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Wall design

dasrks | Posted in GBA Pro Help on

Hello, I am building a house in zone 5 and need some advice. I am using 2 by 4 walls with blown in cellulose, 7/16 OSB sheathing, Henry Blueskin VP 100,  2” GPS insulation, 3/4” furring and then cement board siding. Is this too tight? The house got wet several times  during construction and the wood molded on some of  the walls where the Blueskin was installed even without any foam installed. I Removed the Blueskin to allow for drying and to remediate the mold.  The roof is finally finished and the house is pretty well dried out, but now I’m afraid to put the blue skin back on …..Advice please

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  1. Expert Member
    Dana Dorsett | | #1

    In order:

    1: No. In zone 5 it takes a minimum of R7.5 on the exterior of a 2x6/R20 wall for dew point control at the sheathing (2-1/8" NeoPro HD is R10) and the cellulose will harmlessly buffer and redistribute most of that moisture. The only interior side vapor retarder needed is standard interior latex paint, but on shaded or north facing walls using half-perm "vapor barrier latex" primer would be cheap insurance. A true vapor BARRIER would reduce rather than enhance the resilience of the asssembly- stick with paint.

    2: Yes.

    3: In most cases yes. A 2x6 16" o.c. wall cantilevered 2" off the foundation is at least as strong as a 2x4 wall set flush with the foundation. But it's more common to do this on the exterior of the foundation to adjust for thicknesses in sheathing & siding. Installing the foundation foam at the same plane as the framed wall's foam does a better job of thermally breaking the foundation sill, despite the need for installing Z-flashing at the transition at the top of the foundation. But it takes more than R10- the IRC code minimum climate zone 5 is R15 continuous insulation. A common solution is to use insulated concrete forms (ICF) with EPS on both sides. Even a Type-II EPS 2" + 2" minimalist ICF is nominally R16.8. Many ICF vendors start at 2.25" + 2.25" (~R19)

  2. Jon_R | | #2

    Lstiburek's recommendation here is for a Class II vapor retarder (because of the lower external drying ability of the Neopro foam). But I haven't seen much data showing that this is better or worse than the Class III that code allows in your case. This (page x) suggests that Class III has higher risk. Lstiburek reserves Class III for flow through assemblies (in Z6/Z7 this is exterior mineral wool, in Z5 unfaced EPS can also suffice).

  3. dasrks | | #3

    Thank you so much for your input. Code in my jurisdiction requires a minimum of 10.2R (Welcome to Michigan) for the basement walls. If I go to the exterior I was concerned about the concrete walls that leave the conditioned area i.e the porch and garage take offs that wouldn’t have any thermal break. The offset of the wall towards the inside would allow for the whole 2 by 6 to bare on the foundation. Thanks again this was my first blog post and I appreciate your help.

  4. Expert Member
    Peter Yost | | #4

    Just a quick note on the BSC article: note the representative interior relative humidity used for each climate zone. Results will vary under different assumptions about interior moisture regimes (temperature and RH).

    In reality--and Lstiburek states at the very beginning of the article that the recommendations are based field experience, lab testing, and hygrothermal modeling--it's like the approach of Captain Barbossa and the "parle" code of pirates: "really more of a guideline..."


    1. Jon_R | | #5

      On that issue, Lstiburek writes "not the actual service conditions for typical residential occupancy – but the design conditions for the simple steady state design procedure being used". Open to interpretation, but I take this as "I know these assumptions aren't always true, use them anyway".

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