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Are vapor diffusion ports now required by code?

fromtxwithlove | Posted in Building Code Questions on

  I am in climate zone 2A, building a house with an unvented, condition attic space (ZIP system wall and roof). Roof deck will have continuous exterior insulation placed on top.
  After researching the IRC (, I noticed that if you’re foregoing spray foam (only using air-permeable insulation) directly under the roof deck, that vapor diffusion ports are a requirement (not optional). My original plan was to insulate under the roof deck with mineral wool bats, and forego a vapor diffusion port. The attic will be air conditioned and dehumidified. Reading through some of the questions and posts here made it seem like vapor diffusion ports were optional, but the code appears to be requiring them.

My questions are:
  1) Am I interpreting the code correctly?
  2) If I forego a vapor diffusion port, would dedicated de-humidification and air conditioning of the attic be enough to mitigate any condensation issues?


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  1. Expert Member


    1) The only code compliant methods I know which allow permeable insulation directly in contact with the sheathing are to use a layer continuous insulation above the sheathing, or install a vapour-diffusion port.

    2) Very unlikely.

    1. fromtxwithlove | | #3

      Thank you for the reply Malcolm.

      I updated the question to add the fact that the roof will have a continuous layer of insulation above the sheathing. So considering this, it sounds like I can forego the vapor-diffusion port?

      1. Expert Member
        MALCOLM TAYLOR | | #4

        Yes you are good to go.

  2. crawfordesquire | | #2

    sounds like exterior rigid insulation outboard of the sheathing is the answer, at least to the first part as malcolm said.
    if you're in climate zone 2a, humidity is an issue regardless of your roof assembly.

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