Opinions on (Hypothetical) Double Wall Assembly: GPS = meat, Mineral Wool batt = bread
Ah yes, one of the bread and butter of GBA Q&A – wall assembly questions from a knows-just-enough-to-be-dangerous homeowner. Here we go again!
I’m embarking on a Zone 6 net-zero all-electric build and exploring with my builder what details are most feasible by local tradespersons. Let’s assume for this discussion that dense packing cellulose is just not going to be an option, because that seems to be the situation I’m in.
I’m hoping for some discussion on 1) condensation risk, 2) sequencing issues, and 3) other concerns with execution of a double wall assembly where the two stud bay cavities (bread) are grade 1 installed mineral wool batts and the gap between walls (meat) is GPS with polypropylene laminates on both faces (0.03 perms/inch, R5/inch). My builder prefers Zip System, so we can assume that too, for now. Preliminary thicknesses would be 3.5” batts and 4”GPS, but suggestions are welcome.
In some ways, this is not too dissimilar from Steve Giroux’s 1984 house that Martin Holladay wrote about without much criticism. It was meat = polyiso, fiberglass batt = bread: https://www.greenbuildingadvisor.com/article/a-superinsulated-house-from-1984
The wealth of practical experience on GBA is incredible. Thanks in advance.
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Your proposed assembly will perform something like a wall with exterior insulation, except you are limiting drying of the exterior insulation to the exterior by using a vapor retarder (Zip) on the exterior side. The rule in climate zone 6 walls is that you need at least 35% of the total wall R-value on the exterior, which you essentially have, so as long as you have vapor control on the interior and an air control layer somewhere in the assembly, in theory it should be safe.
However, if moisture makes its way to the outer mineral wool layer, it won't have enough heat energy from the interior to push it through the Zip sheathing, and eventually I would put money on you having moisture accumulation issues (and resulting microbial action). I would go with one of the many dozens of time-tested assemblies instead of inventing a new one.
Thank you, Michael. That's a great point about a different kind of sandwich - a moisture sandwich in the outer wall cavities. Definitely a risk. Probably one not worth taking.
Would a double wall assembly insulated exclusively with mineral wool batts be included among one of the many dozens of time-tested assemblies? I've heard of it occasionally, but I'm not sure it makes the cut for that group. I'm certainly not hoping to invent a better assembly. Double stud dense-pack cellulose and continuous exterior insulation are two excellent products of group iterative process that appear truly optimal, but some of us just won't have the local tradespersons available to achieve either.
Batt-insulating a double stud wall seems achievable with average framing crews plus owner and/or general contractor rolling up their sleeves for some DIY grade one batt install.
I won't do a double stud wall unless it has borate-treated cellulose insulation, or once HP Timber has entered the market, their borate-treated wood fiber insulation. While double-stud walls have been done with other types of insulation--I've been involved in two renovations to 1980s homes with double stud and fiberglass batts, with no obvious issues--it remains a theoretically risky assembly due to moisture accumulation at the sheathing in late winter/early spring and I feel better having the borates protect the insulation and also the framing and sheathing against microbial action. If you really want a double stud wall without cellulose, I would use mineral wool batts (or blown, if you can get it) with a carefully installed interior variable permeance membrane, a sheathing such as CDX or boards that are more vapor-open than OSB, a good WRB and a rain screen.
We are fortunate here in the northeast to have a lot of skilled cellulose installers.