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Heating a 7\" slab

Paul Cooke
Paul Cooke Member Posts: 181
Got a call last night from a neighbor who recently purchased a solid old house built in the 40's. The house was built by a local general contractor who was responsible for the construction of the main post office and town public library many years ago. Like the house, these two old brick buildings have withstood the test of time.

The new owner would like to make the house more comfortable. There is a 7" slab (with half-inch rebar 12 O.C.) suspended over a full basement. He told me it is about 1000' square feet. Before contacting me, he was ready to have the local concrete cutting company cut 5/8" grooves in the slab. His plan was to place PEX in the grooves and then mud them over. I suggested he hold off and consider other less dusty alternatives.

I won't see the place till next week. One thought I had would be to attach plates and tubing to the underside of the slab and then insulate. Any thoughts about the feasability of this approach?


  • heatboy
    heatboy Member Posts: 1,468
    Go with the grooves.

    How would attach the plates to the underside of the slab? Sounds very time consuming at best. Also, consider what the R-value is of 7" of concrete. It's going to take quite awhile, I would imagine, for the heat to get to the surface. Big downward loss.

    Let them groove the concrete. I would opt for Wirsbo MultiCor in this instance. It will stay once it's put down and will be easier to attach to the slab.


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  • Dave H_2
    Dave H_2 Member Posts: 478

    of concrete is about .08 per inch,( SORRY, LOOKED AT THE WRONG SPOT ON CHART, NOT .2) therefore a value of (0.56) total.

    As for going below, is there anything under the concrete, like a plywood or something?

    Also, what will be the finish floor? That could start to add up to a lot of insulating value. With a slb this thick, whether or not you go from below or groove the top, use a thermostat with floor sensing capability as well as air, that slab will take a long time to respond.

    Good luck, Dave H.
    Dave H
  • Paul Cooke
    Paul Cooke Member Posts: 181

    Thanks for your comments. Have you ever heard of anyone grooving concrete like this? I'd prefer the grooving...puts the tube up where it belongs. MultiCor is a great suggestion.

    Dave, to answer your question, as far as I know the slab is exposed in the basement. Finished floor is expected to be concrete either acid etched or stained. The owner was concerned about striping over time.

    The grooving (as long as it is done by someone else) might be the way to go on this.

  • hot_rod
    hot_rod Member Posts: 15,033

    of where the tube is placed top, bottom, or middle that's a lot of heat sink. Plan on slow response. Any way to place some insulation and a new floor over the top?

    As far as grooving concrete Dan Foley recently did this.

    I'd look at a concrete saw with two blades gauged to the tubing OD. The curves would be time consuming with a held held diamond blade. Doable but very messy. Use appropiate lung, eye, and ear protection.

    Here is a sample curve done with a 4-1/2" angle grinder and diamond wheel.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Paul Cooke
    Paul Cooke Member Posts: 181
    Groove width

    The concrete cutter said be could cut a 5/8" wide groove. I'll have to check the O.D. of MultiCor.

    hot rod, I haven't seen the house yet, so I don't know if there is enough room on top to build up the floor. Quick-trac would probably work the best but he wants a concrete
    finish on the floor.

  • Terry
    Terry Member Posts: 186

    What about laying tube down on tracking (over bubble/foil) and puring 1.25" of Gypcrete over slab ?

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Finished concrete floor

    NO WAY you could finish the floor directly after grooving unless the HO wants to see the pattern of the tube...

  • Joe_13
    Joe_13 Member Posts: 201
    What's the final floor finish?

    If wood flooring or carpeting is going down after all this, why not put some sleepers down and just lay tubing between them, 5/8" plywood sheets over that.
  • Paul Cooke
    Paul Cooke Member Posts: 181
    Finished floor

    There is probably not enough room to do anything on top of this exixting slab because ceilings are at 8 foot. I guess that's why the grooving is looking good.

    This Oregon Contractor (link below) offers an extremely thin cement topping that acts as a finished floor and is very durable. He stains designs into the surface and it may be an answer to our problem.
  • John T_2
    John T_2 Member Posts: 54
    We did a job like this

    so I was interested to see what comments were made. We took the job over after the client and the previous contractor parted ways. They had fastened the tube below the floor in the joist bays, joists were also concrete on three foot centers.

    I had concerns about starting out with .2 per inch r-value (some of the floors were over 8" thick) and then adding carpet and pad resistance in some areas. Edge loss along the entire perimiter of the slab was also a concern. The floor was above grade and there was no thermal break between the floor and the exterior masonary walls.

    We opted to use the joist heating for floor warming only in an effort to reduce the temperature difference between the slab and the exterior. Column radiators were installed to act as the primary heat source.

    Everything seems to be working well.

    John Taylor

    Custom Climate Systems, Inc.
    Whitmore Lake, Michigan
  • James
    James Member Posts: 3
    Heating concrete slab.

    Take a look at the Roth Panel system. 3/4" thick grooved foam w/Heat transfer surface attached. Not sure if you can add 1" to your floor surface. The foam takes care of the insulation and gives you a quick response time.
  • Paul Cooke
    Paul Cooke Member Posts: 181

    Thanks for this idea, James. So, this panel has built-in insulation? Sounds like a good product.

    Wonder if any of the other plywood/aluminum panel products come with insulation.
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