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Community and Q&A

Is this a Good Final Envelope Decision?

SLCtinman | Posted in Energy Efficiency and Durability on

G’Afternoon! I am a residential designer in Salt Lake City, Utah and after a year of researching sites as FineHomebuilding, GreenBuildingAdvisor & Bldg Science Corp & debating with a client over the pros & cons of various wall envelope options, my client and I have reached the following decision regarding exterior envelope: 9 ft extr walls are 2×6 wood stud at 24″ o.c. with spray-in fiberglass cavity insulation/DOW Brand 1″ Structural Insulated Sheathing w taped seams/Stucco Finish. Btm & Top plates caulked/sealed & all penetrations foam-sealed. Expected wall R-Value approx 25. Slab-on-Grade w continuous 2″ rigid under slab insul. (client still debating radiant heat). Metal roof ovr Felt/OSB ovr raised heel premanuf Trusses. Min. R-38 blown-in attic insul. Low-e windows, 3 ft overhangs std. FYI, the home is to be built in Southern Utah in a mixed-dry climate (nearly identical to Alburquerque, NM). Total SF: 3154

So, whadda you Folks here think about this combination?

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  1. Expert Member
    ARMANDO COBO | | #1

    It sounds like a good plan. I would add a second WRB for stucco, Tyvek StuccoWrap or #15, and/or maybe a rain screen for best performance. I don’t know what transition you have from 1”SIS panel to 2” perimeter insulation, but it would make sense to have another layer of 1” Dow blue board on top of the SIS panel; or if you are installing radiant heating, you need R10 min. insulation under the whole slab, and then you may not need perimeter insulation.


    I think the 1" SIS on studs 24" OC is a pretty bouncy substrate for stucco. I would want something more rigid to apply stucco to unless you are going with a very thin and very flexible synthetic stucco type product.

  3. Expert Member
    ARMANDO COBO | | #3

    Michael makes a good point... I would look at the insatllation instructions on the stucco you are using and see if your wire mesh nailing pattern will allow you to be nailed at 24" o.c. with a three coat stucco, if it is less than that you'll need to use wood sheathing. In NM we use plywood or OSB with regular blue board on top of it.

  4. GBA Editor
    Martin Holladay | | #4

    I would pay more attention to windows. The phrase "low-e windows" doesn't tell us much.

    Aim for a low U-factor for all your windows. Choose glazing with a high SHGC for the south side; the east and west sides should get glazing with a low SHGC. The SHGC of your north-facing windows doesn't matter much, so install either type there.

  5. SLCtinman | | #5

    Thanks all for the comments. First, I think the comments about the stucco is relevant and I will look at this the realm of windows, we are indeed specifying windows with approp u-vales for region & orientation. I didnt include every bit of info in order to keep down post length...and DOW claims their SIS product does triple duty: "STYROFOAM SIS™ Brand Structural Insulated Sheathing combines structural lateral bracing and transverse load, insulation and water-resistive barrier properties...the 2" rigid insul I mentioned is under-slab & continuous - so there is no "transition" from the 1" SIS panels to the 2"...THANKS AGAIN EVERYONE!

  6. davidmeiland | | #6

    Someone clarify the foam thickness issue for me. Is the climate for this build mild enough (appears to be ~4300 HDD) that 1" of exterior foam is adequately warm on the inside face so as not to condense? I recognize that there is not wood sheathing installed here, just the foam, but if there were condensation it might just accumulate at the bottom plate of the wall.

  7. SLCtinman | | #7

    Im thinking that condensation at btm plate shouldnt be an issue - even if moisture does get in there - since exterior face of stud framing is set flush with fdn wall. Bottom of SIS panel is below that point. Proper flashing details should prevent any further worry. Again, tho, the SIS panel advertises itself as avail with WRB applied to both faces...

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