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Community and Q&A

Slab insulation to meet 2012 IRC

Mitch Theurer | Posted in General Questions on

I am trying to build an insulated stem wall as shown in the Slab Happy article.

My local code official says that it doesn’t meet the current code adopted by our County which is IRC 2012.  He specifically calls out the 6″ minimum stem wall as the problem, but did also complain about the wall assembly not bearing fully on the stem wall.

I would like the 2X6 wall assembly to cover 2″ of slab edge insulation.  Does anyone know of current codes that allow this?
Thanks-Mitch

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Mitch,

    Can you link to the article or post the section you are discussing? I can't find it in the GBA search engine.

        1. Expert Member
          Malcolm Taylor | | #4

          Mitch,

          Thanks. There are two ways to hide the foam which leave you with a wider foundation at the top - remembering that the foam doesn't need to be completely covered by the 2"x6" as you still have drywall and trim increasing the width of the wall.

          - Pour the concrete stem-wall with a 45 degree cant strip on the outside, reducing the top to whatever dimension your inspectors will accept for bearing.

          - Cut the foam at 45 degrees, meaning you don't need the framing above to overhang the foundation wall at all. This does come with a slight energy penalty.

          1. Mitch Theurer | | #5

            Thanks, I don't see how a cant on the outside of a 6" stem wall helps me. They already told me that the wall has to be a minimum of 6". I was just curious if the design is accepted under other code definitions that I could possibly try to persuade my official to accept. The bevel top on the foam is just a place for the floor to crack in my opinion, which was why I would like to avoid that as a solution.

  2. Expert Member
    Malcolm Taylor | | #6

    Mitch,

    There are two separate requirements and we may be conflating them.

    One is the minimum width of the stem-wall, the second is the minimum amount of bearing they require for the framed wall above.

    The minimum width of the stem wall is not affected by incorporating a cant, the way it is when you reduce it as they did in the BSC detail.

    I don't know what the minimum bearing for framed walls is in the IRC (if that's what you are under), but I can't see it being more than 4" for a 2"x6" framing, leaving you space for 2" of foam without angling it.

    You can start with any width stem-wall you want, and leave as much bearing at the top you want by varying the size of the cant.

    Here is the detail I used on the last house I designed.

    1. Mitch Theurer | | #7

      Thanks Malcolm, here is the picture of the plans I submitted and the comment back from the code official. If I understand him correctly the code I have to follow doesn't have a minimum bearing specified only a 6" stem wall minimum. That's why I was asking if someone here knows the code. With that said they have determined that if it is not specifically prescribed in the code it would not be allowed without being professionally designed, and maybe that's what I have to do.

      Here's a link I have for the code we have to follow, I didn't see any of this in there but maybe someone else knows.

      https://codes.iccsafe.org/content/IRC2012P12

  3. Expert Member
    Malcolm Taylor | | #8

    Mitch,

    Hopefully someone with more knowledge of your code can chime in. Otherwise it's off to the engineer.

    Another option which came up in another discussion here was to place your sub-slab insulation so that the bottom of it sits in the top of the foundation, and pour your slab above that. You would need to separate the slab from the wall framing with a 1/2" PT plywood screed-board, but then you then wouldn't need any foam between the slab and foundation, because it it now insulated by the exterior wall. The downside would be you would need longer studs.

  4. Expert Member
  5. Mitch Theurer | | #10

    I have read that but to me it is speaking to the reinforcement specs. It does say there that the narrowed portion of the wall must be equal to or greater than the thickness of the wall above. To me this means I can not narrow the concrete to 4” at the top. It also appears that this is for a brick ledge which isn’t really the slab perch I’m trying to accomplish. Am I wrong?

    1. Expert Member
      Michael Maines | | #11

      Mitch, the section I linked to is labeled, "Concrete Wall Thickness." They note, "The thickness of concrete foundation walls shall be equal to or greater than the thickness of the wall in the story above." The details they list for reinforcement are in addition to this basic requirement.

      Their note, "Concrete foundation walls with corbels, brackets or other projections built into the wall for support of masonry veneer or other purposes are not within the scope...of this section" means, as I understand it, that the kind of shelf/reduced thickness you want to do is not explicitly covered prescriptively, so unless you get an engineer involved, you are stuck with meeting the thinnest concrete foundation wall allowed--6" in most cases.

      When I have worked with licensed engineers on details like this, they have let me reduce the concrete wall to 5" at the reduced-width portion. If you want to do this prescriptively, without an engineer's approval, you're stuck with what the code says.

      Also, the required wall thickness is based mostly on soil type, so above grade you could make an argument that Table R611.3 should rule, which does allow 4" concrete walls. That is a bit of a stretch, though, and would depend on your building code official.

  6. Johanna Carr | | #12

    Mitch,
    I too am in NC and have experienced the same rejection of the foundation “notch” you have — except it was a structural engineer who rejected it. He preferred the 45* on the insulation, though I still don’t understand his reasoning. In any event, without the notch, you’re stuck dealing with some perimeter detail. I chose the 45* because it’s easier to protect that edge with drywall and baseboard than it is to hide 2” of foam. Given the slab is our finished floor, I’m also considering some type of masonry “baseboard” that’s thicker and less likely to wick.

  7. RKaz | | #13

    I just ran into this as well in Massachusetts with an engineer. The funny thing is, this is a detail that the US Department of Energy is promoting: https://basc.pnnl.gov/resource-guides/slab-edge-insulation#edit-group-description (scroll down to "Independent of the Foundation Wall" section). I've done this detail several times, have had it verified by a structural engineer, and never had any problems.

    1. John Clark | | #15

      Fyi...Joe Lstiburek founder/owner of BSC provided input to the DOE

  8. Nate Reik | | #14

    FWIW, Wisconsin's unique/own building code specifically allows this, but with caveats. You can overhang 1", or double the bottom plate and get a 1.5" overhang. This is what I did with a 2x6 wall, and with the sheathing, I've got 2" of overhang, covering my 2" slab insulation. I know it doesn't help in other states, but just putting this out there for others.

    From the Wisconsin UDC:

    b)Extension beyond the bearing surface. All of the following requirements apply to a sawn-lumber sill plate with uniform loading that is partially extended beyond the load-bearing surface of a foundation wall in order to put the exterior surface of an upper-lying wall flush with or beyond the exterior surface of insulation which is placed on the outside of the foundation wall:
    1. The center of any anchor bolt shall be set back from the side edge of the sill plate by a distance of at least 4 times the diameter of the bolt.
    2. The thickness of the concrete or mortar cover around any anchor bolt shall comply with ACI 318 Section 7.7.
    Note: Under ACI 318 Section 7.7, the minimum cover for a 5/8-inch-diameter or smaller bolt is 1 1/2 inches.

    3. Where a stud wall bears directly on a double bottom plate, the plate may not extend more than 1 1/2 inches beyond the load-bearing surface of the foundation wall.
    4. Where a stud wall bears directly on a single bottom plate, the plate may not extend more than 1 inch beyond the load-bearing surface of the foundation wall.

  9. Expert Member
    Zephyr7 | | #16

    Nate beat me to it, I was about to suggest a double bottom plate like his #3. This MIGHT make your building guys happy, so it's worth a call. You end up cantilevered out a little, but it won't really affect the strength of the overall wall since the "lever action" is extremely limited.

    I've normally seen the detail flipped with the insulation on the exterior, the load bearing wall directly on top of the masonry wall (no overhang), and only the exterior siding overhanging the top of the foam. I can't say I work with this particularly often though.

    BTW, don't be too surprised that one gov't agency is promoting something that another rule of a different body doesn't permit. Conflicts like this between rules of different agencies are not particularly uncommon, unfortunately.

    Bill

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