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Where to place the vapor barrier in a concrete slab on grade with radiant floor and integral colored concrete?

user-1109499 | Posted in GBA Pro Help on

We are building a 2,200-sq.- ft. single-story concrete slab-on-grade home with hydronic floors throughout and will be integrally coloring the concrete. Stem walls will be poured and then interior slab will be poured. We are installing hydronic PEX directly to the XPS foam, which will be taped, and then placing rebar on chairs above the PEX. The slab will be 5 inches thick with fibermesh such as Stealth 150.

Reading through previous questions and answers and referring to the many articles on where to place the vapor barrier, it appears the expert opinions are to place the vapor barrier, such as Moistop from Fortifiber or Stegowrap, directly under the concrete. However, where should it be placed when the hydronic PEX is being stapled to the foam?


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  1. davidmeiland | | #1

    My opinion, you should put the PEX on top of the rebar, which of course is already on top of the VB.

  2. CramerSilkworth | | #2

    I can't speak from personal experience, but based on what I've been reading (John Siegenthaler's Modern Hydronic Heating - fantastic resource for all things hydronic), on the bottom of a 5" slab sounds too deep. Somewhere in the 2-2.5" from the top range is where you want to be to achieve reasonable heat output and response time. Has your contractor done 5" depth successfully before?

  3. DWBuilder | | #3

    I agree with David.

    If you are pouring stem walls, does the slab need steel reinforcement?

    I use gravel (no fines), then XPS, then the vapor barrier, then I put down woven wire mesh and zip tie the pex to it. It does a good job of holding the pex in place and is a quick and easy grid system for spacing the loops. The WWM isn't required for reinforcement purposes, but is an added benefit.

    I would also prefer the PEX closer to the top of the slab than the bottom, especially in a 5 inch thick slab. John Siegenthaler has shown, using Finite Element Analysis (FEA), that the deeper the radiant tube is placed in the slab the lower the upward heat output for a given water temperature. So then a higher water temperature is required, resulting in lower boiler efficiencies. His data shows heat output with the PEX at 2" deep is about 35% more than at 4".

    EDIT - Didn't see Cramer's post until after I posted.

  4. davidmeiland | | #4

    For a "show" slab (it's the finished floor), I would go all-out on the prep, as cracks later are really going to hurt. Put #3 rebar at 12" O.C. or better, on 1" dobie blocks. then tie 1/2" PEX to the top of the mat. Use plenty of ties so that the tubing can't float during the pour, especially at the looped ends. If you do it right, the top of the tubing is at 2-3/8" above the bottom of the slab, and more than 1-1/2" deep in a 4" slab, enough so that you can make 1" deep sawcuts without worrying. I prefer to use Combi-form instead of sawcutting.

  5. davidmeiland | | #5

    Forgot to mention... low-water mix with plasticizer, fiber mesh...

  6. jklingel | | #6

    I've always wondered about the depth-of-PEX concern given these conditions: Is it not a moot point if you have a low-temp slab, very well insulated house w/ lots of foam under the slab, and you don't touch the thermostats? I don't think the data in J Siegenthaler's book was generated w/ a ton of foam under the slab, so that needs to be fit into the decision (if applicable here).

  7. GBA Editor
    Martin Holladay | | #7

    A. Lalande,
    I'm putting in another vote for placing the vapor barrier on top of the rigid insulation, so that the vapor barrier is directly under the concrete.

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