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Seeking Feedback from ICF Pros or Engineers on this Construction Detail

alex_coe | Posted in General Questions on
Hey guys, I have a unique ICF design detail I’d love some feedback on. I’ve attached the CAD drawing.

Slab on grade construction. The building consists of ground floor ICF, then second and third floors are wood frame residential construction.

My plan is to pour 12′ high Nudura ICF 6″ walls with a taper top. Next I would build the upper I-Joist floor system as detailed. Then I would pour a 2″ concrete floor topping (25psf load) which would wrap around the rim joist and tie into the ICF core. This would allow  a very effective level of sound & fire isolation that is not too costly in materials or labor. No joist/ledger hangers to deal with and continuous concrete!

Is there anything problematic with this detail, the bearing areas, and loads involved? I know it’s not typical to have the concrete wrap around the wood framing in this manner. Does it look good as is? Is the continuous concrete ok or should I create a full depth joint (or 25% depth control joint) on the inside or outside of the interior double stud wall frame?
I’m going to get an engineer to go over everything eventually, but trying to carve out the details as best I can first.

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  1. Expert Member
    Michael Maines | | #1

    What is your climate zone? You have very little insulation between the concrete floor and the exterior.

    Structurally it looks fine to me but definitely involve a professional engineer.

    1. alex_coe | | #3

      Hey Michael, nice to have you check this out! It's CZ 5. Heating 99% Outdoor Dry Bulb 16F

      I've definitely wondered about how much cold will transfer from the wall through that 2" of concrete. Should I do something about that? Any suggestions?
      I plan to use LVP or laminate flooring as finish.

  2. Expert Member
    Peter Engle | | #2

    A far more common approach would be to build the wood floor platform as is normally done, with joists running out to a rim joist, and the stud wall bearing on the platform. If you want a concrete floor, pour lightweight gypcrete on top of the wood platform. This allows for much better perimeter insulation. Also, your detail will definitely crack at the joint between the floor and perimeter concrete. I'd want a full depth control joint there for several reasons. As long as you're going to do that, I see no advantage to having the concrete wrap around the wood.

    1. alex_coe | | #4

      Hey Peter, thanks for checking this out!
      The reason for the uncommon detail is I need at least 2" of concrete wrapping around the rim joist for strict sound isolation requirements of the ground floor music studio. And the concrete for the floor above must be regular weight concrete. It's being used strictly for the isolation mass, not as finished flooring. A control joint in line with the rim joist does seem like a likely inevitability though I was thinking. Just wasn't sure if full depth or partial depth.. and if it would be good to still have some structural connection there via wire mesh.

    2. Deleted | | #11


  3. charlie_sullivan | | #5

    You've gotten a good summary of the drawbacks of this approach. You could simply go back to a standard approach, but I think it's also worth considering what you hoped to achieve with this, and discuss other ways to get there. You mention sound and fire isolation and cost. Mineral wool boards and gypsum boards are great for fire isolation, and are also good as components of sounds isolation systems. Where to position them depends in part on what your sound isolation goals are--keeping outdoor sound out, indoor sound in, isolation between floors? Similarly, if the first protection objective with regard to wildfires outside or fires propagation between floors?

    1. alex_coe | | #6

      If I could just use a standard approach I would, but I need at least 2" of concrete wrapping around the rim joist for strict sound isolation design requirements of the ground floor music studio (a separate multi-layered inner shell & ceiling not shown). That goes for sound transmission between floors as well as adjacent residential. This building is inches from the property line in a mixed commercial/residential zone.

      If all it takes is making sure to include a control joint where needed that's an easy solution. At that point it just reverts back to the question of whether problematic thermal bridging is taking place at edge of the floor. I hoped with the continuous R11 on the outer wall and a control joint it could be a non-issue. But I'm unsure on that.

      1. charlie_sullivan | | #7

        Thanks for clarifying the priorities! I'm afraid that I don't have enough experience with acoustical control to comment on the specifics of what's needed for that, but given that application, I think the level of thermal bridging is acceptable.

  4. krom | | #8

    At this point, why not do a structural concrete floor, and ditch the wood altogether??
    Many icf companies have a floor/roof product.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      i was thinking much the same thing.

  5. alex_coe | | #10

    Initially what I really wanted was an 8" thick suspended slab, but the nearest shoring rental is a few hrs away and the total costs are looking to be a bit out of budget.

    This could be an option though. I think I might be ok on the sound isolation mass required if I double up the engineered rim board. It seems pretty robust structurally. I've used 3" thick XPS at the rim joist, cut to 12" height so that 2' XPS boards can be halved, avoiding waste.

    1. Expert Member
      Akos | | #12

      Lot of this depends on what sound are you trying to keep out. If you want to avoid footfall noise to the basement bellow, than your only option is a reinforced concrete slab. No light weight wood construction with any amount of concrete topping will stop footfall noise.

      An in-between option, although not common in residential, is a steel composite deck. Needs much less shoring than a full concrete deck and still gives you some pretty long clear spans. You can also look at hollow core pre-cast if you can get a crane on site.

      If you want to limit air borne sound than you are on the right track. I would also suggest a full floating ceiling bellow, that is frame up a structural wall in the basement and put ceiling joists on that completely isolated from the I-joist above. Cover with two layers of 5/8 drywall and not much sound will make it through.

      Also watch all your utilities, for any high STC assembly, it doesn't take a lot of holes to kill the sound isolation..

      1. alex_coe | | #14

        On the sound isolation side I'm sorted everywhere else. The ground floor will be a studio, with an isolated 2x4 framed inner shell and ceiling, multilayered drywall & OSB, resting on a floating slab. Everything will be very well sealed. I just need to avoid compromising the assembly at the rim joist & floor transition too much.

        I also just remembered I have 26' of deck ledger to hang off one of the load bearing walls.. which won't work well with my detail with the 3" XPS on the outside. Probably would need Maine Deck Brackets.

  6. charlie_sullivan | | #13

    If you must use XPS, I hope you will special order the new Owens Corning NGX version that, although it still has a high climate impact, has only 1/10th the climate impact or their regular XPS, and 1/20th the climate impact of the blue stuff.

    But yes, your new design has a lot less thermal bridging. If you stuff the joist cavities with insulaltion, as you may want to for sound purposes anyway, that will help too.

  7. alex_coe | | #15

    Full detail with the Maine Deck Brackets and ground flr shell included. This looks like a workable balance of compromises in terms of structural & airborne sound transmission, thermal performance, structural integrity, cost, & labor.

    1. Expert Member
      MALCOLM TAYLOR | | #16


      Some minor points.

      - The sheathing and WRB should extend down to your mud-sill.
      - Make sure the rim joists supporting your deck ledger connection meet the code lateral force requirements.

  8. alex_coe | | #17

    Are you just thinking the sheathing should continue down and fasten at the mud sill for structural purpose? Certainly that's strongest, but we have no seismic or high wind requirements here, so I don't think it's required by code (but will be checking with the BI shortly). With a double rim joist I would have twice the uplift resistance as well. Or are you thinking along the lines of air sealing? I couldn't think of any necessity for it so I just prioritized R-value and maintaining the wall plane without adding extra steps/cost.

    I'm thinking construction adhesive between the double rim joists and sil plate, sil sealer and acoustic sealant under the mud sill, then tape or fluid flash the seams between mud sil /rim joist /sheathing. Then run the WRB continuous down the sheathing, rim, and ICF.

    1. Expert Member
      MALCOLM TAYLOR | | #18


      You may be fine structurally not running the sheathing down to the sill. I'd be more concerned with the possibility of bulk water intrusion at the Maine Deck Brackets. The rim-joist needs to be detailed so that any moisture that makes it's way back there is channelled to the exterior and can't rot the floor system. Covering that area with a layer of sheathing and an impermeable membrane is good insurance. As a general rule of thumb, having all framing protected by sheathing is a good idea,.

      Under the IRC decks are now required to have lateral resistance at the ledger. The most common approved solution is this piece of Simpson hardware:

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