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Zone 4a wall design – good enough and without unintended consequences?

BrunoF | Posted in General Questions on

Looking for a Good Enough Wall design for Zone 4a (almost 3a), central NC home. I apologize for having so many posts with similar elements but between the forum responses, my online research and my reading, I continue to have more questions and want to be sure that I build the best structure that I can afford.

Part of what I consider a “good enough wall” is one that is overall easy to build and thus not to exotic.  I cannot afford to build a passive house or use a high-performance home builder.  I will be using an experienced contractor who is excited to work with me on building a better home but the option of “just ask your builder” won’t work to address my questions / concerns on the wall design.

The books that I have read so far which I am basing by understanding of building science on are:
– A House Needs to Breathe…Or Does It:  Allison Bailes
– Builders Guide to Mixed-Humid Climates: Joseph Lstiburek
– Essential Building Science: Jacob Deva Racusin

In addition to the books, I have read many articles from GBA and the JLC. I am in 4a, Central NC but climate zone 3a is only 15-20 miles away to my east, west and south.

I planned to have a sealed & insulated crawl space with 2×6 framing supporting the following wall design from outside to inside: fiber cement siding, drainable housewrap, taped and sealed OSB, fibergass batting, drywall.  I plan to use the OSB as the air control layer in the walls and I will use the drywall as the air control layer in the ceiling of the 2nd floor.

In almost all of the example wall designs in the books mentioned, they show some exterior foam board insulation which is not something I have considered and isn’t something I have ever seen done in this area.  In the book about mixed humid climates, it explains that the foam is used to keep the sheathing on the warm side of the enclosure so that it remains above the dewpoint of the interior air and prevents condensation within the wall cavity. Condensation within the wall cavity is not something that I have considered yet but I also do not know how relevant it is in my geographical area with the wall I outlined above.

The wall I specified isn’t really that fancy…it will just be thicker and have better air-sealing than what is considered “typical” here…so does it really need a sheet of foam over the sheathing?  Will this wall I specified perform better than a code-mandated 2×4 wall?  Are there any unintended consequences of this wall that need to be addressed?

Again, I am trying to keep it simple so that I don’t run into problems during construction where the crews are unfamiliar with the techniques and that i don’t spend money unnecessarily. Thanks!

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  1. Expert Member
    Michael Maines | | #1

    Bruno, it would be a lot easier to read your question if you could break it up into paragraphs.

    Have you read this article:

    Unfortunately, in many areas, building better than code-minimum IS exotic, enough that you may not have seen any examples. That doesn't mean you shouldn't pursue better building. Somebody has to be first.

    I'd suggest that Zip-R is made for your situation. I don't love it in colder climates because installation gets tricky, but you could do Zip-R6 over a 2x6 wall insulated with fiberglass batts and have excellent results. A big advantage of Zip-R is that it barely changes standard operating procedures. Zip R-6 over a wall with R-20 batts meets 2021 IRC requirements and is a safe ratio of interior to exterior R-values for condensation concerns.

    1. BrunoF | | #2

      Michael, thank you. My original post had paragraphs but I'm not sure where they went so I have edited it and hopefully now it is easier to read.

    2. BrunoF | | #3

      Michael, I just had a look online about the Zip R...isn't the foam on the wrong side of the sheathing?

  2. walta100 | | #4

    Consider the “pretty good house” It seems to me to be more about making investments in the house that make economic sense without gatekeepers collecting fees and awarding plaques.
    This would add the pretty good house book to your reading list.

    I would not want Zip+R on a house in a hurricane and or seismic zone.

    My zone 4 wall from outside to inside is insulated vinyl siding, 1 inch foam, Standard Zip sheeting. 2x6 frame filled with damp spray cellulose, drywall, paint.

    A nice thing about zone 4 is there are no bad wall that can collect moisture and rot.

    From an economic point of view, no to houses are the same and X R value is optimal for every house in that zone.

    I used a computer model of my home for almost every choice called BEopt.

    BEopt program is free to use but requires about 20 hours of time to learn and use.

    BEopt Training video


    1. BrunoF | | #5

      Thanks for the links! I do actually have the PGH book but I didn't find it as useful as the ones I listed. The book probably should have been titled "How Create a Better New England House", I think all the examples and cases were in super cold climates in New England.

      I will have a look at BEopt..thanks!

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