Exterior rock wool insulation with batts?
First, I’d just like to say how thankful I am for the invaluable resource that is GBA. I spend A LOT of time reading the blogs and Q&A and I find that pretty much any question I might have has been covered here already. Though sometimes my understanding gets a little vague regarding certain details so I am hoping for some input on a potential wall detail.
I want to build a “Pretty Good Wall”, ideally with a whole wall R-value of 30-ish, which I feel is a good target for my location in CZ 4A (Although I think we may be closer to CZ 5A here in the mountains of SW VA. Elevation 2400 ft)
I have done the exterior foam thing on a few projects (thanks to GBA) but I tend to lose sleep thinking of carpenter ants and termites in places I can’t see and I would like to limit the use of foam when I can. I try for reclaimed goods when possible but it is somewhat limited in my area.
I really like the idea of the Roxul ComfortBoard IS but availability in my area seems to be a no go from a cost and quantity standpoint.
Double wall construction with dense pack cellulose is also appealing but the installers in my area don’t give me the confidence that I need and they are very resistant to dense packing a large open void. I have used damp spray cellulose a few times and I am very comfortable for a standard 2×6 depth, as well as a 2×6 wall with horizontal 2×2 cross hatch. I am hesitant to go any deeper with damp spray but if anyone can offer any info doing this than I am all ears.
So here is my proposed wall description and an attached image(hopefully):
From inside to out:
1/2″ GWB primed and painted
2×6 24″ OC load bearing wall with 5 1/2″ damp spray cellulose in bays
1/2″ CDX plywood sheathing taped and caulked for air barrier
2×4 24″ OC non load bearing wall with 3 1/2″ Roxul ComfortBatt
WRB Typar or similar
1×4 vertical battens for rainscreen
My uncertainties with this wall are in regards to the 2/3 to 1/3 rule, which I don’t really understand, as well as how this wall performs in summer with very occasional AC usage.
Any comments, critiques are appreciated,
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