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Community and Q&A

Is the Vapor Diffusion Port still recommended by experts?

2screws | Posted in General Questions on

I’m planning a new slab house build in CZ3 with OC spray foam conditioned unvented attic. I’m getting resistance from the spray foam sub about having a vapor diffusion port at hip roof ridge.

In April 2020 article, Dr. Lstiburek lists 3 ways to avoid humidity problems in conditioned attics— and Vapor Port is NOT mentioned? Is it no longer used?
Is the spray foam man right?

(3 methods in article are fan outflow balance with air intake, dehumidifier, erv/hrv.)

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  1. JC72 | | #1

    I suspect it's not code approved.

    Other than Fig 10 in your link an alternative is a dehumidifier (allowed by code) or HVAC supply run (may be allowed by code) would mitigate moisture issues.

    Fig 10 relates to what Joe discovered as the "ping pong effect". He discovered that as water vapor was absorbed and de-absorbed it would crawl along the surface of the foam migrating to the high points in the roof line. This is why in Fig 10 he has the exhaust inlet at the ridge line. Remember when you spray foam the roof line in your attic the space becomes part of the interior of your house.

  2. Expert Member
    ARMANDO COBO | | #2

    Vapor diffusion ports are allowed in the 2018 IRC (R806). I happen not to have jumped into the VDP’s wagon. My main reason is labor quality here in TX (CZ3), especially when there is so much at risk. For shingled roofs, I specify 2” R13 min. ccSPF against the roof decking plus 7” ocSPF R25 min., for a full R38 min. For metal, tile or flat (TPO) roofs, I specify 1.5” R10 min. rigid foam on top of the rood decking and 8” R28 min. ocSPF under the roof decking.
    Many insulator contractors con builders and owners into spraying 5.5” R20 min. ocSPF under the roof decking, because under the performance and AU codes, you are allowed to reduce the insulation amount, but I believe the “laws of physics and chemistry” do not care what codes you are applying. That’s something I learned from Dr. Lstiburek 25 years ago, and I believe that is very risky. I won’t do that to my clients.

  3. Expert Member
    Michael Maines | | #3

    Vapor diffusion ports are allowed in zones 1-3 starting with the 2018 IRC:

  4. Jon_R | | #4

    Vapor diffusion ports are only for air permeable insulation (like cellulose - see 5.2 in above code). With spray foam you don't need one (5.1.1 is sufficient).

  5. 2screws | | #5

    OK thank you all. I'll skip the port idea.

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