Wall options
user-7124595
| Posted in General Questions on
Looking for input on the following wall choices in zone 5.
Double stud wall with two 2×4 walls and a 4″ cavity (11″ total depth). Exterior sheathing is 1/2 plywood with blueskin and 3/4″ rainscreen outboard.
All things being equal from an environmental impact (I realize they are not), but comments from a performance/risk vantage point would be greatly appreciated.
Option 1: Dense pak cellulose in two lifts, the first in the outer stud wall, with netting, the second in the remaining 7-1/2 inches with Intello.
Option 2: Three r-15 layers of mineral wool with Intello inside.
Thank you for any comments and advice
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
User...595,
I'll be interested to see what people favour. The moisture buffering capability of the cellulose makes the wall more resilient, but I wonder how important that is with a rain-screen and variable-perm interior Vb to help dry things?
No idea how cost stack up, but my own preference would be the three layers of mineral wool. No worries about future settling, and much easier to adapt and renovate over time.
Agree on moisture buffering, it is my biggest concern. Mineral wool is about 5% less in my subs quote, significantly less if I do the install myself. This surprised me. In addition the cellulose install offers little opportunity for visual inspection through the Intello. Thank you for the feedback Malcolm
Whereabouts are you building?
Southern NH
It probably won't be until Monday, but I'll be interested to see what the New England crew of guys like Brian, Dana, and Michael think. They seem to really favour cellulose for the moisture buffering.
What is your exterior cladding and interior side perms (drywall/paint)? Probably OK, but Summer moisture drive can be an issue.
User 595,
I opted for a similar wall in Sunapee, NH. I'll share what I've learned in hopes that it helps:
Just like you stated, the mineral wool carries a much larger carbon footprint than cellulose. Much, much larger...
But with that aside, the mineral wool gives you a high R value but is far more air-permeable than dense pack cellulose. This may elevate the "cold sheathing" risk that may or may not even be an issue. Dense pack cellulose is certainly better from a thermal mass perspective. Both are excellent in terms of their flame-spread rating, if you care about that stuff. For what it is worth, I have toured a couple of homes in NH with double stud walls filled with 2-3 layers of Fiberglass batts in the early 80s. Both are still warm and comfortable (and standing!).
I installed mineral wool batts into a basement stud wall for added insulation. Although it isn't nearly as bad as fiberglass, mineral wools batts are still itchy as hell and a pain to install properly unless you do it all the time. I wouldn't trust just anybody to install mineral wool batts and achieve a Grade 1 install. I found that the thicker batts were actually easier to handle.
Blown-in insulation almost always achieves a Grade 1 install. However, I don't care what anybody says- Intello complicates this. This was the case in my house (with dense pack fiberglass) and I've seen it on houses that I have visited while under construction (cellulose). Intello is so air-tight that it can leave air pockets in an otherwise dense packed wall if adequate slits/holes are not punched. I would still include a smart vapor retarder like Intello or Membrain (ESPECIALLY if you opt for an air permeable batt) but add it AFTER they have blown in the insulation behind netting.
I would go with cellulose or dense pack fiberglass over Rockwool. In fact, I would rather be water-boarded than install three layers of Roxul for an entire house.... lol.
Hi user-7124595 (It would be great to get your actual name...)
Seems you already know the potential pros and cons, including cost of your options.
I advocate dense pack cellulose in these assemblies for four reasons: the potential to buffer moisture, the lower carbon footprint compared to other options, the recommendation of builders who have built, monitored, and had the opportunity to open up cellulose insulated double stud walls (as you have described), and finally the potential for a little extra air sealing.
That said, whichever insulation you choose, the key to building this wall well is great air sealing, at the sheathing and the interior, in your case, with the self-adhering Blueskin and the Intello membrane. Plus all of the other necessary air sealing details.
Not sure it offers the advantages you seek, but have you considered DP cellulose in outer portion, then finishing with mineral wool?
Possibly with the smart retarder to the exterior of the interior wall if you feel comfortable with the build sequence on that. That gives you a full 3.5" service cavity with the air barrier protected and if you fill it with batts it may be more conducive to future retrofit work. Also no bulging cellulose issues for drywall (perhaps not really an issues if rolled or if you're strapping anyways).
The borate in cellulose kills mold. But perhaps spraying the interior side of the plywood with mildewcide would offer similar benefit.