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Main Floor Cantilever

user-7049485 | Posted in Green Building Techniques on

Hello. I am working on a project where the main floor is cantilevered over the concrete foundation wall. I am having a hard time finding details showing the sealnt, barriers, and insulation for the cantilevered floor. The details I do have show a cantilever over a wood frame wall, not a concrete foundation wall.

any suggestions would be greatly appreciated. Thank you.

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  1. Expert Member
    Michael Maines | | #1

    User-7xx, can you share your name? The software isn't showing the name you registered with for some reason.

    There are many ways to handle a situation like yours. What is your climate zone? How is the rest of your house built? How is the foundation currently detailed? What types of products are you currently using?

  2. user-7049485 | | #2

    My name is Brandon, I am located in Zone 7A which is Alberta, Canada. the house is constructed using conventional platform advanced framing techniques. the project is currently in design. Exterior cladding is metal and stucco. the foundation wall has no exterior insulation, just bitumen below and parging above. we typically use a ladder sill instead of an anchor bolt and sill plate.
    Im just not sure about the path of barriers/retarders because of the ladder sill.

  3. Expert Member
    ARMANDO COBO | | #3

    Here are few options from the internet. On your final detail, replace the top plate on top of a frame wall with a sill plate and sill sealer over a concrete wall.

  4. Expert Member
    Michael Maines | | #4

    Brandon, Armando's images should be a good start. You should also read these articles:
    If you have additional questions after that, let us know.

  5. user-7049485 | | #5

    Great, thank you. My concern now is the sealing the interface between the top of the foundation wall and the floor frame since i am using a ladder sill which is embedded in the concrete rather than a sill plate.
    Here is the detail i am currently working on.

  6. Expert Member
    Michael Maines | | #6

    Brandon, I was not familiar with the term "ladder sill" but it looks like it's probably framed like a short wall, laid horizontally? Or is it just the two 2x ledgers? It's not critical to the discussion, I'm just curious.

    I assume your I-joists run through to create the cantilever, and the inner LVL is blocking between joists? Or does the outer LVL rim carry the load of the floor overhang?

    In any case, I think you're on the right track. As it stands, you have a large thermal bypass between the foundation insulation and the floor overhang insulation. You need to bring the floor insulation in to meet the interior plane of the foundation insulation.

    As for an air seal, you are showing various dashed lines. It's a good idea to use separate lines for the air barrier, vapor retarder and WRB, unless two of the three provide the same function.

    At the top of the foundation wall, you need a capillary break and an air seal; there are various membranes you could simply lay over the top of the foundation wall, with some extra material to then tie other membranes to. My first choice for this would be 12" Vycor, but there are other options. It doesn't matter which way the Vycor adhesive faces.

    I don't see much of an advantage to using XPS between the floor joists as you have shown. There is hardly any additional R-value above what you would get from thicker batts, and as people here probably get tired of reading, the blowing agents are potent, persistent global warming agents (aka carbon polluters) and the insulating value of XPS drops over time so there is no good reason to use it.

    At your above-grade walls, polyethylene is not the best choice, because it prevents inward drying. A variable-permeance membrane meets most building codes and makes for a more resilient assembly. Considering the amount of insulation you are using elsewhere, I'm curious why you're skimping on insulating the above-grade walls. A layer of exterior insulation would significantly improve thermal performance.

    At your basement walls, you are probably safe with the mix of foam and batts, though again, conscientious builders would use either EPS or more often polyiso at that location. The interior poly on the interior of the basement walls is a very bad idea, at high risk of causing moisture accumulation, leading to mold and rot. As long as the seams in the rigid foam are well sealed with tape, simply painting the interior with standard latex paint should be enough to prevent moisture problems. If your building code requires a class 1 vapor retarder, any of the variable permeance membranes meets that requirement. If your code specifically calls for polyethylene, and I think I've read here that Canadian code does just that, I don't know what to tell you.

  7. user-7049485 | | #7

    Thank you for your recommendations Michael.

    The ladder sill is just as you described it.

    My joists are continuous and the lvl's are just blocking.

    My code requires an RSI value of 3.46 for below grade foundation walls and 3.08 for above ground and i am just coming in at slightly above the minimum with 3.51 (below grade) and 3.2 (above ground). if i go any higher the developer will complain about unnecessary material and labour costs resulting in me having to do more revisions. if it were up to me i would have exterior insulated walls as you recommend.

    based on your recommendations i will find a alternate to the poly, remove the XPS from the joists and increase the batt size, provide a capillary break and air seal at the top of the foundation wall, and remove the poly from the basement walls.

    Thank you for all your help. energy codes are fairly recent in Alberta.

  8. Expert Member
    Michael Maines | | #8

    That all sounds good, Brandon. Feel free to post your updated assembly when you're ready. There may be other fine points we can help you tune. As for insulation, "developer" says it all, unfortunately. I'm curious why the below-grade R-value requirements are higher than above-grade; the opposite is true in the US and makes more sense to me.

    A couple more notes:
    You didn't ask about framing, but you're probably aware of the need to beef up the I-joist webs at cantilevers, right?
    OSB or LSL rim stock is cheaper than LVL rim stock.
    If the design allows, it would be good to add some continuous insulation below the joists. If it's available where you are, Zip-R is good for this. Any thickness will help and shouldn't cause problems in this situation. But "developer...."

  9. BlackHawkDesigns | | #9

    Brandon, just finding this now in trying to do a similar thing but to have a 4" real stone wall facade in front of my home. Would the same thing apply?

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