Air sealing
I am in the process of building a new home in zone 5. We are doing a passive solar house with vaulted ceiling in the living room, kitchen and dining room area and the rest of the house will be flat ceilings. I plan to do blown in cellulose insulation on the flat ceilings and closed cell on the vaulted ceiling. I plan to do 3” of rigid foam insulation on the exterior walls with spary in cellulose (2×6) walls. I am also using Prosoco seam filler for the sheathing and Fast Flash for the flashing. 1.) Since I have the air and vapor barrier to the exterior, should I not seal the drywall to the wall? 2.) I don’t have rigid foam on the roof, so should I airseal the interior drywall there (on the ceiling) on the flat ceiling?
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Michael. See this article and video on stopping air leaks: https://www.greenbuildingadvisor.com/green-basics/video-how-hang-airtight-drywall-1-3
Also. How are you segregating the flat ceiling areas from the cathedral ceilings? I ask since it sounds like the flat ceiling areas are ventilated.
Are you installing enough closed cell foam on the interior to reach r-49?
1: You absolutely SHOULD air seal the drywall. It will have plenty of drying capacity through standard latex paint (3-5 perms.) Air leaks from the interior would only serve to transport more moisture into the cavity during the winter via convection when the sheathing is cold, creating a convective drive.
2: Similarly ALWAYS air seal the ceilings, no matter what type, since stack effect drives can move a lot of higher humidity air into colder portions in winter, causing damaging levels of moisture accumulation. This is even more important than with walls.
You have more than adequate exterior foam for dew point control a the wall sheathing, but you haven't specified the vaulted ceiling stack up. If the closed cell foam is directly on the underside of the roof, in zone 5 the foam needs to be at least 40% of the the total R in foam + fiber stackup behind the ceiling gypsum for dew point control. Closed cell foam is very expensive R and not very green (high polymer content per R and uses climate damaging HFC blowing agents.) To hit R49 with closed cell foam costs about $8 per square foot. But with ~3" -3.5" of foam ($3 -$3.50 per square foot) you can do the rest in fiber (say 8" of damp sprayed cellulose) at a much lower total cost. That fits just fine in a 2x12 rafter bay.
Yes, thank you, I have seen that video. I was concerned if I had a barrier on the exterior(R-guard and rigid foam) and the interior(caulking the drywall) if wouldn't let my wall dry to exterior or to the interior.
Segregating the ceilings is going to be interesting, I am building the walls all the way up into the scissor trusses and I haven't decided but I thought those walls would need to be closed cell. The flat ceiling area would be ventilated.
We do have raised trusses so the R-49 won't be a problem.
Michael,
You wrote, "I was concerned if I had a barrier [an air barrier] on the exterior (R-guard and rigid foam) and the interior (caulking the drywall) if wouldn't let my wall dry to exterior or to the interior."
It sounds like you assume that you would be depending on air leaks to help your wall dry out. But that's not a good strategy. Air leaks through your wall are just as likely to make your wall wet as they are to make your wall dry.
No one depends on air leaks to dry a wall or ceiling assembly. You want your wall to be as airtight as possible. The wall components dry by diffusion, not air leakage.
Ventilation drying is occasionally useful -- most obviously in a rainscreen cavity. But that cavity is entirely on the exterior of your thermal envelope.
Thank you for your input. Dana, you are right about the closed cell, I think I just wanted to do that because I didn't get rigid foam on the roof but it would be more cost effective and green with less foam and more cellulose. Martin, thank you for clearing that up.
Michael: one option is to use the bottom chord of the trusses as the air barrier location and having vaulted ceilings throughout. Then build flat ceilings where you want them and use the space between the flat ceilings and the underside of the trusses for attic storage or utility chase. You can then blow as much cellulose in the truss space as you want and easily ventilate the whole roof.
Thanks Stephen, but we already have the trusses in place. I really haven't gave much thought to the wall I need to build up but it will be exposed to a ventilated attic area so I probably just need to treat it like an exterior wall. Drywall, sprayed in cellulose, and a couple of inches of rigid foam? Here is a picture of the ceiling, the room jetting out is our first bedroom. That is one section that I will need to build up to enclose it from the attic.