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Community and Q&A

Attic Humidity and Closet Vents

archmg | Posted in Mechanicals on

For a house in a hot, dry climate (Vacaville CA), the contractor is proposing an unvented roof system.  While I have never done one of these as an architect in the Bay area, I see the benefit from both an energy efficiency standpoint and WUI fire code standpoint.  However, a while back when I was skeptical and researching unvented attics, I remember encountering one study that stated something to the effect that humidity in unvented attics was consistently higher than the rest of the house and long term effects of this was not yet know.  I can no longer find this study on the world wide web now that I am looking for it again.  I thought it was one done on the west coast, so not in a hot humid climate like the SE, and within the last 10 yrs or so.  Does anyone recall this?  Secondly, because of this article which I can no longer find, I had specified small 8×8 vents in the attic access doors, which are located in the backs of closets, to equalize some of the humidity. Given the access doors are in closets, would there be much heating and cooling loss through the small vent through the door in the back of the closets (there are 4 access doors into attic under a steepish gable roof on the second floor)?  Thanks in advance.

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Replies

  1. Jon R | | #1

    Some good info here:

    https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned

    Note the paragraph containing "50 cfm ...". But IMO, if you measure attic humidity and it's always low, then you don't need any airflow into an unvented/conditioned attic.

  2. Charlie Sullivan | | #2

    Can you clarify the planned insulation of the roof? Assuming that your insulation and air sealing are at the roof, the proposed vents are a good idea and would not lead to heating/cooling loss.

    1. archmg | | #3

      yes, we are doing open cell spray foam R38 at the underside of the roof, R21 open cell spray foam at exterior and knee walls and R21 cellulose at gable ends.

    2. archmg | | #4

      Charlie, assuming we have a good seal at the roof, and mech heating and cooling equipment is located in the attic space, but we are not heating/cooling the actual attic space (i.e. no vents/diffusers to this space), do you still think there will be no loss? Are you a mechanical engr or contractor?

  3. Jon R | | #5

    Conditioning the attic less (ie, no 8×8 vents) will use a little less energy. If measurements show that the attic gets too humid, then the hvac in the attic makes the fix easy.

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