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Vapor diffusion ridge vent worthwhile for a conditioned attic?

ddbear | Posted in General Questions on

I’m in the middle of having a new roof constructed for a conditioned attic, with 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.  I was reading this article,

https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned

and found the “vapor diffusion port” (vapor diffusion ridge vent) section interesting.  I didn’t think of this before.  Now is my last chance add this feature.  Is this something worthwhile to consider?  Where would the roofer obtain a special material that allows vapor to exit but prevents airflow?  Any particular brands, item numbers of this special material? 

My initial thought is that this may be overly complex and I can just allow a little bit of the air conditioning/heating into the attic space.  Or maybe this is useful?  Right now it wouldn’t cost that much to add because the roof is nearing sheathing stage.. except for trying to get some exotic diffusion material. 

Also would this have a big effect to reduce the R value of the assembly?

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Replies

  1. Patrick OSullivan | | #1

    What is your air barrier?

    In general, once you have exterior insulation on the deck of a sufficient amount, a vapor diffusion port would be unnecessary because you wouldn't have a condensing surface you're trying to make sure has an opportunity to dry out.

    > Where would the roofer obtain a special material that allows vapor to exit but prevents airflow?

    This describes most WRB materials (e.g. Tyvek) if taped.

  2. Expert Member
    Peter Engle | | #2

    If you use foil-faced polyiso insulation and tape the seams, that would be your air&vapor management layer. There is no need for a vapor diffusion port with this design.

  3. ddbear | | #3

    Thanks, it sounds like I don't need to worry about a diffusion ridge vent in that case.

    >What is your air barrier?

    I was thinking that all of the layers of underlayment (FR10 fiberglass, G40 fiberglass modified asphalt underlayment) might be sufficient as an air barrier. The polyiso is standard Atlas 2 polyiso (organic facers) - I was originally trying to get some with fancy facers (glass faced, foil, etc.) but couldn't get these because of supply chain unavailability. The seams are not taped.

    Half the roof now has the plywood upper sheathing on it. Should I add an extra air barrier layer of material on the top of the plywood in addition to the underlayment layers? I was originally thinking that the combined medium permeability of all these layers of underlayment might be enough, but not sure if it has to be totally air tight.

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