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Interior air barrier over batts

MattJF | Posted in General Questions on

So I am getting near completing my flash and batt attic project. It is 3″ (r21 turned out 3″-4″) Gaco OnePass Low GWP, R30 Rockwool, 1-3″ polyiso furring strips on framing to increase depth. 

There are areas behind knee walls and above the collar ties that will not have drywall directly on them (see photos). I am told batts should have a 6 sided air barrier. This will need to be a flexible product in my case at there is limited access to these spaces. 

I am considering using Membrain or Tyvek. 

Is there any benefit of one vs the other from a moisture standpoint? The foam is 1.7 perm @ 1″. 

It is about $180 in materials for either product. Is an interior secondary air barrier worth it in this application?

(Sorry, all the photos need a 90deg clockwise rotation, not sure what happened)

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Replies

  1. Jon_R | | #1

    I would follow the recommendations here (and code of course).

    IMO a second air barrier is beneficial, even more so with cold sheathing.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Matt,

    Your insulation strategy needs to be able to dry inward, because water vapor could possibly condenses on the inside surface of the closed-cell foam. I also think a secondary air barrier could be beneficial. Both of the products you are considering will allow for inward drying. So, for $180, why not go for it? Just pay attention to the details--tape the seams, maybe tape over the fasteners. To be effective, air barriers need to be continuous.

  3. MattJF | | #3

    Thanks for the input.

    How tear resistant is Membrain? The spaces I would be putting this are a bit convoluted, so it would be nice if it was not fragile. How's it compare to 6-mil poly?

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