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Position of outside plywood sheathing

deerefan | Posted in General Questions on

Have finally completed board formed concrete walls on the house…will begin placement of metal columns and beams later this week and then onto framing.

I have a question regarding the exact structure of the wall where the stone will be placed. I plan on using 2×6 studs, 1/2″ plywood, 1″ XPS or polyiso rigid foam, 4″ stone. My stone ledge is 5.5″ wide and so I am trying to figure how to make this work. Can I move the base plate in by 1/2″, having the 1/2″ plywood sit at floor level and then overlap the rigid foam on the stone ledge(using it as a rain screen)? Or does the plywood sheathing have to sit over the stone ledge, which would mean I would have to use thinner rigid foam insulation? How much space behind stone (it is all 4″ saw cut so no irregularities there”) is enough? Thank you for your help.

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  1. Expert Member
    ARMANDO COBO | | #1

    Your plan for pushing the sheathing on the floor side by 1/2" would work as long as you have good flashing details. The rigid foam can be a WRB if taped with approved tape. Typically, you want 1" space to allow proper mud cleaning space, plus the mason's fingers. You can overhang the stone outside of the foundation by up to 1" with proper tie installation. Usually, masons overhang stone/brick when foundation is wavy anyways.
    You can always go to 2x8 brickledge if you feel more comfortable.

  2. deerefan | | #2

    Thank you for your response.

    The brick ledge is 2x6 (5.5") already so too late to change it. Given that, do you think it is worth it to add the 1" rigid foam on the outside of the sheathing or not put it at all? Do you think the potential problems created outweigh the benefit? I appreciate your opinion on this.

    If I do go ahead, my plan would be to use an EPDM gasket under the base plate, 1/2" plywood sheathing even with beginning of brick ledge sitting on a z-flashing, cover sheathing with delta vent house wrap, 1" rigid foam insulation outside house wrap.

    If you agree, is there a particular z-flashing that you would recommend? Plastic or metal? Also would I need to add a gasket/seal between z-flashing and side of baseplate at the perimeter? Thank you again.

  3. Expert Member
    AKOS TOTH | | #3

    I'm in Zone 5 and 2x6 with 1" of exterior foam is a very common assembly. You do need an interior vapor barrier.

    The 1" foam bumps up your assembly effective R value by 20% to 25%, depending on your heating fuel costs, this could be worth it.

    I would go with one of the vapor permeable exterior foam options, either EPS or permeable polyiso. You don't need any fancy WRB, a simple house wrap works well enough, one of the crinkly ones create a bit more space for an extra bit of drying capacity.

  4. deerefan | | #4

    I am in zone 2, so no interior vapor barrier here. I plan on using XPS or polyiso for rigid foam. One of the reasons I would like to use it is to decrease condensation on the framing. I think this may be more important for me as I have over 20 structural metal columns and beams that may be particularly susceptible. I look forward to hearing about some real life experiences from builders on here. Thank you.

  5. Expert Member
    ARMANDO COBO | | #5

    To start off, you are correct, in TX we don’t use interior vapor barriers, and also, around Austin’s CZ 2, 1/2” polyiso would be enough to do the job, but 1” is always better. That’s what I spec on my jobs too.
    Now, if you are going to use Z-Flashing, I would use a 1.5” aluminum z-flashing and set it all back. You can use the EPDM gasket, then install some calking or sealant under the flashing edge to make sure bugs don’t get in. Then install the sheathing and 1” of foam/polyiso about 1/4” off and be flush on the brick ledge, now you have the full 5.5” for the stone and plenty of space to adjust for any foundation waving.
    Even though using an approved tape with the foam/polyiso makes it a WRB, I always like to cover the foam/polyiso with a taped housewrap on top to protect the foam till the mason gets there, and it’s also cheap insurance.
    Here is a good reason why I develop full specs along with my set of drawings BEFORE the building permit is pulled, in that way, we try to minimize most, if not all issues, ahead of time. It saves a lot of downtime and possible mistakes on the jobsite. Also, when a client wants to use Austin Stone or any other cladding wider than 3” or 4”, then I go to a 2x8 formed brick ledge.

    1. deerefan | | #6


      Where I am, when I even mention outside rigid foam, people look at me like I am crazy. I had this house designed along with an architect and structural, MEP engineers - I described what I wanted in detail, but am still running into issues because the tendency is for people to design the same way they have always done... I ended up firing my builder as he simply refused to follow plans or pay attention to any kind of detail. He just wanted to do the same as always, patch the problems at the end and make money quickly. Now, I am organizing and supervising everything on my own, I am confident that with help and time I can get this done well.

      As far as setting everything back, I think that would be out of the question as the anchor bolts would be way too far off the center of the base plate. If you agree, would you then suggest a 1/2" flashing, setting sheathing flush and rigid foam overlap the brick ledge?

      Second, I read a lot about the house wrap being placed on sheathing and rigid foam outside. Why do you suggest the wrap on the rigid foam? Does that change if the foam overlaps the brick ledge?

      Thank you sincerely for your help.

  6. Expert Member
    ARMANDO COBO | | #7

    If you agree, would you then suggest a 1/2" flashing, setting sheathing flush and rigid foam overlap the brick ledge? YES
    Why do you suggest the wrap on the rigid foam? Because you would be using the wrap to protect the foam from UV till the mason gets there, plus it helps stopping the inward solar vapor drive once the stone/brick is up. I'll add that it also makes it clean to wrap the window and door install with the WRB.
    Does that change if the foam overlaps the brick ledge? NO

    1. deerefan | | #8

      Thank you very much. What tape do you like to use at seams of rigid foam. Also, what sealant would you use under the flashing edge?

  7. Expert Member
    ARMANDO COBO | | #9

    The tape for the foam depends on which tape the manufacturer has approved to make it a WRB. Dow, Owens Corning and other foam mfg. have their own tapes. If you use a foam that has no approved tape to make it a WRB, then you can use any tape, from zip to Tyvek to 3M, etc.
    Any polyurethane calk will work well under the flashing edge... Loctite, Tremco, Master Seal are good brands.

  8. deerefan | | #10


    Have you had experience with using Advantech for wall sheathing. What is your opinion about it. I have been researching different 1/2" plywood options and there is some concern regarding delamination. Is there a particular type of 1/2" plywood you prefer?

  9. Expert Member
    ARMANDO COBO | | #11

    In my opinion, Huber’s sheathing, subflooring, tapes and flashing are some of the best in the business, but expensive. So you could say ”you get what you pay for”, “if money is no object…” etc., etc. In my world, most of my clients, Builders and homeowners, want the high-performance of the building envelope at the beer budget price, bearing in mind that all products are as good as the installation is done.
    In my market, their 7/16” wall Zip sheathing costs the same or more than a regular OSB + 1/2 rigid foam board, which helps me with thermal bridging. You still have the same costs for taping and quality installation. Same goes for the roof sheathing.
    I don’t specify the ZipR product line, because I believe, the foam is on the wrong side of the sheathing and it makes the quality installation to be more rigorous than normal OSB + foam. Besides, when the foam is taped with an approved tape, it becomes a WRB.
    Delamination can happen to all OSB and plywood when they get wet. OSB takes longer to get wet and dry out than plywood, plywood returns closer to its normal shape than OSB, and that’s why both need good installation detailing. In general, I believe plywood is better than OSB. Not all OSBs are the same, since different manufacturers make it with different specs, just like every manufacturer of integrated air and water-resistant barrier board makes their products different. You need to talk to your sheathing provider for the pros and cons of different products, and what they can get in your market.

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