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Community and Q&A

Possible to add Vapor Diffusion to Unvented Attic?

bwsct | Posted in General Questions on

Is it possible to add a vapor diffusion port to a unvented attic that is closed cell spray foamed at the rafters?  
The roof is tongue and groove and in need or replacement.

In the summer the 2nd floor of the house has very high humidity and I was thinking that would help.  Or another option would be to add HVAC supply to the attic.  

There is currently ducting to the bedroom in the attic hence the reason the roof has CCSF at the rafters..

Your opinions are appreciated.

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  1. user-5946022 | | #1

    Closed cell foam is generally considered a vapor barrier, so two things:
    - the high humidity in the attic is not nearly as much of a threat of rotting the decking as it would be with open cell foam, because the vapor can't get through the foam to the decking.
    - the vapor diffusion port would not reduce the attic humidity much because again the humidity can't get through the foam to get to the port.
    Now if your attic were open cell foam, yes a vapor diffusion port can be added from the exterior and is a relatively simple thing to do. The caveat is the people doing it need to know what they are doing so it gets done properly.

    Multiple articles and posts here indicate humidity rises so if you have higher than desired humidity in your attic you could try lowering the humidity in the space below the attic and see if that helps. Also your idea of cutting in a supply is sometimes also done.

    1. bwsct | | #2

      I was wondering if cutting into the CCSF to create the vapor diffusion port is ever done?

      Its + - 2 inches of CCSF.

      1. Expert Member
        DCcontrarian | | #4

        You're on the wrong track with a "vapor diffusion port." The outside air has more moisture than the inside air, you'd be bringing in humidity.

      2. Expert Member
        Michael Maines | | #5

        You say your roof is T+G and in need of replacement. Do you mean the sheathing needs to be replaced, or just the roof cladding?

        2" of CCPSF is usually around 0.5-0.6 perms. As a class 2 vapor retarder it will allow moisture to diffuse through slowly, but I have never heard of it being used in conjunction with a vapor diffusion port. It probably wouldn't hurt anything and might help a little, but if you're experiencing high humidity, air-sealing, ventilation and dehumidification are probably your best options.

        1. Expert Member
          DCcontrarian | | #6

          I disagree with ventilation. If the house is conditioned the outside air is going to have a higher dew point in the summer.

          1. Expert Member
            Michael Maines | | #7

            DC, I agree, after re-reading the question and seeing that it's a summertime problem. If the wintertime relative humidity was too high, ventilation would help.

        2. bwsct | | #9

          The cladding needs to be replaced and areas of sheathing but not all of the sheathing.

    2. bwsct | | #8

      Thanks for the info. I need to begin elsewhere in the house with air sealing and insulating. Most of the rest of the house is uninsulated which I plan to try and fix soon but having cellulose blow in.

  2. Expert Member
    DCcontrarian | | #3

    In the summer the outside air probably has a higher dewpoint than the inside air so bringing in more outside air makes the humidity worse.

    Is the house air conditioned? If the air conditioning is not removing enough humidity I'd see if it can be adjusted, slow the fan down to dehumidify more. If that doesn't get it then a dehumidifier may be necessary.

    I'm not sure why you're focusing on the attic, it isn't going to be a source of humidity. Humidity is coming from outside air getting in, and occupant activity -- cooking, bathing, breathing.

  3. bwsct | | #10

    Yes. The house is airconditioned.
    I've realized from the replies that I need to focus on air sealing elsewhere in house before worrying about the attic right now.
    Thank for your reply.

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