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Least evil of my wall system options

Bruce Davis | Posted in Green Products and Materials on

Border of Zone 4/5. I’ve read much of the material on this website about wall systems. While I have my desires for advanced framing techniques, including 2×6 24OC and thick foamboard outside insulation, these things aren’t a realistic option with builders/subs in my area undergoing a building boom.

The path of least resistance is 2×4 or 2×6 16OC. The only other builder/framer reluctant possibility is Zip-R with built-in polyiso. My city uses the 2012 IECC, but has amended the walls from R20 or R13+5 down to R13. Yes, you read right, here in 2018, the Zone 4/5 wall code is R13.

My preference is for using the Zip for the R13+5. My question is: does the R20 of wall fill equal the R13+5? If so, it seems the easiest (and maybe the least expensive) of the two options is to spend a little extra for 2×6 (vice 2×4) and put in R20-21 fiberglass batts or R23 mineral wool batts. I think the house will already be using a fair bit of 2×6 anyway due to high ceilings on both levels of reverse 1.5 story.

The city has also amended 3 ACH code down to 5 ACH. I presume I still want to shoot for 3 ACH. If I use regular Zip or OSB+Tyvek, would closed cell foam be required for 3 ACH air sealing? If so, what’s the minimum depth to be used with wall fill? Would you recommend, say, R15 fill + 1″ R7 closed cell or R11 fill + 2″ R13 closed cell or some other value combo? Thanks.

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Replies

  1. Mitchell Costa | | #1

    I've done a lot of research for a home I'm building on the border of zone 4/5 in CA, and have settled on a pretty good version of Joe Lstiburek's "Perfect Wall" which allows more standard building methods due to more closely matching the standard 2 x 6 wall thickness. Durability is key for me since we'll be living in it for the next 50 years or more, but the design also has R value and sealing benefits over the designs proposed in your query. The wall utilizes 2 x4 s on 16" centers, but 2 x 6s to frame around the doors and windows with the extra 2 inches towards the exterior to match the thickness of 2" worth of exterior foam sheathing on the exterior. ZIP sheathing over the 2 x 4s for shear strength and as air, water and vapor barrier. Flashing of the window and door openings at the sheathing layer for the water barrier is probably the most challenging portion for the builder not used to it, but excellent articles on it are available on GBA. Two layers of 1" thick exterior foam board with each layer's seams offset from the layer below is ideal to account for possible shrinkage of the foam board over time, and increase the long term protection of the structure. However, one 2 " layer of tape sealed foamboard would still be superior to a thin ZIP-R layer, or the other walls described, due to better sealing redundancy and less thermal bridging. Furring strips of 1 x 4 screwed through the foamboard into the 2 x 4 framing secure the foamboard and provide a rainscreen while providing a good base for most sidings. With 2" of Polyiso and R-13 in the stud cavities, it counts as R-23 for the county, but whole wall performance would be closer to R-20 counting thermal bridging. Whole wall performance of the standard 2 x 6 wall proposed is closer to R-13 per the following link http://www.homeenergy.org/show/article/nav/walls/page/3/id/1517 . The standard 2 x 6 wall also has much less protected structure due to no rainscreen, and exposure to temperature extremes. Framing with 2 x 4s, and well secured sheathing for shear strength, should be fine for 1.5 stories, but higher wind or snow loads, or walls with lots of windows, or windows or doors close to corners probably warrant engineering evaluation.
    Mitch

  2. John Clark | | #2

    R13+5 is a 2x4 wall with R5 exterior insulation.

    R-20 is a 2x6 wall w/out exterior insulation.

    https://www.greenbuildingadvisor.com/blogs/dept/musings/overview-2012-energy-code

    Builders do NOT want to deal with exterior foam. It's a non-starter.

    Personally I would keep it simple and perhaps try to go for 2x6 on 24"OC if code allows (seismic zone?), use regular ZIP, skip the exterior foam. Make sure the ZIP tape is applied correctly and use their liquid flashing for overdriven nails as well as rough openings.

    IMO air sealing is of primary importance, along with airtight ceilings and lots of attic insulation.

  3. Expert Member
    Michael Maines | | #3

    Mitchell, your wall design sounds pretty good (pun partially intended) and something like the NAHB's extended plate wall system (https://www.greenbuildingadvisor.com/articles/dept/musings/extended-plate-and-beam-walls). Have you run your design past an engineer, though? The frames around windows are not just for gravity loads; they also work in conjunction with the wall sheathing to provide lateral resistance as well, so engineers can get a bit twitchy about alternative window and door frame designs.

  4. Expert Member
    Michael Maines | | #4

    Bruce, one other option that people rarely consider is to frame with 2x8s. Builders may look at you funny, but they don't have to change any of their standard practices, aside from adapting to heavier walls. There is thermal bridging through the studs but a 2x8 is about R-9 total, about the same as a 2x4 with an inch of foam on the outside.

    Looking at just the center-of-wall, including studs but not plates or frames, and R-3.6/in for insulation:

    2x4s 16" oc and R-5 exterior insulation will perform at about R-16.8, or U-0.06.
    2x6s 16" oc and no exterior insulation will perform at about R-18.4, or U-0.05.
    2x8s 16" oc and no exterior insulation will perform at about R-24.5, or U-0.04.

    Actual values will vary with insulation type and total framing fraction, bringing the 2x4 wall up and the 2x8 wall down, but maybe this will steer you one way or the other.

  5. Expert Member
    Michael Maines | | #5

    Bruce, regarding air sealing, projects I've been involved with routinely get below 1.0 ACH50, down to 0.25 ACH50, with virtually no spray foam, so it's definitely not required to reach 3.0 ACH50. Building in a relatively airtight manner may mean changing some habits, though. There are many ways to improve airtightness, and many products that can help. ZIP sheathing with all seams sealed with ZIP tape makes airtightness pretty easy; you just have to then put some attention into connections at floors, roofs, windows, etc.

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