Zone 6 – Best wall without exterior foam
Hi All,
This is a great forum. I’ve been reading a lot over the past year but finally signed up for access to all the additional information.
I need to make a decision about the wall assembly in my 1.5 story new-build house in Climate zone 6a.
My first choice would be to use Zip R-6 with dense pack cellulose in 2×6 walls. I don’t want to go any thicker on the Zip because of concerns about racking resistance.
My second choice is just going with osb/plywood with taped seams or standard Zip system. Then I would do 2” of closed cell + dense packed cellulose in the cavities. This is what several of the builders in my area are doing. But they aren’t using an interior vapor barrier.
In both cases, I would use LP Smartside with a rain screen, taped seams on the zip sheeting (if using zip) or well-installed Tyvek (if using osb or plywood). Well sealed and painted 5/8” drywall on the interior.
I realize these aren’t super high performance, but they are in my budget and my contractor is comfortable executing them.
I’ve read until I couldn’t think straight. It seems that with the small amount of exterior foam I’ll have with either of these options, that I will need to exclude vapor from the wall cavity using a semi permeable barrier under the drywall, to limit condensation when it is very cold outside. But I would like to avoid it if possible. My concern with the interior vapor barrier is that with OSB or especially the Zip R on the outside, it won’t allow enough vapor passage and I’ll end up with a wall that can’t dry.
Once again, thanks to all here. This is an excellent forum.
Dan
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Replies
Great question real world meets FHB LOL- you will probably have some responses insisting on min 1/3 of your wall insulation being exterior in climate zone 6. Zip sheathing has come way down in price- and i don’t see any sheet wrb that isn’t self adhered that is competative. Price Grace,VCR and Henry blueskin for SA wrb that arent insanely expensive if you don’t want to usezip they will be vapor permeable then you don’t have to “sweat” no pun intended- the above concern.
Thanks, Deuce. Those wrbs would relieve the sealed wall issue.
For the inside, it seems I will need the vapor barrier on the inside of the studs, because I don’t have enough external insulation to keep the sheeting temp (be it zip, zip R, or plywood) from dropping below the dew point. Is this correct?
You would meet code with a variable permeance membrane (VPR), but if you want to use R-6 exterior sheathing without additional foam or a VPR, you will likely have moisture accumulation at the interior of the Zip-R polyiso. If you're concerned about racking and you have enough wall area, you could use diagonal metal straps or let-in 1x4 wood as bracing. Or you would apply a flash coat of closed-cell foam between studs to get you up to the 1/3 ratio of exterior to total insulation R-value.
Thank you, Michael. You both have confirmed my understanding. I realize I should find a way to increase the foam on the outside. If that’s with the thicker zip R, I could do as you suggested with strapping or in set 1x4.
"My first choice would be to use Zip R-6 with dense pack cellulose in 2×6 walls."
Maybe I'm wrong here, but I don't think the Zip R would be considered exterior insulation. But maybe that doesn't matter because the inboard side of the foam on the Zip R won't get cold enough for condensation to form?
https://www.greenbuildingadvisor.com/article/working-with-zip-system-r-sheathing here's some good info in the article comments.
Between the two options, the Zip R seems like the better choice.
I'm sure by now double stud wall has crossed your mind at some point, but in case you wanted to read up on an article that nearly directly compares your options:
https://www.buildingscience.com/documents/insights/bsi087-chubby-checker-fat-man-do-permeance