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Depth for Radiant Heat Pex Tubing in Slab on Grade

alanferg | Posted in Energy Efficiency and Durability on

What is the typical or recommended depth of radiant heat pex tubing in a monolithic slab on grade?

Everything is ready for concrete to be poured, but the mason says the radiant tubing is too high. Tubes are ziptied to wire mesh. The top of tubes currently range 1-1/4″ to 2″ from the expected slab surface. The slab will be 5″.

I’ve read that radiant tubing should be placed nearer to the surface and 1″ to 2″ is recommended. But the mason is concerned.

Thank you in advance from a nearly panicking owner-builder.

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  1. Jon_R | | #1

    I've seen recommendations for 2" cover from PCA and down to as little as 3/4" in code. Apparently optimal for heat transfer might be about 1" without much difference if you keep your 1-1/4" to 2".

  2. Expert Member
    Akos | | #2

    About the only problem with tubes close to the surface is the concrete will crack. Provided you have insulation under the slab (which you should) there is very little effect on comfort and efficinecy.

    If your finished floor is not polished concrete, I wouldn't worry about it.

  3. BirchwoodBill | | #3

    John siegenthaler in his Coffee with Caleffi you tube channel recommends 2 inches. He has pictures of working with the concrete crew having them lift the pex tubing. I prefer warmboard r which installs over the slab. It react much quicker to temperature variances. I like running the system between 85 to 105. Typically with outdoor reset the system runs at 93f. Lower temperature water means higher efficiency.

  4. alanferg | | #4

    Jon, thanks for sharing the link.

    Akos, finished floor will be tile. Yes, insulation is installed and 10 mil vapor barrier. I also installed rebar 2' OC and 6x6 wire mesh. Concrete mix will include fiber.

    Intention is to place tubes nearer the surface to run lower water temps. Tho I can not find practical info/field experience of tubes installed less than 2" below surface.

  5. jberks | | #5

    I think the fiber is key to minimize cracking with higher tube placement. I did my last slab with tubes at 1.75" oc from the surface and it's fine.

    Things to note though, make sure you have good zip tie hygiene, you don't want any of them poking through the surface (or a folded down tail end of a zip tie being mistaken for a PEX tube breaching the surface, yes that happened). So when installing, practice clipping the tail ends as you go vs leaving it as a last step. Keep track of your height with a laser and a storypole as your zip tieing as well. The height of the rebate should be all the same but that's your last chance to notice and correct any inconsistencies.

    Just my bit of personal experience


  6. alanferg | | #6

    Jamie, thanks for sharing your experience. Yes, I clipped zip tie tails while installing pex. We are ready to pour concrete, just waiting on weather; but the mason decided last minute he's uncomfortable with tube depth.

  7. Jon_R | | #7

    6.3.10 here says min 3/4" or 1.5" cover, depending on the meaning of "contact with ground".

  8. alanferg | | #8

    Jon, thank you for the code reference. This is helpful.

    Here is recommendation from a contractor stating minimum 1" concrete cover:

    And this states radiant tubing should be no more than 2" below FINISHED floor surface (I assume that means 2" below the tile surface):

    I've found several references to John Siegenthaler's study recommending tube placement at 25% below surface. In the case of my 5" slab, the current placement of tubes is optimal.

    I've not found information about structural/cracking issues with tubes placed at this depth, unless someone can enlighten me.


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