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Can I pour a new radiant slab on an old slab?

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We have a 800 square foot outbuilding with a solid concrete slab from the 1940s. It has some sloping to old drains that have been decommissioned. We have an 11’ceiling.

Is it unwise to pour a new reinforced 4-6” radiant slab directly on top of the old slab? I would place a vapor barrier, 2” hardcore foam insulation, then about a 4”-6” reinforced slab with tubing. Would this work? I’d still end up with a ceiling height of about 10’6”. I want to tie it in to my new existing boiler that was sized and zoned to accommodate this space.

My question is more about avoiding jackhammering and removing the existing slab. It’s not only a pain, but access is difficult, and I worry about how this slab ties in to the converters perimeter walls. I’d like to leave them alone. Also, removing the existing slab would tear up a mature garden and make a huge mess, etc. We need the slab to rise up at least 4” off the patio outside anyway.

I think the best way to do this would be to remove the existing slab, but there is literally no problem with this slab. As I said, it’s a little uneven in places. The goal is to make this a nice comfy workshop and office with radiant heat. We considered building a level subfloor on top of the old concrete, and adding Warmfloor or Ecofloor, but I really prefer the efficiency of concrete slab radiant. Thoughts?

Any opinion is welcome! Thanks.

Comments

  • hot_rod
    hot_rod Member Posts: 22,483
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    I would look for any large cracks or shifting of the old slab. That could indicate the sub grade was not compacted or prepared properly.

    Good drainage around the slab, no water pooling below? Does it have rebar, do you know?
    You would want to insulate with 2" foam between the old and new. The foam serves as a cleavage membrane and insulation.
    Think about how you insulate the edge, that is a big heat loss area.

    Pros and cons of slab radiant.

    Slow to change temperature with all that mass.
    Flywheel effect can work against or for you. Any large glass, high solar gain area, that can cause a slab to be a bit harder to control.

    If you are in an area with wide frequent temperature changes, that can be an issue.

    Durable long lasting floor. They can be stamped, stenciled, colored, stained. Saw cut and grouted to look like large tile. So many finish options.

    If you go decorative, look for a pro with experience and past job photos.

    All concrete cracks, if it is the final surface, know that it is possible to crack.

    My shop slab, going on 3 years old just recently developed some "road map" hairline cracks. Right through the expensive epoxy shop floor finish :(

    I always suggest a tight tube spacing for the most comfortable floor with the lowest possible supply. 6" on center is doable with 1/2" pex. Getting the tube 2" below the top surface helps speed up and lower supply temperature also. Mesh lifted on rails makes this possible.

    Tight, raised tube opens the door for water to air heat pumps and thermal solar.

    You only get one chance to tube it. Spend the $$ to do a top quality installation.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • realliveplumber
    realliveplumber Member Posts: 354
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    Make sure you use the correct foam, with a high psf rating.

    The stuff in the home centers isnt it.
  • pecmsg
    pecmsg Member Posts: 4,967
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    I’m trying to picture 4 or 6” of slab inside an existing structure?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,167
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    The old slab is not properly insulated for radiant heating. Ideally you would want to have at least 2" foam below the heated slab to drive most of the heat UP. That does not mean it will not work. If the old slab is at least 4" thick and you are adding 4" minimum of new concrete, you will have a very large mass that will be slow to heat up and slow to cool off. A good portion of that heat will go into the ground. However, once you get the ground warm below the building, you will get a very comfortable space. @hot_rod has a good handle on it. Re-read what he points out.

    Here is what he said:

    I would look for any large cracks or shifting of the old slab. That could indicate the sub grade was not compacted or prepared properly.

    Good drainage around the slab, no water pooling below? Does it have rebar, do you know?
    You would want to insulate with 2" foam between the old and new. The foam serves as a cleavage membrane and insulation.
    Think about how you insulate the edge, that is a big heat loss area.

    Pros and cons of slab radiant.

    Slow to change temperature with all that mass.
    Flywheel effect can work against or for you. Any large glass, high solar gain area, that can cause a slab to be a bit harder to control.

    If you are in an area with wide frequent temperature changes, that can be an issue.

    Durable long lasting floor. They can be stamped, stenciled, colored, stained. Saw cut and grouted to look like large tile. So many finish options.

    If you go decorative, look for a pro with experience and past job photos.

    All concrete cracks, if it is the final surface, know that it is possible to crack.

    My shop slab, going on 3 years old just recently developed some "road map" hairline cracks. Right through the expensive epoxy shop floor finish :(

    I always suggest a tight tube spacing for the most comfortable floor with the lowest possible supply. 6" on center is doable with 1/2" pex. Getting the tube 2" below the top surface helps speed up and lower supply temperature also. Mesh lifted on rails makes this possible.

    Tight, raised tube opens the door for water to air heat pumps and thermal solar.

    You only get one chance to tube it. Spend the $$ to do a top quality installation.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • hot_rod
    hot_rod Member Posts: 22,483
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    Make sure you use the correct foam, with a high psf rating. The stuff in the home centers isnt it.
    Home Stores usually have 150 or 250 foam board Dow and other brands

    250 is 25 psi rated. Which comes out to 3600 lbs per square foot. Unless you have some very high point loads that covers most typical residential slabs and driveways, even some commercial slabs

    Good chance the dirt below most residential slabs and driveways is not compacted to a 3600 psf load!

    Around here they are using those 8’ white foam geo blocks to build on and off ramps instead of compacted fill. They look like  white beadboard cubes to me😗

    Dow high load 60 psi runs around $160 per sheet. Ive never had a structural engineer call for more than 25 psi foam even in heavy equipment shops that I did for John Deere and Caterpillar. 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • RobinInCali
    RobinInCali Member Posts: 38
    edited August 2023
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    I guess my question is, should I spend the time and money to jackhammer out the old slab? The old slab is in great condition. No cracks, no movement. It’s just sloped in one area and not perfectly flat. I have an 11 foot ceiling so if I put 2 inches of foam on top of the old slab and poured a new 4 to 6 inch radiant slab on top of that, I’m assuming it would work 

    However, if I were to jackhammer out the old slab, install a vapor barrier, gravel 2 to 3 inch foam core, and pour on top of that, I know that would be superior.

    It’s a workshop in office. I estimate jackhammering out and hauling, as I live in a city, would cost about $30-$40,000 to the job. This is San Francisco. Not Minnesota. But due to the fog and chill, I would be keeping that slab warm about nine months out of the year  This is why I was considering just pouring a slab on top of the old slab. And, of course, of course of course I’m going to put a vapor barrier, sufficient thick foam to minimize heat, loss, etc.. 
  • Jamie Hall
    Jamie Hall Member Posts: 23,657
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    Don't bother to take out the old slab. Lay your foam insulation and concrete on that. I'd use mesh reinforcement in the concrete -- and fibre reinforced concrete as well.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    hot_rod