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Raised slab – insulation and vapor barrier

jdchess | Posted in General Questions on

I have some raised slab questions. 

New build in Pender County, NC.
Climate zone 3a…warm humid.
Simple rectangle envelope.
Front door facing N (slightly NE).
52’x40′ exterior frame dimensions.
Wrap around porch (front and east).

I want the aesthetics of a crawl space (raised) but without the potential issues, so I would like to build on a raised slab like this…

Pros/cons of this method? Would insulating the slab be necessary for my climate zone? If so, how should insulating be done? Rigid foam underneath like with a non-raised slab?

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  1. GBA Editor
    Martin Holladay | | #1

    Unless I'm confusing your question with someone else's question, I think I already answered. Yes, you can build a slab foundation with frostwalls, and you can elevate the slab to any height your want.

    Details are in this article: "Insulating a slab on grade."

    See the illustration below.

    1. jdchess | | #2


      Yes you did and thank you. I had some follow up questions that I think may have gotten lost in the shuffle.

      I seem to have overlooked the article link when you answered previously. My apologies. The article you posted does in fact answer a lot of my questions. Much appreciated.

      Also, earlier you mentioned builders in NC still use blocks instead of poured walls. What are the advantages of poured walls over using blocks?

    2. jdchess | | #4


      I have re-read the article. At the beginning, you said...

      "The most important factor is climate. In climate zones 1, 2, and 3, most builders don’t bother to install any insulation. While it could be argued that insulation might be useful in climate zone 3, it really isn’t needed in warmer climates, where an uninsulated slab helps lower air-conditioning bills compared to an insulated slab."

      Assuming the warm-humid portion of zone 3 (southeastern NC), how does one decide whether or not to insulate the slab? Also, does the fact that it is a raised slab play into whether or not to insulate?

      1. GBA Editor
        Martin Holladay | | #5

        Yes, the fact that this is a raised slab makes a difference. In your climate zone, you can omit the horizontal insulation under the slab if you want. But you need vertical insulation at the slab perimeter.

        1. jdchess | | #7


          Would the edited attached detail be correct when choosing to omit the horizontal insulation under the slab? Or should the vertical insulation at the slab perimeter extend below the bottom of the slab into the compacted fill?

          Can you clarify how deep the vertical insulation should go in CZ3?

          Thank you again for your help.


          1. Expert Member
            MALCOLM TAYLOR | | #8


            The insulation should extend down to the footings, or if they end up being very deep, a couple of feet below the exterior grade.

            The only reason to bevel the slab is to hide the foam, and provide backing for the finished floor. Another way of doing this to decrease the width of the foundation wall at the top, in a similar way to Martin's illustration in post #1.

          2. MAinspector | | #12

            Your pictures show the foundation wall being the same thickness as your stud wall. Will this truly be the case? Typically a poured wall has a notch formed into it just like Martin's detail shows. I like to leave the vertical slab insulation full width (no bevel) and sticking out past the wall plate slightly.
            Use perfectly straight cut foam pieces and set them dead level and it makes a great gauge to screed the concrete floor to. I can't see the exposed foam edge effecting any flooring install and it will mostly be lost under the drywall edge and baseboard.

          3. jdchess | | #14


            Thank you for pointing this out. I was curious as to the difference in the pictures from Martin's article. One had the "notch" while the one showing the bevel cut was the full width of the stud wall. Certainly appreciate the insight on that point.

            What would the detail look like with the notch at the top of the foundation wall if you chose to omit the horizontal insulation under the slab?

          4. MAinspector | | #15


            The notch would still remain the same. Regardless of whether you fully insulate under the slab you will still need a small strip of insulation where the slab sits on the foundation wall. The goal is to thermally isolate your slab from your foundation wall.

          5. Expert Member
            MALCOLM TAYLOR | | #16


            The detail would look exactly the same as the one Martin posted, but with the sub-slab insulation replaced by fill.

          6. jdchess | | #18

            Jon and Malcom,

            So you create a "shoulder" of insulation for the slab to sit on? Am I seeing this correctly?

          7. Expert Member
            MALCOLM TAYLOR | | #19


            Yes that's it.

          8. jdchess | | #22

            Slightly different question for you guys and Martin regarding the vapor barrier...I understand a polyethylene vapor barrier should go above the rigid foam and be in direct contact with the slab. Is this correct?

            I assume this is the case whether or not you choose to include the horizontal insulation under the slab. Either way (horizontal or no), where does the vapor barrier end? At the bottom edge of the slab or the top edge (see attached)?

          9. GBA Editor
            Martin Holladay | | #23

            Q. "I understand a polyethylene vapor barrier should go above the rigid foam and be in direct contact with the slab. Is this correct?"

            A. Yes. For more information, see "Polyethylene Under Concrete Slabs."

            Q. "I assume this is the case whether or not you choose to include the horizontal insulation under the slab."

            A. Your assumption is correct.

            Q. "Either way (horizontal or no), where does the vapor barrier end? At the bottom edge of the slab or the top edge (see attached)?"

            A. Either way will work -- the difference is minor. The polyethylene is a vapor barrier, not an air barrier. In any case, it's a good idea to seal the perimeter crack (at the perimeter of the slab) with caulk after the concrete is cured, and you don't want a flap of polyethylene in the way when you apply the caulk.

          10. jdchess | | #24


            With a non-monolithic slab like this is there any need for a vapor barrier against the foundation wall or footers assuming vertical insulation down to the footers? Or can the vertical insulation sit directly against the foundation wall?

          11. GBA Editor
            Martin Holladay | | #28

            You don't need polyethylene in that location. You can install vertical insulation directly against the concrete wall.

          12. jdchess | | #25


            Aside from budget, how do you determine whether or not the horizontal insulation would be beneficial since there is the potential cooling benefit of an uninsulated slab in the summer? What does the math on this look like?

          13. Expert Member
            MALCOLM TAYLOR | | #26

            The argument for not insulating slabs in some climates to take advantage of the temperature of the soil underneath doesn't apply in your situation, because with a raised slab such as y0u are proposing, the insulation on the walls is not buffering the slab from the surrounding soil, but from the outside air. You still get whatever advantages there are to be had by not insulating underneath the slab.

          14. GBA Editor
            Martin Holladay | | #29

            Malcolm gave you a good answer. You don't need the continuous layer of horizontal rigid foam under your slab in your climate. And there isn't really an easy way to do mathematical calculations to answer this question. It's a judgment call -- one that has been made by building scientists smarter than me.

          15. jdchess | | #30

            Malcom and Martin,

            Thank you both for your insight. I have much better understanding now.

        2. jdchess | | #10


          Martin's article that he linked to mentions the vertical only extending to the footers in colder climates (CZ 4 and higher). My understanding was that this wasn't necessary in CZ 3 and lower. However, I was thinking that it possibly should extend below the bottom of the slab. I just don't quite have a clear understanding yet.

          1. GBA Editor
            Martin Holladay | | #11

            It sounds like you are planning to elevate your slab, so that instead of the top of the slab being (for example) 8 inches above grade (a typical slab), the top of your slab might be 2 feet above grade. Your slab will be atypical, and that changes my advice.

            I agree with Malcolm on this one.

          2. jdchess | | #13


            Thank you for the clarification. You are correct. My intention would be to elevate the slab 18–24 inches above grade.

        3. Deleted | | #21


  2. krom | | #3

    The only reason to use block is that sometimes its cheaper.. Other than that, poured is superior in every single way.

    The foam keeps the wall from turning into a giant radiator that sucks heat from the slab and transfers it to the outdoors. in the winter, and the opposite in the summer.

  3. aaronbeckworth | | #6

    Trever states, “Other than that, poured is superior in every single way.” Every single way meaning what? I would expect block would be less expensive AND may be preferred by some builders. Without sound reason why ask a builder to deviate from what they are comfortable with?

    Also, block may be less intimidating for a DIYer working without a crew.

    I really hope to hear some experts chime on on the question of block vs. poured foundation walls for a raised slab.


    1. Expert Member
      MALCOLM TAYLOR | | #17


      The two reasons to use blocks are cost and adherence to what is local practice. Neither of those should be discounted, but Trevor is right: objectively, a solid concrete foundation wall performs better in every respect.

      Is a block foundation easier to build that a concrete one for DIYers? I'd be interested to hear people's views on that.

  4. Deleted | | #9


  5. Chris_Duncan | | #20

    A block Foundation is harder to build for owner Builders and probably for professionals too. And I think a poured Foundation is stronger unless you do a lot of extra work with rebar and filling the block openings with concrete, even then it's not as strong.

    All the examples I've seen of failed Foundation walls were block walls

  6. rockies63 | | #27

    This article has some good information on concrete slabs, as well as how to finish them with flooring.

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