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Radiant heat questions

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Mallon3194
Mallon3194 Member Posts: 1
edited 12:45AM in Radiant Heating

I’ve done pretty much every type of Uponor radiant install over the years, from joist-track systems and Infloor-type heating systems to Quik Trak, Warmboard, PEX tied to rebar, and PEX stapled over foam.

On a current residential project, the GC is proposing putting the vapor barrier over the top of the PEX tubing before the reinforcing steel and concrete are placed.

That’s obviously ultimately the GC’s call, but I can’t say I’ve ever seen a vapor barrier installed over the radiant tubing, especially going into the pour. Typically I’ve seen the vapor barrier placed under the XPS/insulation, with the tubing and reinforcement above it.

Has anyone installed it with the vapor barrier over the PEX, and is there any Uponor guidance supporting that assembly?

I’m also interested in the recommended tubing depth/cover in the slab. I normally try to keep the PEX at least 2 inches below the finished concrete surface for protection against future drilling, anchors, saw cuts, etc., while still maintaining good heat transfer.

Curious what others are using for:

  • Vapor barrier location relative to the XPS and PEX
  • Minimum/recommended concrete cover above the tubing
  • Whether placing poly directly over the PEX creates any issues during the pour
  • Best practice for protecting the tubing while still getting efficient heat transfer

Comments

  • hot_rod
    hot_rod Member Posts: 28,106

    It seems the barrier over the tube may result in voids around the tube, no direct contact between tube and concrete?

    Steel over the pex?? That will put the tube pretty deep.

    In a 4" pour with 1/2 rebar, pex over the bar ends up about 2" below the top.

    What about just taping the seams of the XPS?

    I also see spray foam used for under slab insulation, giving you an excellent barrier and whatever R value you want to pay for. 2" is common around here.

    Concrete gets poured and pumped over concrete, maybe on a daily basis. I don't know what it needs protection from?? 6 mil plastic vapor barrier would not offer much protection?

    2" of typical concrete mix is fine. Pea gravel mix, 3/8" aggregate can be 1-1/4- 1-1/2" over the tube.

    One rule of thumb is 3X the aggregate size over the tube.

    You have to really work to keep all the tube exactly 2" below. Mostly it ends up deeper unless you use a lot of bar chairs to hold up the mesh.

    Tube 2" below allows for a 1" or so deep control joint saw cut

    Spray foam and mesh

    Screenshot 2026-08-26 at 7.39.07 PM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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