Ceiling sealing to walls – Membrain still worth it?
Building a house in zone 6A central Minnesota.
Wall assemblies are 1/2″ drywall, 2×6 walls (r-19 batts), tyvek house wrap, drain wrap, two layers of xps totaling 2.5″, 3/4″ furring strips and Ascend siding. Air barrier is taped OSB
Roof trusses are 24″o.c. cathederal parallel trusses… The cavity is 22″ deep. My plan was using extra 1/2″ eps foam I have on some leftover 2″xps spacers for a vent cavity from bottom to top. I am planning to stretch insulation netting to the bottom first to blow in the fiberglass. Then put my vapor barrier on top so it’s not holding all the weight.
The energy heal is close to 24″ from the top of my plate. Planning to use Vulken XTR for it’s supposed stretchability to run a bead where the top plate meets the OSB (OSB runs up the energy heal to the bottom of the 2×4 tail) and the horizontal seams of the top plates. Then seal my ceiling vapor barrier to the front of the top plate with accoustic sealant and tape if I need to.
Will seal and paint the drywall on the walls for class III retarder. Main question is the ceiling.
First, does my plan to seal to the wall work? Anything else to consider there?
Then the question is – is Membrain absolutely worth it in this case? I am hoping to die in this house someday – but not because of it! So want it to last. Problem is I have all the poly I could ever use. To do it in Membrain would be another $7-800, which just hurts a bit more… If it’s going to be worth it, I’ll definitely do it, just getting conflicting answers. Nobody around here thinks it’s worth while (local lumber yard had no idea what I was talking about and had to explain the whole concept to him…he’s getting back to me on pricing). Of course they all think I’m crazy for having 2.5″ of foam on my house also! Other option I’ve been told is to just do some r13 faced batts for the vapor retarder there.
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I live just north of you, in climate zone 7. I'm involved with a few organizations across the state and this year there have been more investigations of summertime moisture forming in wall cavities where poly is present. Of the investigations I've heard about, the thought is the increased use of air conditioning during the summer is beginning to cause issues in homes with polyethylene. It sounded like most of the homes were production built, but they are still hitting the required 3 ACH50 minimum air leakage rates. This probably isn't a major issue yet, but I don't think AC use in the coming years will be decreasing in cold climates. I'm using a smart vapor retarder on my home renovation.
One other note, the Minnesota Energy Code requires R-20 in walls in climate zone 6. The other option is building 2x4 and adding R-5 continuous. Personally, I'd build 2x4 (R-13 cavity) and add R-10 exterior. This would eliminate the need for a class I or II vapor retarder on the wall. Painted drywall would be all you need. You'd still need an air barrier at the ceiling, this is where I'd use the Membrain.
Good luck on your build.