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Slab on grade angled insulation and vapor barrier

joenorm | Posted in General Questions on

I know this has been covered a lot here and I have read most of the articles associated. I am wondering what the best way to detail the little bit of under slab insulation that ends up at a 45 degree angle down to the base of the thickened edge footing? It’s never as perfect as the computer drawings show. 

Does any one have tips for this area? 

Also to clarify, does the insulation go above or below the vapor barrier?


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  1. Expert Member


    I'd like to hear some opinions on whether that angle is necessary.

  2. Expert Member
    Michael Maines | | #2

    That angle is one of the main reasons I never do conventional thickened-edge slabs. I have tried a few alternatives, including running the insulation all level and then adding fill over the foam to reduce the amount of concrete needed. I often use separate stem walls of some sort to avoid this issue. My favorite new idea, after using Glavel in conjunction with the WarmForm system (, including the dreaded slope, I recently heard that WarmForm is adding a new profile to more easily contain the Glavel so it will be more of a step than a slope. Glavel has lower embodied carbon emissions than foam and can double as fill and a gas-permeable layer for radon mitigation.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      It brings up several difficulties here.

      The first is that as soon as the footings are integral to the slab, under our code the design moves from the prescriptive Part Nine, to Part Four which requires an engineer.

      The second is that foundations are required to go down to 18" below grade. To do that, and still have an adequately wide thickened portion for bearing, you end up using a lot of concrete on what is a disproportionately large footing.

      1. Expert Member
        Michael Maines | | #7

        Malcolm, do you not have frost protected shallow foundations as part of your prescriptive code? We can use the bottom of the foam insulation to meet the required depth, or often building officials allow WarmForm's concept of including the non-frost-susceptible crushed stone under the form as part of the system.

        1. Expert Member
          MALCOLM TAYLOR | | #8


          Here the depth is both for frost and seismic anchoring.

          Practically I find that 18" is barely adequate to run perimeter and downspout drains, rock, and a bit of cover to grade.

          For most projects the size of that thickened slab edge would probably be around 18" wide by 12" deep. With a clearance to the framing from grade of 8", that would leave the bottom go the concrete 4" below grade. Increasing the depth of the footing to get down lower is basically using concrete as filler.

          1. Expert Member
            Michael Maines | | #9

            Malcolm, if you're talking about the WarmForm system, the height of the foam is fixed at 16", with 4" of that in the sub-slab portion, so the concrete is always 12" deep at the perimeter. The IRC allows 6" clearance to grade but I prefer 8", leaving 8" below grade. I didn't dimension the 8" below grade on this drawing but that's what I'm showing.

          2. Expert Member
            MALCOLM TAYLOR | | #11


            WarmForm seems like a great solution, but for me it crosses what for now is a bright line against using any assembly that needs foam under the load-bearing portions of the foundation.

            All rural lots here are home to carpenter ant colonies that can make short work of foam, and I just spent a good part of the week before last fortifying the crawlspace of a 1911 house against rats that had tunneled under the 12" deep footings and established nests in the batt insulation.

            Until I find something I can rely on to survive uncompromised for the life of the house, I can't see using anything under the footings. Here is a piece of foam I removed from a roof nearby last fall:

          3. Expert Member
            Michael Maines | | #12

            Wow, I see why you aren't comfortable with foam under footings!

          4. Expert Member
            MALCOLM TAYLOR | | #13

            They are the devil incarnate, split into a thousand pieces.

  3. joenorm | | #4

    I am building a little shop building so I am not as concerned about perfect details the way I would be with house.

    Still I wonder how that angled portion works at all, would love to hear from people who have DIY's a slab and come up with a successful way to detail this.

    And so the order would go: Gravel layer, insulation, vapor barrier, rebar and mesh, concrete?

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Yes that's right.

    2. Expert Member
      Michael Maines | | #10

      I got sidetracked a bit above but you can use Glavel (or other aerated glass aggregate) instead of foam. It's relatively easy to compact it with a slope. It's about R-1.7 per inch so you need more of it than you would foam to get the same R-value, but that's rarely a problem under a slab.

  4. JustusM | | #5

    Yea, that slope never seems to turn out very well. Typically the foam ends up broken in lots of places with large gaps in it. Your best bet would be to try and build up the slope fill using 3/4" down to fines crushed rock using screed rods and hand tamping. If you put enough time and effort into making it in plane it could be done.

  5. joenorm | | #14

    Perhaps for a small shop building leaving out that problematic portion will not be the end of the world

    1. Expert Member
      MALCOLM TAYLOR | | #15


      If you want the insulation to be continuous, and don't want the complication of the angle, I would do what Michael suggested and layer the foam, overlapping them a couple of inches.

      1. joenorm | | #16

        Interesting. That leaves my footing sitting on insulation which I was not prepared for and do live in a similar climate as you do so ants may be a concern here also(not sure on that).

        It also leaves me wondering how the vapor barrier gets detailed in these assemblies to not create small pockets of unfilled area where the vapor barrier bridges an inside corner.

        1. Expert Member
          MALCOLM TAYLOR | | #17


          That's the problem with integral footings: If you want continuous insulation with no thermal bringing, you need to run it under the slab, footings, and up the outside too. That's why, like Michael, I always use stem-walls.

          If your climate is similar to ours, I would either omit the piece under the footings, or omit all the foam except that on the outside of the footings.

          You can eliminate most voids by using smaller pieces of poly and letting them slip a bit during the pour. There is no advantage to using one large piece over the steps, or taping it. Vapour-barriers don't have to be entirely continuous. Small voids are inconsequential anyway.

          1. joenorm | | #18

            Thanks for the replies.

            So you don't think the interior slab insulation is worth doing in this case? Climate 4C.

  6. Expert Member
    MALCOLM TAYLOR | | #19


    If it were me I'd run the foam down the outside of the thickened slab, and maybe a few inches further into the ground, and be done. With sub-slab foam the slab would feel a bit warmer, but I can't see you using the shop in your socks, and the energy savings are tiny.

    1. Expert Member
      Michael Maines | | #20

      Here in climate zone 6 I like to include a little foam insulation under garage slabs, to help them stay close to the air temperature and avoid condensation that results in a musty smell. But it's probably not as effective in CZ4.

  7. joenorm | | #21

    Thanks Michael and Malcom,

    At this point I could remove the 2" of pink foam(only 7 pieces) and have a thicker concrete slab of between 5-6" or leave in the foam as planned. In the latter case the concrete is on average 4" thick and I'd hope for some benefit from the insulation.

    Suppose I should have asked these questions before I started digging and forming but so it goes.......I thought insulation was a no brainer if I wanted to heat the space. But all the points made here about it having minimal impact make a lot of sense.

    Thanks again for the conversation

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