubing is tied to wire mesh, not suspended above the rigid foam insulation on rebar, so the tubing will come in contact with polyethylene vapor barrier in some places. "

    Unless the only reason you have the mesh is to keep the Pex in place, and are not intending to use it for tensile strength in the slab, the mesh needs to be suspended on chairs, or raised up as you pour to the same position as rebar would be.

  4. user-2988384 | | #4

    Stumbled upon this old post, so maybe nobody will see this, but I'd like to see references to support the claim of VB on top of insul being the clear choice. Typical projects for me (several feet of NFS soil beneath the slab), it seems better to have the poly beneath the foam to avoid punctures from the PEX staples, rebar tie wire, and a million boots walking on it during construction. Ultimately, I'm not sure how much it matters (at least in my climate). It's mostly serving as a capillary break and vapor barrier with very low moisture flux potential since the soil is granular and the site is always well drained. If we are relying on it as an air barrier (even though it's beneath 4-8" of concrete), avoiding the aforementioned staple holes would be a key reason to put it below the foam.
    If you want to talk about using mesh instead of rebar, that's a different story, and I'll put my structural engineer hat on and explain why we never use mesh on commercial projects (for building slabs), but whatever your reinforcing is, we've come to the conclusion that stapling to foam is the best installation method from several perspectives (mechanical, maintenance, future renovations, construction cost)

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