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I’m feeling lost on wall detail for a Stucco build and airtightness

Uavmx | Posted in Energy Efficiency and Durability on

I’m bought into the benefits of an airtight home. Our codes are also shifting to 2021 IECC, which requires the exterior insulation and airtight build, less than 3.0 ACH. I’m not trying to go passive haus, but just want a well built home, and it seems like ~1.5ACH would be a good target of benefits to cost. It also doesn’t seem that hard to achieve that number with a good envelope…which is where I’m now stuck on determining the best wall detail. I have read so many articles, blogs, this website, youtube, etc and I cannot come up with a clear answer. Once I think I know what the stack up should be, I come across the next thing that makes that seem incorrect. 

I have a GC that I like and trust, he’s building quality homes, but is certainly on the “lower” end of the cost spectrum, which is needed, but generally building to code which for our region is a good home at a good cost 2018 IECC. He knows these new codes are coming, he’s not a naysayer, and is willing to do/explore whatever I would like to build, but just hasn’t done them and doesn’t have a strong opinion about the best way. I’m working with a draftsman that has done a great job for me, as I already had the design mostly figured out, however again, he hasn’t been designing to the new standards yet and doesn’t have a strong opinion. I’m fine with all of this, I love researching and learning, but this airtight wall details has me unsure of what way to move forward. What is the best way to stack up this wall detail to handle the requirements of WRB and Airtightness with all the options to do so: Zip/FF, WRB/Wrap, Exterior insulation/thermaltight, stucco air gap (not required), weep screed, exterior foundation protection, etc?!?

The home will be in Zone 5B (less than 20″ of rain, 285 days of sun), slab on grade (single pour requires insulation on exterior), 2×6, fully sheathed with external insulation and stucco exterior. Windows will be more innie (attached to OSB). What is the best approach (wall detail) for this style of build that you want to be airtight? What’s best VALUE for costs considering the ACH target? 

To me, simplifying the install seems to be smart. Meaning if you have to tape the CI exterior, do all the airtightness out there (like Thermaltight) – but thermaltight and code requires a layer between it and the stucco. Then read that the WRB should be at the window plane, so then move the WRB to the OSB (zip/ff) or house wrap -but then you’re taping again at the insulation. However the insulation is acting as the additional layer between the stucco and WRB (zip or housewrap) which is more simple. Just doesn’t seem to be a clear answer, appreciate the help!

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  1. freyr_design | | #1

    if you can afford it i would incorporate a rainscreen+lath combo system over the foam for your stucco. something like delta dry and lath

    1. Uavmx | | #3

      Thank you, I think that's overkill for 5b and less than 20" of rain isn't it? If the CI is also water proof?

      1. freyr_design | | #5

        This is a good resource. Overkill is a personal decision.

  2. Expert Member
    Akos | | #2

    Keep the air barrier at the seahting plane (so taped seams there, ZIP/CDX/OSB all work) and move the drainage plane out to the foam. This means small nail on window/door bucks (2x lumber ripped to the thickness of the foam) which is pretty simple and not much extra work.

    Once you move your WRB outside to the foam layer all the rest will now come together. You can have the two layers of WRB with drainage for the stucco.

    Exterior foam also lends itself to EIFS install. Not real stucco, but simpler and very durable. Bonus is extra insulation is pretty much free compared to install costs.

    With EIFS, in that case you can keep the windows and WRB at the sheathing plane, so closer to standard house build. EIFS does have specific details for dealing with transitions and openings. They are not hard but if not followed, the wall can fail:

    1. Uavmx | | #4

      Is the insulation taped considered a WRB? Do I have to spec a specific kind? Seems like EPS is preferred on this site, is that always WRB?

    2. Uavmx | | #6

      Just bumping this up as I'm curious on if I need to be spec'ing a specific exterior insulation to gain the WRB. I think I'm pretty set on what you're saying: Airtight OSB w/ tape>exterior insulation taped, stucco system will work well and not be overly costly. I think I'll do the airtight barrier carrying over the topplate to the inside and seal that across the ceilings. Can't imagine drywallers love doing the amount of caulking required for airtight drywall....

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