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Community and Q&A

Raft slab edge insulation detail

trad4life | Posted in Energy Efficiency and Durability on

Hi – I’m building a home in Central Vermont (Zone 6) this summer and had a question regarding slab edge detailing.

I’d like to get R30 under and on the edges of a raft slab to meet some energy goals in the state. That would mean roughly 7″ of eps outboard of the slab edge. I’m framing a conventional 2×6 wall and will overhang my bottom plate a 1.5″ from the edge of the foundation + 3″ of exterior roxul. How could I cleanly make that transition over the remaining 2.5″ of foam? Should I go to 4.5″ of foam on the edges and and have it plane out nicely or does anybody have a clever work around? Any and all suggestion are much appreciated. Thank You.

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  1. GBA Editor
    Martin Holladay | | #1

    As you probably realize, most people who choose raft slab foundations install the same thickness of rigid foam on the exterior side of their above-grade walls as they have at their slab perimeter.

    You want the continuous insulation on your above-grade walls to be thinner than the slab-edge insulation.

    You're worried about aesthetics, not structure. In the realm of aesthetics, there is no correct or incorrect answer. You need to come up with a detail you can live with.

    At a minimum, you need metal Z-flashing to make the transition, with the upper leg of the Z-flashing under your WRB or your siding, and the lower leg covering the vertical part of your slab-edge insulation. You also want at least a little bit of slope to the "horizontal" leg of the Z-flashing. So if I were you, I would shape the top of the slab-edge insulation to include that slope.

  2. brp_nh | | #2

    We recently built a house in central/northern NH with a double stud wall on top of a uniform 8" thick slab. There is 4" of XPS underneath and on the slab edge. Our framing doesn't overhang (except for the sheathing) so there is 4" of foam beyond the wall. We did bevel the top of the edge foam, Grace Vycor extends from the sheathing over foam, sloped piece of pressure treated wood on top of this to protect, and then aluminum flashing. Reference photos and info:

    This bare aluminum flashing isn't a great long term solution, so I'm entertaining replacing it with thicker coated aluminum ( or even trying some type of cementitious/stucco coating. If building again, I'd lean towards some type of slab on grade foundation that doesn't rely on so much exterior foam.

    We found this area time consuming and difficult to detail, partly because there are a lot of things to keep in mind: wall to foam transition, keeping foundation and wall/siding dry, protecting foam, detailing around doors, thinking of insects, and more. It was also complicated in that we needed soil on top of the horizontal foam for protection, yet as you put soil there, the closer it gets to the wall/siding. Not sure of your building experience level, but these are just things to keep in mind:
    protecting the horizontal and vertical foam from foot traffic and insects (just saw some carpenter ants exploring around our house the other night)
    protecting the wall/siding material if the grade level is closer than standard foundations

    I think reducing to 4.5" of edge foam would make this area easier to detail and maintain in the future. If you did this, you could do some type of cementitious/stucco coating on the face of the edge foam with simpler metal flashing.

    Long post, hope my thoughts help. Keep in mind, just a first time home owner with a large part in the build, take my advice for what it's worth.

  3. trad4life | | #3

    Thank you Martin and Brian for the advice. Its nice to hear from the GBA community with real world experience. Brian - Thanks for mentioning the door as a tricky transition, I hadn't put much thought into that yet. The aluminum flashing detail on your house look nice and clean. On my build I had forgot to include full 1" board sheathing thickness, so the proud foam is actually only 1.5". I think as Martin mentioned, a nice custom piece of Z-flashing could meet my needs. Thanks again for the advice.

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