IECC code question
Hi Everyone! First time long time! Im building an addition to a home on Peaks Island Maine (Climate zone 6) and am finding myself confused by a few things.
The addition is a 2nd floor dormer over an existing bump out. Ill start by saying that I have never worked with Zip-R but plan to for this project. With my lack of experience with it, I was not taken aback by the fact that my engineer called for a layer of 1/2″ sheathing under the zip-R12. Truthfully I can see the benefit from a shear strength perspective but in practicality this wall is getting pretty thick. It will be framed with 2×4 to match what is existing below and my plan was to put rockwool r15 in the bays and zip r12 on the exterior to satisfy code. This puts me at needing 4″ nails even without the 1/2″ initial layer of sheathing. So my questions are:
My understanding is that that zip-r satisfies the same shear strength requirements as good ol regular zip, is this true?
If I downsized to the zip-r9 with my r15 in the wall would this satisfy IECC code of 13+10?
If Im overlooking anything please feel free to point it out! Im fairly new to the building world, but trying to do the best I can to build a healthy home for my family and I, thank you!
Note: Peaks Island is a neighborhood of Portland, Maine who adopted IECC as law of the land, hence my compliance with that as opposed to IRC.
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Replies
Zip-R has some limitations on where it can be used as a shear wall: https://www.huberwood.com/uploads/documents/technical/Code-Recognition-for-Insulated-ZIP-System-R-sheathing-ESR-3373.pdf.
Even without the additional layer of sheathing, Zip R-12 requires longer nails than most nail guns can handle; the nails need to penetrate the framing by at least 1 1/4". Zip R-12 is about 2 1/2" thick so the nails need to be at least 3 3/4" long. With the additional layer of 1/2" sheathing, you would need 4 1/4" nails. There are a few nail guns available that can shoot those nails but they are powerful and can be hard to place the nail heads at the right depth. I have talked with multiple Huber reps about this, including one just a few days ago; they know it's a problem and they are working on a solution. I like Huber and think that Zip-R is a good solution for certain situations, but I currently do not spec or recommend Zip R-12 because of this issue.
Zip R-9 is a nominal R-9, so it does not meet the prescriptive R-10 requirement for continuous insulation. In fact as it (or any polyiso foam) ages the R-value drops, and it drops in cold weather, so for long-term resistance to moisture accumulation (aka condensation) it's safer to consider it R-7.5 to R-8.
For energy code compliance, you can use the U-factor alternative. https://www.greenbuildingadvisor.com/article/all-about-u-factor
Maine's building code, MUBEC, references the 2015 IRC and 2015 IECC. Portland adopted the stretch code which still references the 2015 IRC for most things but the 2021 IECC (which is part of the 2021 IRC) for energy-related things.
Note that the energy code requirements do not necessarily result in an assembly safe from condensation. For that, you need to cross-reference with this section: https://codes.iccsafe.org/content/IRC2015/chapter-7-wall-covering#IRC2015_Pt03_Ch07_SecR702.7, and/or find someone who knows how to do an accurate hygro-thermal model of the assembly to show that it will be safe from long-term moisture accumulation.
". I have talked with multiple Huber reps about this, including one just a few days ago; they know it's a problem and they are working on a solution. "
I hope their solution isn't 4.5" structural screws at 3" OC :|
Thank you Michael! This is super informative. Im trying at all costs to avoid spray foam for both health and environmental reasons, but am struggling to think of a better way to achieve compliance. Maybe if I frame 2x6 will be the answer, but the unsightly overhang, or loss of space on the inside gives me pause. Back to the drawing board I suppose, thanks again!
Unfortunately, sometimes spray foam is the best choice. I'm wrapping up design on a major renovation on Spruce Head that will have Zip R-9, a flash coat of closed-cell foam and the remainder of the existing 2x6 studs filled with cellulose.
I have come to like a bit of overhang; the shadow line provides some visual interest and old homes in Maine often have the siding 2-3" over the foundation. On some Passive Houses I have seen up to 8" of overhang, which can look fine but it risks a "floating wall" look.
If you decide to build inward, you might consider the Bonfiglioli approach (https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf). I have done it successfully with existing 2x4 walls. In fact we did it on this project: https://foursevenfive.com/blog/from-post-office-to-postcarbon-future-with-emerald-builders/, plus we added exterior wood fiber insulation. The code does not require the continuous insulation to be on the exterior or continuous so it might be a good option for you, with your 1" exterior EPS.
Thanks again so much for your insight here Michael! We’re going to end up using the r-9 plus an additional layer of 7/16 zip for shear. The distinction I wasn’t making was that you only need 1.5” of penetration on your shear layer, which in this case will be the 7/16, then I should be able to get just over an inch on the r9 layer. We calculated the u factor on the r9 zip and r15 wall to be .44 juuuust meeting code. I really appreciate your explanations if all this stuff, it’s a new world and hopefully leading to a better one!
Look at the zip -R approval report- if used for shear - all panel edges must be blocked and edge nailed -even on the east coast.