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Community and Q&A

Installing a vapor diffusion port in zone 4

pdx_homeowner | Posted in General Questions on

My house has a cape cod style upper floor where the roof creates the ceiling for a portion of the living space.  The current attic is vented by 3 box vents and the eave space is unvented.  There is little insulation and in some places no insulation is provided at the eaves.  Rafters are 2x4s at 24” O.C., roof geometry is a cross gable with “T” footprint, and roofing material is asphalt shingles with hopes to replace with a metal roof in the future.


My plan is to demo the attic ceiling, add some skylights, and do an unvented roof assembly.  Exterior roof insulation is out of budget, so I plan to use plywood gussets to extend the rafter depth at every other one to deepen the roof cavity to provide the code minimum R-38 (zone 4C) insulation requirement using the flash and Batt method.


One option I’m considering is using spray foam (R-20 air impermeable required by code) and use blow in insulation for the remainder.  But have mixed feelings on using spray foam.  So, my other option is to use rigid foam.  But I worry about the installer error (me) and potential air leaks.


And then I read about the research on vapor ports and it makes me wonder….


I understand that fundamentally a vapor port is not needed if I am providing air impermeable insulation against the sheathing up to the dew point , but would it be prudent to install one at the ridge anyway?  Would it help mitigate the risk if I did flash in batt or cut and cobble with rigid insulation?


Influential articles:

Insulating a Cape Cod House

Vapor Diffusion ports (Rats, I’m zone 4C)

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Natalie,

    The International Residential Code only allows vapor diffusion ports in climate zones 1-3. I think you know that. Also, cut and cobble is not recommended for unvented cathedral ceilings. Follow the recommendations in this article for your flash-and-fill assembly: How to Build an Insulated Cathedral Ceiling

  2. pdx_homeowner | | #2

    Thanks for response and link. I think the heart of my question is asking for thoughts on adding a vapor port outside of zones 1-3 to lessen the risks associated with unvented assemblies with historically bad track records like cut and cobble.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Do cut and cobble assemblies have bad track records when diligently executed, or is is simply that they aren't recommended because the risk of a poor job is much higher that with spray foam?

      Put another way: Are there problems associated with well executed cut and cobble roofs? It's still recommended on GBA as a way to insulate rim-joists.

      1. GBA Editor
        Brian Pontolilo | | #4

        Hi Malcom.

        It's my understanding that the method has a poor track record in roofs and is not a reliable method, potentially for a few reasons, but primarily because the risk with cut-and-cobble is framing movement that breaks the air seal. This is more risky in an unvented roof assembly than in other assemblies.

    2. GBA Editor
      Brian Pontolilo | | #5

      I'm not convinced that vapor diffusion ports should be considered best practice in any climate and therefore I wouldn't rely on them to make a risky assembly less risk. My opinion is to just go with proven methods.

  3. Expert Member
    Akos | | #6

    Some interesting read about cold climate diffusion vents:

    Generally anything is better than unvented with cut and cobble, but why take the risk?

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