Faced or unfaced wall insulation?
ken_o | Posted in Energy Efficiency and Durability on
Wall design: Conventional 2×6 framing, R-19 fiberglass in stud cavities, (3) 2″ EPS layers outside OSB sheathing, which will be covered by some kind of low-perm WRB. All rigid insulation and OSB seams taped. Interior drywall with Airtight Drywall methods.
Location: West Yellowstone, Montana, right in the middle of Zone 7 at 10,900 heating degree days (65° F).
Simple question: Since walls will need to dry to the inside, would I be better off using unfaced fiberglass in the stud bays? The big concern here is to maximize drying to the inside, as drying to exterior will be minimal. Airtight drywall should block most air infiltration. I’m thinking the craft facing would only slow drying to the interior and offer no advantages.
I’ve done a lot of research to come up with this design, and I’m not interested in changing it at this point. There’s too many other factors involved, such as big snow loads, seismic, wind, soils, etc. The heating system is already designed and the plans are ready for submission. Thanks for your thoughts.
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"I’ve done a lot of research to come up with this design, and I’m not interested in changing it at this point."
Begs the question: Why ask a question if you're unwilling to hear anything that might differ from your plans?
I cannot offer any input to your question other than there has to be one way out and that you've got to pick a "side" (what's prescribed in your area should be pretty common, and that would be "the side" to pick- and then one would select the materials and application of them to meet that).
And... my guess at an answer would be: un-faced; but, again, I'm not one of great experts here (who should be solicited/approached with openness). Vapor III spec paint on sheetrock on perimeter walls, I believe.
I'd like to look at this as a final refinement to a design that has evolved through much research. Unfortunately most of the building going on in the West Yellowstone area (the only real Zone 7 area in Montana) appears to use minimal insulation and just "pay the heating bills," so I'm not finding any answers there. I'm hoping for some here. I want to keep discussion focused on this one question.
Kraft facers act as a sort of old school smart vapor retarder, which means they are more vapor open when there is more moisture in the wall. This is a Good Thing, meaning that they allow for more drying when it's needed, but help to slow moisture ingress into the wall when you want the wall to stay dry.
With nearly R25 worth of exterior rigid foam, and R19 in the walls, you don't need faced batts at all -- you would be perfectly safe with unfaced insulation here. The exterior rigid foam is going to keep the sheathing warm enough to minimize moisture issues in the wall. I would recommend an interior side smart vapor retarder as some extra insurance, but it's not required. I would absolutely install the drywall airtight as you're planning to do. You could use the kraft faced batts as as way to get that interior side smart vapor retarder, but if you do that, make sure to tack the facer on the edges of the studs and not to the inside faces of the studs. when you're done, the facer should look like a flat sheet installed on top of the wall prior to drywall going in, it shouldn't dip in at each stud. You could use vapor retarding paint too if you want a little extra protection, or use the paint instead of the faced batts or vapor retarder membrane.
The only suggestion I would make in terms of a design change is to consider using mineral wool instead of fiberglass. This would get you from R19 to R23 in the wall, and it's a better product to work with in terms of getting a good installation.
Bill, Thanks for the detailed thoughts. I was leaning toward unfaced and your vote helps me to move in that direction. Mineral wool has been suggested before, but, believe it or not, more insulation (greater R value) inside the walls is a negative, not a positive in this situation. It changes the ratio of outside vs. inside insulation, moves the dew point closer to the inside, and increases chances of condensation in winter. In Alaska, they build this design with 2x4 walls. I'm only going with 2x6 walls because of snow loads, seismic, and wind issues. Thanks a bunch for your thoughts.