Looking for help on a wall system
We are building north of Ottawa, which is zone 6a (I think). Pretty chilly in the winter. We have been given a good quote to frame up our house with typical 2×6 framing, so would like to go ahead with it. The wall system we are thinking of for insulation would be, from outside to inside:
Tyvek over Plywood sheating (taped, sealed)
cellulose or roxul in a 12 inch space
tyvek (taped and sealed)
Although I would prefer that the sheathing be on the warm side of the wall and function as our air barrier, I don’t see how we can do this. We are a little concerned about the sheathing absorbing moisture but I understand that it can dry out too. We don’t want to wrap the house in foam so I guess we will have to just hope the sheathing holds up.
Does this wall sound like a good system?
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If you provide ventilation openings at the bottom and top of your rainscreen gap, you're good to go. This is a fairly standard double-stud wall, and it should perform well.
The use of interior Tyvek is a little unusual, of course. Your cellulose installer will probably suggest a different fabric -- something like InsulWeb. Remember, you can always create an interior air barrier with your drywall layer.
I have often wondered about punctures in the drywall (i.e. hanging pictures) allowing mass flow of air and vapor into the wall with the airtight drywall approach.
Would you recommend using the sheathing as the air barrier? We are also considering applying roxul over the sheathing (3.5" at r-14) to ensure that it stays warm.
One air barrier is good (for example, at the sheathing layer). Two air barriers are even better (for example, at the sheathing and drywall layers).
I don't think a small nail in the drywall used to hold up a picture on the wall will create a significant air leakage path, especially if the nail stays in the hole. Once you move the picture to a new spot, you'll probably repair the area with spackle anyway, sealing the hole.
Would you recommend not using the interior Tyvek ?
Assuming that you are insulating with cellulose, as I recommend you do, you need to coordinate the decision with your cellulose contractor. Most cellulose contractors like to use an air-permeable fabric like InsulWeb because it allows air to escape from the stud cavities when cellulose is blown in.
Once you have an insulation plan, you can decide how to install an interior air barrier. Options include:
- OSB + service cavity (nice, but expensive);
- A European air-barrier membrane like Pro Clima DB+ or Siga Majpell 5;
- An American air-barrier material like MemBrain;
- Airtight drywall, following the methods of the Airtight Drywall Approach.
Thanks again for your responses.
Given that we would prefer not to apply foam to the outside of the sheating, do you think that we should put roxul board (comfort board?) on the outside of the sheating to ensure it stays warm and dry? Or in your opinion can we risk not taking this step?
As I wrote in my first response, "If you provide ventilation openings at the bottom and top of your rainscreen gap, you're good to go." I don't think exterior Roxul is necessary.
ok great. thanks again!