Least evil of my wall system options
Border of Zone 4/5. I’ve read much of the material on this website about wall systems. While I have my desires for advanced framing techniques, including 2×6 24OC and thick foamboard outside insulation, these things aren’t a realistic option with builders/subs in my area undergoing a building boom.
The path of least resistance is 2×4 or 2×6 16OC. The only other builder/framer reluctant possibility is Zip-R with built-in polyiso. My city uses the 2012 IECC, but has amended the walls from R20 or R13+5 down to R13. Yes, you read right, here in 2018, the Zone 4/5 wall code is R13.
My preference is for using the Zip for the R13+5. My question is: does the R20 of wall fill equal the R13+5? If so, it seems the easiest (and maybe the least expensive) of the two options is to spend a little extra for 2×6 (vice 2×4) and put in R20-21 fiberglass batts or R23 mineral wool batts. I think the house will already be using a fair bit of 2×6 anyway due to high ceilings on both levels of reverse 1.5 story.
The city has also amended 3 ACH code down to 5 ACH. I presume I still want to shoot for 3 ACH. If I use regular Zip or OSB+Tyvek, would closed cell foam be required for 3 ACH air sealing? If so, what’s the minimum depth to be used with wall fill? Would you recommend, say, R15 fill + 1″ R7 closed cell or R11 fill + 2″ R13 closed cell or some other value combo? Thanks.
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