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Sub-slab insulation under a bearing wall?

JONATHAN SCHOLL | Posted in Building Code Questions on

There was a previous post very similar to my situation posted back in 2012….I am looking for any new or additional clarification on this question. I have two interior 2×6 bearing walls, 30′ long which my engineer says must be able to support 700 lbs per foot. He requested a footer with the slab turned down to sit directly on the top of that footer. I would prefer to run my 2″ thick EPS insulation… rated at 40psi….continuously across the whole bed of 4″ crushed stone which would also run across the top of the footing. My math indicates that the 40psi insulation can handle 2600 lbs per ft applied by the 2×6 wall above…almost 4x’s the load capacity needed by my engineer. His concern is that he believes CODE requires that the slab be connected to the footer…..can anyone confirm or deny this for me? He says that if I can get the local inspector to OK this detail than he is fine with it.

Any comments or suggestions welcome!

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Replies

  1. Stuart Friedberg | | #1

    That's an interesting question, and I don't know your code-specific answer. If you were talking about the perimeter footings, there would be no question that you'd have to tie the footings to the perimeter stem walls. Whether or not it's code, it would be equally inconceivable to make an interior footing with a stem wall resting directly on the footing, without tieing the two together.

    But you're got the intervening slab, insulation and rock. It's my impression that we generally don't tie a separately poured slab to a underlying footing. But that may simply be because it is much more common to thicken a monolithic slab when the slab needs (perimeter or interior) footing support, which renders the question trivial. Certainly we don't tie a separately poured slab to underlying perimeter footings, and we often insulate between the two. However, in that situation, there's no question about the slab bearing any significant load right up against the perimeter walls.

    IRC chapter 4 wants your interior 2x6 load bearing wall to be anchored to the foundation. It's not obvious that the slab is the foundation in your situation where an interior footing is required. And the required length of anchor bolts may require you to anchor into the footing anyway! 7" is a typical requirement. A typical 4" of slab with 6" of underlying insulation and rock isn't going to meet that. And in a seismically active area (relevant for me, but maybe not for you), even if you went with 13+" anchors that 6" weak gap (weak relative to concrete) is going to require an engineer to scratch his/her head a bit. For commercial construction where things must be securely anchored in relative thin slabs, there is engineering detailing for "shallow podium anchorage", but relatively thin in that context means at least 10" thick slabs. However, an engineer could probably scale that down for you as loads and forces for a house will be much less than for commercial construction with parking floors, etc. If your engineer is happy with the anchorage of the load bearing wall to the slab, you could discuss this with your building department as a non-standard but engineer-stamped detail.

    The other concern is the stability and load bearing capacity of your rock layer. If you are using round rock to maximize voids and radon/drainage permeance, the rock layer is quite capable for shifting slightly out from between the insulation and the footing below. If you need the extra load bearing capacity of the footing, you might not have it, and you can't inspect for it.

    It's a bit fussier than your intended plan, but can you excavate about 6" deeper and 12" broader for the interior footings, put down your 4" rock layer, then your 2" insulation cut to form fit the footings (use a few rebar stakes to keep the insulation slabs in place), then vapor barrier, then pour the slab and footings as a monolithic thickened slab? Then nobody should have any issues.

  2. JONATHAN SCHOLL | | #2

    Thank you for your detailed response Stuart. I am a mechanical engineer by schooling and never saw this concept as being so involved. I have built a lot of structures over the years and I tend to over build but this issue just seems to take things to a level with which I am not familiar. For a crazy frame of reference, the house we are building was intended to use some existing masonry walls from a previous house which had stood without compromise for 40+ years. When we went to demo the slab...because it was not well done and we wanted to put plumbing underneath....we came to find that the foundation was non-existent! There were no frost walls or footers!! The house had been built directly on the 4" slab which had been poured on virgin soil and some stone dust. So with that as a guide, why is my desire so hard to sell?? Unless someone else comes alone to assist, I suppose I am left with no alternative but to interrupt the insulation and turn the slab down onto the footers to satisfy my engineer. Oh well.....

  3. GBA Editor
    Martin Holladay | | #3

    Jonathan,
    It would seem to me that you need to talk to your local code inspector next. Perhaps some information from this article (or the comments under the article) will prove useful in that discussion: Foam Under Footings.

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