Substitute for ZIP R
I’m working on a project of several structures in climate zone 7 where ZIP R-6 has been installed on everything enclosed currently – however the builder is having trouble sourcing more ZIP R and is proposing an alternate assembly to keep closer to schedule. They have proposed putting a different integrated sheathing (i.e. Advantech, OSB with wrap, or similar) over 1″ rigid. While this approach is similar to their current assembly, it seems like it may be best to change course to a more standard rigid over sheathing? This would change some of the cladding attachments, but wondering if there are any other considerations I am overlooking.
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Replies
I think matching what you have might be the best approach in this situation, even if it's not the route I would have taken in the beginning.
The insulation portion of Zip R sheathing is polyiso. I would try to match that at least, so that it has the same properties as the rest of the material. That way they can keep the same nailing schedule and continue their work without mental interruption. I'd request that the seams of the OSB be taped with ZIP tape as well, and then covered with an appropriate WRB.
You have to be careful, unless an engineer signs off on it, you can use DIY panels for shear walls.
The simpler option is to use let in bracing on the interior of the walls where there is no Zip R.
I think you meant "can't".
I really dislike the argument that because it's two separate peices, that an engineer has to click a box to allow it, but such is life. The contributions from the bonded insulation to the panel shear capacity is negligible, so what were really considering is a geometry problem and not a material problem, if you assume that all category rated performance panels surpass the same standards. A panel stamped 7/16 meets 7/16 standards, and so on.
However, Akos is probably right. Throw some diagonal bracing in on the corners and call it a day.
> The contributions from the bonded insulation to the panel shear capacity is negligible
I'd go so far as to say it's zero, because the bonding between OSB and polyiso in a Zip R panel is very negligible, so much so that sometimes they readily separate if you cut a small piece of a panel.
It makes me think that if the code making folks were so inclined, they could come up with a set of prescriptive design options for field assembled sheathing over rigid foam over studs. But, someone would have to put the time and money into designing and testing such options.
cnrwood,
I wonder if it might be less work overall to spend the time attaching the rigid foam to the sheathing with adhesive? Then you could install it the same way you did the Zip-R, rather than having to make two passes over all the walls.
Cnrwood,
The main thing that jumps out is that by using polyiso (assuming foil face) you are going from a vapor open wall with ZipR to a vapor closed wall. The foil face is a vapor barrier and I believe the ZipR has a perm raiting of 2. The polyiso is fiberglass faced on the ZipR, not foil faced. Depending on the whole assembly, this may or maynot be something to consider as well.
Shawn
ZIP-R isn't the only manufacturer of this type of cladding system. I would look for an alternative rather than attempt the proposed mock up which is typically reversed (OSB-WRB-Foam-Cladding).
https://isolofoam.com/us/product/isobrace-osb/