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Community and Q&A

Air Barrier in Conditioned Attic

ddbear | Posted in General Questions on

My conditioned attic roof currently under construction (almost done with plywood sheathing stage) has the following layers: rockwool (inside attic between rafters), existing skip sheathing, FR10 fiberglass slipsheet, 3″ polyiso insulation, plywood sheathing, FR10 fiberglass slipsheet, G40 underlayment, lightweight concrete tile.  I’m in climate zone 3, Southern California.

On a different question that I posted, people asked “where is your air barrier?”  Originally I was planning on a glass or foil faced polyiso, but it was impossible to find because of supply chain issues, so I had to go with a more air permeable regular facer.  The seams are not taped.

What are everyone’s thoughts about adding a layer of DuPont Tyvek Roof Protector on top of the upper plywood sheathing as the impermeable membrane?  Would this be useful in my climate, or would this be overkill/over-engineering the assembly?  I’m not sure if the cumulative impermeability of all my medium-impermeability layers may be just fine.

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  1. Expert Member
    Michael Maines | | #1

    It's best to have a single air control layer, mainly so you can connect it to the air control layer at the walls. Plywood is airtight if you tape the seams, though low-quality plywood can have a lot of edge voids to deal with. Using the Tyvek product for air control should work.

  2. ddbear | | #2

    Since I don't have an air control layer at the walls, and I'm in a mild climate (Southern CA) I ended up not specifying an additional air control layer (might be a scramble to try to find Tyvek at the fast pace the roofers are constructing).

    I figure that the cumulative low permeability of plywood and all the other layers may be ok, and if moisture does get trapped somewhere it can slowly escape.

  3. ddbear | | #3

    I told the roofers to tape the seams of the 1/2" CDX plywood upper sheathing. Good suggestion, thanks!

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