Moisture Considerations with Exterior Board Insulation
woodguyatl
| Posted in Green Building Techniques on
We started construction on a home in Atlanta designed using Zip R6 sheathing. Contrary to what our supplier promised months ago that sheathing is not available in the Southeast. A possible change we can make is to use regular Zip sheathing and 1″ exterior foam or Comfortboard 80 mineral wool (also hard to find in the area). Assuming the Zip siding is installed and taped properly are there any vapor barrier/moisture considerations with either the foam or Rockwool. Either choice will be covered by a 3/4″ air gap and wood siding.
Thank you for your insight.
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Replies
1" polyiso over Zip sheathing will get you up to the same R value as Zip-R in this case, since Zip-R is also using polyiso for the "-R" part of the product. The big change is that you have to put the Zip sheathing up against the studs first, THEN the polyiso. You can't field assemble Zip-R by putting up polyiso and then sheathing due to shear on the nails, and you don't have the testing that Huber has to show that your homemade assembly is safe. I would tape the Zip sheathing as your primary air barrier, then tape the seams between polyiso panels too to help keep moisture and critters out of the polyiso foam core of the panels.
I would not use Comfortboard here. Polyiso will get you more R, and is likely to be easier to get (there have been reports of Comfortboard being hard to find too).
Your 3/4" rainscreen will greatly improve the robustness of the assembly in terms of moisture tolerance regardless of what you end up doing for your insulation.
Bill
I had a similar situation when building in the Atlanta metro in 2013. R3 ZIP was delivered to the job instead of R6, and getting R6 would have required required putting the project on hold for weeks (and this was before COVID).
My biggest concern was that the Manual J had been calculated based on using the R6 ZIP. So I decided to ditch the ZIP and install OSB with one inch of XPS.
The substitution worked fine but was a lot more time consuming to install. Because the house was two stories over a full basement, the installers had to make multiple passes to complete the process. The exterior foam also created some unexpected challenges that would have been avoided with the ZIP panels.
You are likely to experience similar constructability and cost issues on your project. Opting for rockwool, which is challenging to keep coplaner, will make things even more challenging.
If I encountered this issue today, I would keep the R3 ZIP and simply revise the Manual J. I don't think the small difference in R value is going to make much of a performance difference in our fairly mild climate.