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Insulated slab on grade design

| Posted in Green Building Techniques on

I have several questions about the Insulated slab on grade design. I have seen it done several ways on the internet and really need some advice. A little background on the project.

First this is a timber frame home that will be built in Searsport, Maine. I believe it is in climate 6A and is fairly close to the ocean. It will have 12″ double studded walls with dense pack cellulose blown in the walls. A layer of 1 x 8 pine as the interior sheathing for the roof covered by a vapor barrier. On top of the sheathing will be 12″ I joist running from ridge to overhang and will have dense pack cellulose blown into that cavity and then covered with 1.5″ of polyiso with firing strips attached above to lay down ZIP sheathing for the cool vented outer sheathing of the roof and covered either asphalt shingles or metal roofing.

Now to my question of the slab with apologies for the long prelude.

One method I have seen used is first laying loose flowable fill with 6-8 inches of EPS placed on top. The plumbing pipes are protruding through the EPS and sealed with foam. Then a 15 mill vapor barrier is placed on top of that and sealed with tape. After that ICFs are placed on top to form the continuous footer. After footer has dried, gravel is filled in the open cavity to within 4 inches of the top of the footer and then the slab is poured.

The second method starts with the same first step as above except the plumbing is placed in the gravel above the EPS and appears to go though the ICF before the concrete is poured for the footer and then the slab is poured like above.

I am confused about this and would appreciate anybody’s advice on this and their thoughts

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, most residential slabs don't require flowable fill. There is nothing wrong with flowable fill; if your soils engineer advises it, or your concrete contractor likes to use it, it works fine. But its use is rare in residential construction.

    The usual base for a concrete slab is about 4 inches of crushed stone. However, every site is different, and some soils are unusual -- so if you have an engineer, listen to your engineer.

    The second scenario you describe -- one in which gravel is installed above a layer of rigid foam -- is also unusual.

    The usual layers are as follows, from the bottom up:
    4 or 5 inches of crushed stone;
    2 to 6 inches of horizontal rigid foam insulation, with additional vertical foam insulation at the perimeter;
    a layer of 6-mil polyethylene, or a similar vapor barrier;
    the concrete.

    Depending on your frost depth and local soil conditions, this can be a thickened-edge slab (with an integral footing), or it can be a slab placed on compacted fill between frost walls.

  2. davidmeiland | | #2

    I'm confused by the OP's description of ICF stacked on a slab over EPS. That would have to be a thickened edge slab as Martin describes, with the thickened edge bearing on grade, wouldn't it?

  3. Tom_Maine | | #3

    Thanks Martin. Our Frost line is 4 feet so I would need thickened edge slab and where two posts are located in the center of slab.

    David, sorry that I was not clear on my description. From what I could tell from video was that there was rigid insulation placed horizontally then about 2 feet in from the edge of the rigid insulation there was placed one course of ICF forms all around the perimeter of the insulation maintaining the 2 feet distance from the edge. The ICF form was about a 12 inches wide and 12-16" deep with rebar inside. So the ICF was filled with concrete which profided the perimeter footing. Inside the footing would have the gravel/dirt fill to within 4" of the top of the footing and then the remaining 4" depth within the footing would be filled with cement to make the slab. The two feet of insulation extending outward beyond the footer would be backfilled with dirt to provide the flange of insulation around the perimeter of the building.

  4. GBA Editor
    Martin Holladay | | #4

    It sounds like you are describing a foundation similar to the one shown in this article: Owner-Builders Get a Tight Shell With SIPs. That house has a frost-protected shallow foundation with ICF stemwalls and a slab.

  5. Tom_Maine | | #5

    Martin and David,

    This is like what I am talking about. It is located about 10 miles from where I am located.

  6. user-940291 | | #6

    That sounded like Alan Gibson's details from the beginning to me. Congrats on the project Thomas. If you're questioning anything you can probably get a few minutes with Alan or Matt for a few dollars and get some experienced input strait from the source. We're using the same method on a project in Westbrook, ME starting this spring. We've chosen to put the plumbing runs in the crushed stone above foam as the stone acts as a limited drain water heat recovery (very limited). Flowable fill is optional; it's a nice aid for almost perfectly flat foam and speeds things up a bit but can be expensive.

  7. Tom_Maine | | #7

    Thanks Matthew,

    Yes it is Alan Gibson's details. In the post above yours I provided a link to GO Logic website's video of the foundation. Do you have concern about condensation from the plumbing runs being trapped between the vapor barrier and the cement? Would like to hear more about your project if you don't mind.

  8. user-940291 | | #8

    We're running radon mitigation in the crushed stone which keeps the slab and stone dry.

    For Martin's reference, Alan wrote an article on his foundation details for JLC but I'm not sure it's appropriate to link to it here.

    The project is a 2,100sqft TFA 3 bedroom 2-story single family home. We're building a double stud wall with both walls bearing loads; the outer walls carry the roof and the inner wall supports the second floor. It makes for a very large footing but it allows us to terminate the floor framing over the inner wall and put 11.5" of dense-packed insulation outside the rim joist.

    Total wall is R40 with Intus windows at 10% glazing ratio. 6" EPS subslab and 24" loose fill in the ceiling. Our peak heating load for 70* is about 15kbtu/h at -5*F. Using one mini-split was tempting but we're using one Fujitsu RLS2H up and down due to cooling concerns in a large media room on the second floor. Venmar HRV. Lots and lots of Siga tape.

    The most unique thing is that the interior air and vapor membrane, Intello Plus, is on the outside face of the innerwall, leaving the innerwall free for utilities. I really like the security of the location for barrier continuity and the utility chase but it makes for quite a few construction details and some workflow challenges.

  9. user-1135248 | | #9

    That seems like an awesome project, and hopefully the foam base
    will be able to hold everything else up long-term! Now all you
    have to worry about is the whole thing floating away like a cork
    in the next hundred-year flood...

    Not to derail the thread, but out of curiosity what are the
    thermal probes you're using?


  10. Tom_Maine | | #10

    Thanks H, The project is on a hill 206 feet above sea level. So if I have to worry about a flood, well we all are in for a big surprise. : ) The research that I have done has shown that the EPS that has been used in Alaska highways and the Nordic countries for decades has performed extremely well. So I am not too concerned about it supporting the structure. As far as thermal probes, I am not trying to officially get this house PassiveHouse certified. However, if I need probes and not aware of it, I would appreciate your input on this matter. Please let me know if I need to read up on these as well. Thanks!

  11. user-940291 | | #11

    I think Hobbit is referring monitoring probes describe in the video from GOLogic. If I had to guess, it'd be HOBO but I'm not sure specifically what they're using.

  12. Tom_Maine | | #12

    Thanks Matthew,

    I noticed that you said you were using Intus windows on your project. Are you using uPVC or another material for the frame. I am also looking at Intus windows as well as Fibertec and Marvin Integrity fiberglass windows. What thoughts do you have on these?

  13. Tom_Maine | | #13


    In your article on Shallow Frost Protected Slabs the diagram shows the perimeter (or larger depth) of the slab with no insulation below. It did have a the 6 mil poly. I am wondering if adding insulation under the perimeter or footing would be ok or if there was a reason to not place the rigid foam there.

  14. GBA Editor
    Martin Holladay | | #14

    The perimeter of a thickened-edge slab is a kind of footing. It's possible to install foam under footings, as you suggest, as long as you can obtain approval for the practice from your local building inspector.

    The practice of installing rigid foam under footings is somewhat controversial, so if you go that route, you may find that you are blazing a new trail with your local building department.

    For more information, see Foam Under Footings.

  15. user-1072251 | | #15

    i don't think you want to be using an interior vapor barrier in your celing assembly. The polyiso is an exterior vapor barrier; you need to allow drying in at least one direction.

  16. Tom_Maine | | #16

    Thanks Bob, that is a good point. Would it make a difference if it is a cold vented roof? There will be firing strip on top of the polyiso then the Zip sheathing on top. If it is not a good idea to use the polyiso on top and the vapor barrier on the pine board ceiling, then I wonder how I should approach the roof from the inside going to the top.

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