Continuous Exterior Insulation over Zip Sheathing
brentwilson
| Posted in General Questions on
Can you give me some types and brands of insulation that would serve well as exterior continuous insulation installed outside of zip sheathing on 2×6 walls in climate zone 6?
If the stud cavity insulation is R20 cellulose or similar R-value batt insulation, do I understand correctly that the exterior continuous insulation should be at least R11.25 in climate zone 6?
For energy efficiency and cost, how does a build-up as specified above compare to closed-cell spray foam or flash and batt on the interior of the sheathing only?
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Replies
I would use either polyiso (my preference here) or EPS/GPS. GPS is graphite infused EPS, and has very slightly better R value per inch as a result, but at higher cost. Brand doesn't matter much, use whatever is available in your area. These days just getting product is the most important concern, brand is secondary. Everything is harder to get than it used to be right now. Lead times on materials are making it very difficult to schedule projects!
Spray foam in a wall is pretty much wasted money. Closed sell spray foam is only partially filled, since it's difficult to trim flush. The resulting partial fill gets you an R value not all that different from what you'd get with a full fill of open cell, since open cell spray foam is typically overfilled and then trimmed flush. Either type of spray foam gets you a total R value in a studwall not too different from what you'd get with either mineral wool or high density fiberglass batts. With a good installation of exterior rigid foam, with "good" meaning you take the time to tape the seams and caulk the perimeter to get a good air seal, you'll end up with a higher performing wall using batts on the inside and rigid foam on the outside.
Flash and batt is just a more expensive way to do air sealing. You can do just as a good a job of air sealing by doing it the old fashioned way: canned foam in all the holes/penetrations, and a bead of sealant around the perimeter before the sheathing goes up. I would detail the interior drywall airtight too, since that helps to make the wall assembly more robust and doesn't really add much in the way of additional cost.
Bill
When I remodeled a couple of bedrooms a few years ago I filled the stud cavity with closed cell spray foam an the few areas it was over filled wasn’t really that difficult to trim. I found it was easier to control the spray and prevent overfill compared to open cell
Asked the same question today in a different manner. Looking at doing the as you mentioned.
This article goes into more detail.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Seems the below requirement (from this article) is rarely followed when exterior foam is used...
" It spells out the minimum R-values for exterior foam to be sure that moisture won’t accumulate in a wall."
" Zone 6 R-7.5 for 2×4 walls; R-11.25 for 2×6 walls
If polyiso were used as the exterior continuous insulation, would it need to be more than R11.25 due to the fact that it's r-value degrades in cold temperatures? If so, how much should it be bumped up? Is 2 inches enough to cover requirements in a case like this?
If you want to derate for cold weather, the commonly used value is R5 per inch for polyiso.
Bill