Can someone explain the science behind a mid-wall vapor barrier on a double stud wall?
In order to save money, I’ve switched my wall assembly last minute from using exterior rock wool board to a double stud wall construction. I would like to speak with my county inspector about using this approach to make sure I don’t have problems during construction. I’m struggling to understand how I’m going to explain how a vapor barrier on the back side of the inside wall is a good idea.
I’m in climate zone 5, and here is my intended wall assembly:
Insulweb netting (affixed to stud to hold cellulose)
2×4 stud wall (with cellulose) (also serves as utility chase)
Certainteed MemBrain (affixed to backside of stud. Taped and sealed)
5″ gap filled with cellulose
2×4 stud wall (with cellulose)
Insulweb netting (affixed to outside of stud to hold cellulose- blown-in from outside?)
Housewrap (brand yet to be determined)
1×3 rain screen gap
cement board siding
So, I have several questions.
1. How is a mid-wall vapor barrier a good idea?
2. Do I need to install MemBrain on the inside of the inside stud wall as well? Every house I’ve dug into has a 6mil standard vapor barrier directly behind the drywall. I’m wondering if my county inspector will want a vapor barrier here. If they do, I imagine ill want a smart vapor barrier here as well rather than standard plastic sheeting.
3. How do I install horizontal fire blocking to separate the two stud cavities? At what point in the framing process should I do this?
4. Can I fill the full 12″ depth full of blown-in cellulose as planned?
Sorry, so many questions.
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