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Slab poured inside framed wall above foundation wall

Last_but_not_least | Posted in Energy Efficiency and Durability on

I am currently in the planning stages of building a small retirement home (not passive but pretty good) and working on wall details. I’m considering a single story with double wall construction and trussed roof spanning the full width of the house requiring no internal walls prior to being closed in. Under-slab mechanicals and the floor itself will be installed once the house is closed in. The walls and scissor trusses will have interior OSB as an air barrier and vapor retarder. With that background, my real question concerns the pouring of the slab above the concrete stem wall, up against the framed wall. The top of the under-slab insulation will be flush with with the top of the foundation wall. The plan would include a 4″ tall piece of 1″ foam up against the base of the studded wall and a 1×3 screed sandwiching the foam against the wall. This entirely separates the slab from the stem wall which will only be insulated on the inside of the foundation. At any rate, does anyone see any issues with that approach, structural, moisture, etc. There will also be a firred out service cavity inside the OSB air barrier on the walls so the foam and concrete screed will not be exposed. The wall detail at doorways also provides an insulated wooden threshold further isolating the slab from the exterior.

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  1. Expert Member
    Michael Maines | | #1

    Roger, what is your climate zone? The only aspect of your proposed assembly that concerns me is that the bottom of the wall will be relatively cold and close to moisture sources, so there is a risk of mold and fungal growth.

    1. Last_but_not_least | | #2

      Hi Michael, thanks for your feedback. I currently live in the Chicago area, but we will be building this spring in Colorado. The environment is still new to me but the home will be built in zone 5. 6200 feet - dry plains - 1.5 hrs southeast of Denver.

  2. Expert Member


    Its an interesting idea. A section would be useful to understand all the implications.

  3. Jon_R | | #4

    Consider not using a slab at all. Just plastic sheet, EPS, plywood, then whatever flooring you want. Preferably the plastic sheet would extend over the stem wall and to the exterior (so it doesn't let soil moisture get to the wood). Being completely above grade also reduces moisture concerns.

    Probably best to ask the inspector what he thinks first.

  4. onslow | | #5

    Mr. Erber, I would urge you to have a soil analysis done of the site with a profile of the materials you will be building on. There is significant risk of being in an area with expansive clays. There is also a risk of radon presence, though it is much higher risk over here on the west slope. Here is a link to the Colorado Geological site that publishes a book of likely clay profiles.

    I would caution that from personal experience that soils can vary wildly over very short distances. Think ancient stream beds and river bends that are covered over by time. The book maps may only be useful as a general guide. However, if you are the lucky winner in the expansive clay sweepstakes, you will find it more properly called expensive clay.

    I do wonder how you will achieve a double wall construction without pouring a very wide stem wall and why you would want to in the first place. If the idea of an isolated slab is the goal be aware that cracks will likely become part of any floor treatment. Adding fiber to the pour for cracks may affect stamping processes. Not sure about the risks with stains or colorants. You will still need to insulate the slab for practical energy reasons.

    I do not subscribe to the utilities under the slab approach having been involved with some very difficult to deal with failures in a slab house NW if O'Hare. Jon R's idea of using plastic, foam, subflooring over soil doesn't improve the access to pipes and drains and additionally requires great care during assembly to not violate the moisture protection barriers. Construction workers are not ballet artists.

    There is a common form of low crawl space used this side of the mountains that I imagine is also in use on the front range. The perimeter stems walls are about 30" and one or two ground beams are placed where needed to keep the joist spans suitable for 9 1/2" TJI. I would spring for the 12" personally. Convincing the crew to rake the area inside the perimeter and have a radon grade plastic barrier set before the floor deck is done would be a better approach in my opinion.

    As Mr. Taylor notes a section view would help a lot. I would also plan for more wind and very high summer temps in your planned area. It would also be a good idea to ask about termites despite the drier than Chicago environment. You will find it much nicer than the muggy summers and soggy winters I am glad to call a memory now.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Mr. Taylor is my father, I'm just Malcolm. Cheers, and Merry Christmas!

    2. Last_but_not_least | | #7

      Soil analysis is underway. My son who already lives in the area sent me a photo of the backhoe out there two days ago. Yes I've read about the expansive soil. Hope to dodge that bullet. I will put up a wall section later today. Right now I'm taping drywall at my other son's home.

    3. Last_but_not_least | | #8

      Muggy summers, soggy (cloudy) Winters and year round taxes. But good soil!

    4. Last_but_not_least | | #11

      Here is the wall section at the slab.

      1. Expert Member
        MALCOLM TAYLOR | | #12

        Thanks Roger,

        The only downside I can see is one of the advantages of using slabs is it affords the opportunity for flush entries, which this precludes.

        You might consider using a 1"x2" screed and bringing the interior 2" insulation up to the underside of it, or eliminating the wood screed altogether and using the foam instead.

  5. onslow | | #9

    To Malcolm,

    I was raised by old New Englanders, and yet after too many decades I still expect to get a clomp in the head for using someone's first name without permission. Not having met yet, the classical conditioning response kicks in. Merry Christmas to you as well as Roger, the pending Coloradan.

    1. Expert Member
      MALCOLM TAYLOR | | #10


      Try moving out here to the West Coast where people you have just been introduced to give you a hug!

  6. Jon_R | | #13

    +1 on being concerned about differential movement between the slab and the walls - especially with some of the wall overlapping the slab. And +1 on considering the usual trade offs of slab vs crawlspace. Avoid XPS (use EPS).

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