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Community and Q&A

Damp Sheathing with Unvented Conditioned Attic

home26 | Posted in Energy Efficiency and Durability on

Climate Zone 1a    3300 sq ft home
I am planning on installing open-cell spray foam on the roof rafters and seal the attic. I read the article about damp sheathing and was hoping to avoid it.
1) Do I need to add HVAC supply to the attic to avoid humidity? If so how many CFM?
2) By creating a tight envelope will I need an ERV or a whole-home dehumidifier to bring in fresh air and keep moisture low?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Home.

    Hopefully some of our more mechanically-oriented members will jump in with some thoughts. As you read in the article, damp sheathing with open-cell foam is more of a concern in colder climates than your. In general though, you'll want to watch humidity levels in the attic. Treat it like the rest of the conditioned space.

    A tight home needs balanced ventilation, so an ERV is a good idea. Also, in a tight and well-insulated home, the air conditioner may not run enough to dehumidify the space, in which case, you may need a dedicated dehumidifier.

    A good HVAC designer should be able to spec all of this for you.

  2. Jon_R | | #2

    > the air conditioner may not run enough to dehumidify

    Imagine the day that is very humid and 75F - the air conditioner won't dehumidify at all. So unless that never happens, install dehumidifiers(s) and an ERV.

    On the other hand, whole-house is expensive and in most cases, a couple portables will provide good enough comfort.

  3. Carhus | | #3

    Would vapor diffusion ports at the roof ridge work? Dr. Joe Lstiburek created the concept. Wondering if that would work here with open cell foam.

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