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Control Joints with Radiant Tubing

idahobuild | Posted in General Questions on

Hey all,
I’m getting ready to send out my plans for bids on the slab.  I had been considering  1/4″, relief cuts to control where the conc. cracks. Recently, I’ve encountered some inner turmoil around the idea due to the radiant heat tubing in the floor.  I have in the plans that the tubing will be placed at the vertical mid-point of the slab – using some 2″ chairs.  That should leave about 1.5″ to the surface.

How would you folks handle this scenario?

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  1. Expert Member


    For control joints to be effective they need to 1/4 of the slab depth, otherwise the cracks just occur elsewhere.

    1. idahobuild | | #2

      Eeek, that sounds dangerously close to the tubing. I suppose we'll just stick with the 'natural' settling cracks. What are your thoughts on perhaps adding some additional rebar support at specific edges to reduce cracking in those areas? Is that even necessary?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        You would be better off adjusting your concrete mix and adding fiber.

        1. idahobuild | | #8

          Problem with fiber is that we plan on polishing and from what I've read, fiber makes the polishing less effective.

  2. Expert Member
    Akos | | #4

    This idea the pex tubing needs to be in the middle of the slab is left over from the days when the slab was not insulated. With modern insulated slabs, it doesn't matter where the pipe is.

    My $0.02 is to simplify your life and use one of the insulated PEX panels for one of your sub slab insulation layers. This is much quicker install for the pipe and also protects the pipe during pour. Since the pipe is now at the bottom of the slab, relief cuts are not an issue.

    1. idahobuild | | #6

      Akos - do you have any specific product web-links?

      1. Expert Member
        Akos | | #10

        Take a look at Amvic ampex or Isorad. There are other similar panels out there.

        1. idahobuild | | #11

          Akos - I found a local supplier for the Ampex. It looks like the cost is a bit higher than Owens, Foamular 250. Not too bad though. Do you know if using the Ampex changes the overall depth of the conc. slab (from the mushroom pex holders)? I guess what I am asking is; would the conc. needed be 4 inches above plus enough to fill the voids in the Ampex? Or are the valleys in the Ampex included in the 4" slab? Make sense?

  3. user-723121 | | #5

    There is also what is called Zip Strip for concrete, don't know if it is still available. Provides a control joint without sawing.


    1. idahobuild | | #7

      the Zip Strips look like a viable/inexpensive option. THanks ...3121

    2. Expert Member
      MALCOLM TAYLOR | | #9


      Yes, I used them on a shop last summer. Relief cuts are best reserved for slabs that are being left exposed, and where looks matter.

  4. jberks | | #12

    If you're a fastidious planner, you can preplan where the control joints are going, and don't put any tubing near there.

    Other than zip strips, my slab I'd like to get my hands on leave in place screed rails, basically a strip of aluminum set to the correct height that I could screed off of, and leave in place as a control joint.

    Otherwise, I find mircro fibers or "stealth fibers" don't show at all. Do a good placement float well to push the fibers down and get a good cream and power trowel.

    Avoiding cracks altogether is a lot of prep work. I find really tampening the gravel later important so it doesn't move. Extra layers of insulation acts as a buffer. And no air entrainment really helps.

    Slabs I've skimped on the base gravel later or use air because they were outside have cracked.

    I'm just a concrete fanboy, and don't do this everyday so take what I say with a grain of salt.

    Also, if you're going to do tubing mid-high in the cross section of the slab, consider your spacing and your water temp. I've done mid high tubing and i find it just dissipates the heat into the air and overheats the room, and doesn't heat the slab uniformly. Ya it's a super reactive heating system, but I call it a failure cause the point was for floor warming not necessarily space heating.

    I think lowering the tubing, or decreasing the spacing from 12" to 6" would have helped for uniformity, otherwise adding a mixing valve to control the water temp to slow the thermodynamic exchange might also help.

    Another thing that most don't talk about is the anxiety caused with fasteners in the the slab later in the finishing stage. I have used demonic 100 to adhere walls or panels to the slab vs using tap cons. Lowering the tubing to the bottom, and having a max 1.5" fastener depth policy can really help your sanity later.


  5. idahobuild | | #13

    Thanks Jamie,
    Sanity is important and cracking secondary - I can deal with cracks in the finished floor if need be. With that in mind I think I am going to take Akos' advice and go with the Ampex R10. That product appears to allow the tubing to be set deeper in the slab; which would make the relief cuts and, as you point out, the fastener installation much less worrisome. I'll add a note, and talk to the framing crew, regarding NOT penetrating more than 1.5" to 2" into the slab with the partition walls -- though I believe that typically the tubing doesn't run under partition walls.

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