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vermiculite under slab

The 2 " pink insulations hard foam on well drained hard packed gravel with ploy sheet in between.. 6" oc sheet wire mesh... is the proven method. Have heavy equipment on it , just hair line cracks, that's concrete, no matter what..


  • Boilergeek
    Boilergeek Member Posts: 42
    Vermiculite insulation

    I am in the process of tearing down an old house and building a new one. The old section was full of vermiculite insulation circa early 50's. I have bags of this stuff that was recovered and I recall a snowmelt project years ago that the engineer specigied vermiculite as under slab insulation. My Project is slab on grade radiant in the mountains at 7000'. I want to make sure we properly insulate, none of that bubble crap! Has any one out there ever used vermiculite and what do you think of it under a slab? I would also be interested in what you are doing on your jobs. I am a rep and see tons of jobs out there, few properly insulated. I welcome your thoughts!
  • jp_2
    jp_2 Member Posts: 1,935
    seems to me

    it would compress and then your slab cracks!

    can't imagine how it would work?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Some of the properties of vermiculite would bother me if used as under-slab insulation.

    1) It's literally like a sponge to water--it's this property that makes it so useful as a soil conditioner for potted plants, etc. Asphalt coated vermiculte is available but I doubt that's what you have.

    2) It's loose, extremely light and has very low compressive strength. Couldn't be fun to pour a slab over a bed of the stuff and the weight of the concrete would almost certainly compress it significantly thus reducing the insulation value.

    Vermiculite is a common ad-mix for concrete as it reduces the weight considerably and adds some insulation value without much affect on strength.

    Perhaps someone else will comment about the use of vermiculite in concrete for a radiant slab.
  • Boilergeek
    Boilergeek Member Posts: 42

    I was thinking of using a vapor barrier above and below and rolling it to compress I know it will loose R value, but an inch of compressed vermiculite would still be much better than many other methods I can think of. The question remains the compressive strength of this method. I will have wire mesh and rebar on 24" centers, but the concrete is the finish surface. Cracking will occur I understand, I just hope to minimize it. Mostly I am just questioning the standard and trying to explore new things.

    What methods are you using?
  • Frank M Alleva
    Frank M Alleva Member Posts: 20

    Please make certain the insulation is not zoneolite, which was mined in Libby, Montana and was contaminated with asbestos. It also was used as a "pour type" insulation. Just my 2-cents.
  • Couderay
    Couderay Member Posts: 314

    Concrete to me is only as good as its base. Pink is the way to go, high density is available. Use a vapor barrier. Drainage is also very important.
  • John Monson
    John Monson Member Posts: 1

    Vermiculite, Zonolite, Zelite, all the same thing. I have been told that it comes out of the same mines that asbestos used to come out of. You have bags of an unwanted expensive waste to dispose of on your hands that would be a horible thing to put under your slab. We have installed miles of pex tubing on 2" blue styrofoam. It's a special 2" blue that is specific for the purpose. It has a different rating than most and is designed for that type of installation. We use 1/2" pex with an o2 barrier in it and a special stapler to staple it down.
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