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Radiant PEX in Concrete Slab

tsloss | Posted in General Questions on

There seems to be several opinions on this questions. I value the opinions on this forum more than some of the other places. In a 5″ slab for a garage floor, using 1/2″ pex spaced 12″ apart; I have 4″ under slab EPS with 4″ perimeter EPS, with 2″ of the perimeter cut at 45 degrees to reduce the “channel” width around the floor.

Is it worth the time, effort and money to use rebar with chairs to place the pex near the center of the slab? I understand there is some benefit; just whether the benefit vs. risk of damage from cutting is significant. Thanks for any advice.

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  1. Expert Member
    NICK KEENAN | | #1

    The purpose of the concrete is to have something with high thermal conductivity around the PEX to even out the temperature of the floor. I don't see the position of the PEX in the concrete making a significant difference.

  2. user-6184358 | | #2

    The rebar & chairs are worth the effort to make sure the reinforcement is in the correct depth in the slab. The wire mesh and pulling it up during the pour is wishful thinking.

  3. Expert Member
    BILL WICHERS | | #3

    It’s worth using the chairs to keep remesh near the center of the slab. If you don’t use something to keep the remesh in position, it will get pushed down during the pour and end up UNDER the slab where it won’t do any good. I’ve seen this happen many times in commercial buildings. Reinforcement needs to be positioned correctly within the concrete to function correctly.

    The PEX just needs to be completely inside the slab. The exact position isn’t as critical as it is for remesh. If you are using remesh, then I’d anchor the PEX to the remesh with zip ties to keep it within the slab. This will put your PEX near the center of the slab which is a good spot. You don’t want the PEX loops poking out the top or getting under the slab position during the pour.

    Don’t expect anything to stay in position during the pour unless it’s anchored. Concrete is heavy stuff, and as it slops out during the pour it impacts things a bit like a hammer and will force things around more than you might expect. It’s really hard to fix things WHILE you’re pouring, so it’s best to just go a bit overboard with ties ahead of time.


  4. Jon_R | | #4

    Some numbers that can be used to estimate the increase in efficiency of midpoint vs bottom. Lower temperature improves heat pump and condensing boiler efficiency.

    1. Expert Member
      NICK KEENAN | | #5

      Thanks for the link, I always love Siegenthaler.

  5. pnwbuilder | | #6

    For control joints you can use a product called zipstrip, it's a plastic strip that is installed in wet concrete and eliminates the need for cutting control joints.

  6. MAinspector | | #7

    I did basically the same thing in my slab on grade family room addition 3 years ago. I used 1 1/2" chairs, rebar grid, remesh, and then 1/2" tubing 12" o.c. with a ton of zip ties to lock everything in place. The tubing ended up about 1' - 1 1/2" down from the top of the slab which is what I wanted because I wasn't going to be using high water temperatures. Going into my third heating season (just fired up the slab 2 days ago) and it seems to work really well. The water temp going into the slab is 95* and it takes about an hour to raise the slab 1*.

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