R=Max Thermasheath (aluminum faced) Zone 6
dondev
| Posted in Energy Efficiency and Durability on
Building a home with 2×6 studs, 1/2″ cdx plywood (for shear) and apply 1.75″ Thermasheath(aluminum faced)R-12 with 7/16″ OSB over the plywood. Them applying Blueskin over the osb 3/8″ airspace and 1×10 shiplap as the exterior siding.
Question I have is what to use in stud cavities for insulation. Glass or Mineral wool?
Have I created a moisture problem?
Thank you for any help
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A couple of articles I recommend reading:
https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
In your case, in CZ6 at least 35% of the total wall R-value should be on the exterior to keep the plywood from dropping below the dewpoint temperature, which is what would allow condensation to occur.
Thermasheath's data sheet shows that at 1.75" of thickness it performs at R-11.4, using the ASTM C518 test method. https://static1.squarespace.com/static/57a39be0579fb3a3a8540f56/t/63dab1d4f0ac1f05aed5f109/1675276757183/Rmax-Thermasheath-Product-Data-Sheet.pdf That test is done after six months of aging which is supposed to represent real-world scenarios but manufacturers are known to age their samples very carefully so as to get the highest possible R-value, and the testing is done at mean 75°F which does not represent the temperatures at which condensation can be a problem.
If your only goal was to get a permit, this assembly would meet code, but if you want long-term durability, I recommend consider the foam to be about R-5 to R-5.5 per inch, which would put your proposed foam layer at about R-9 to R-9.5.
At 5.5", fiberglass batts are about R-20 and mineral wool is about R-24. Mineral wool is typically denser than fiberglass and many people consider it easier to install, but it costs more than fiberglass and some people find that fiberglass is easier to install. Either one will work well and both are in the same category when it comes to installer health, occupant health and environmental impact, though there are fine differences between brands and products.
If you go with fiberglass at R-20, I recommend using at least R-10.8 on the exterior, or 2" of polyiso. If you go with mineral wool at about R-24, I recommend using at least R-12.9 on the exterior, or 2.5" of polyiso. It's safer to go with more exterior insulation but not less.
You can be code-compliant with less exterior insulation if you use a class 1 or 2 interior vapor retarder, but it's not what I would call a resilient assembly--if anything goes wrong you will end up with moisture problems.