“Picture framing” stud bays with sealant.
I am retrofitting my house in climate zone 6, which was built in the 80s. I had to open up most of the walls due to rodent infested fiberglass insulation throughout the house. I’m taking this opportunity to improve the insulation in the walls.
The original wall assembly (from outside in) was: cedar shingles, on plywood sheathing, 2×6 stud bays filled with fiberglass, and then a layer of poly sheeting under the drywall.
I did notice clear signs of mold on the interior of the poly sheeting.
My plan was to upgrade the fiberglass insulation to mineral wool, replace the poly sheeting with a smart vapor barrier, and in the future add more insulation to the exterior.
However, I came across some discussion on GBA about air sealing sheathing on the interior: https://www.greenbuildingadvisor.com/question/picture-framing-the-stud-bays-with-knauff-ecoseal. I don’t think my sheathing is taped or sealed on the exterior, and I was thinking of using a caulk like Contega HF to seal the studs to the sheathing from the inside, i.e. “picture framing”.
Does this make sense? or is this redundant with the interior smart membrane?
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Some on this site will disagree, but I do not feel that your framing needs to be sealed, unless you are taping your sheathing. Sheet goods such as poly or smart membranes can be installed and detailed so as to be very air tight, and the nature of a sheet good is only the perimeter and penetrations need to be sealed.
Would you get higher performance by sealing every stud cavity? Probably. Is the juice worth the squeeze? I think not. The time and energy is better spent in areas with better performance payback, such as detailing penetrations, box joists, windows, etc.
Good luck!
I sealed the stud bays to the sheathing with a little canned spray foam. There are better products nowadays as you suggest. Sealing the stud bays is only a partial solution to stop air leaks through walls (doesn't stop leaks at top plates or band joists for example). Plumb_bob suggested a better method which you can't do until later when you add your exterior insulation.
The foam (or caulk) is cheap, and I applied it quickly, about 30 seconds per stud pay to quickly move the nozzle up, across, down and across the edges of the sheathing to the interior corners with the studs/plates. My girlfriend was perfectionistic about it and spent about three minutes per stud bay. (Obviously she was paid by the hour! ;-) In any case, not much time and materials for this work to reduce air leaks, a crucial job. If air leaks through stud bays, it reduces the usefulness of insulation.
Most people suggest its a good idea to create multiple air barriers, as air can find some unpredictable path through the wall that we mortals can't quite comprehend during design and construction! ;-) I tried three separate air barriers although each one wasn't perfect I'm sure. Didn't have blower door tests back then 40 years ago, so no data to share.
I will add that I have seen very good ACH/50 results with a variety of different approaches- exterior air barrier, interior air barrier, and hybrid methods. Although the system you choose is important, more important is the thoughtfulness and attention to detail when actually implementing the job.
You can air seal the interior. Installing air tight drywall is pretty common in high performance homes, and it's not that difficult to do. I like to seal the exterior sheathing too, but that's belt and suspenders since I know I'll also be sealing the interior side drywall too. The idea is that if you seal both sides, you have some redundancy in your air barriers so you'll have less chance of leaks, and a more robust assembly that will hold up better over time. That doesn't mean you MUST seal the sheathing. If you can only seal the interior side, seal that. I like to use polyurethane sealant here (which I use on the exterior too).
Note that a power caulk gun can really make things easier when doing these very long beads of sealant.
Bill
Instead of trying to caulk everything up and hoping to seal up the house, I would do a bit of blower directed air sealing instead while the drywall is off.
Chances are you won't have to caulk as much as you think plus you can seal all the other areas besides the sheathing that can leak. I wouldn't aim for this to be perfect, just good enough to seal up most of the big leaks. Blower door is the best for this but you can do a good enough job with a box fan mounted into window blowing in a couple of smoke sticks.
Once that is done you can insulate and install a properly detailed membrane as the main air barrier. Make sure to install vapor tight device box (these are the ones with the gasketed flange) as regular boxes can be a big air leak.
Thank you everyone for providing more context. You've convinced me to do some DIY blower door testing with a smoke machine before I put up the smart membrane, to seal up major holes. We will see if I end up "picture framing" the stud bays, probably will be based on whether I see the smoke leaking out there.