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Community and Q&A

Controlling humidity levels in unvented attic space

user-509810 | Posted in Energy Efficiency and Durability on

We have three attic spaces that are sealed with open cell spray foam. There are HVAC systems in all three spaces. We live in Herndon, VA. I checked temperature and humidity levels the other day and found the attic spaces to be running about the same temperature ( 76 degrees) but 10% to 12% higher in humidity (54% first floor vs. 65% attic space). Is there a need for us to be concerned about this differential and if so how would we reduce the attic space moisture levels? We do use the attic spaces for storage. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    The problem you report is common. Here is a link to an article with more information: Open-Cell Spray Foam and Damp Roof Sheathing.

    In that article, I wrote, "Here at GBA, we have been receiving an increasing number of questions from homeowners who are complaining that their unvented conditioned attics (most of which were created by installing open-cell spray foam on the underside of the roof sheathing) have problems with high indoor humidity. Building scientist Joe Lstiburek has investigated a number of attics with similar problems. “How come the moisture ends up in the attic space and not in the main part of the building?” Lstiburek wrote. “Moisture-laden air is lighter and less dense than dry air. Moisture-laden air ends up in the attic due to this ‘hygric buoyancy.’” Although Lstibuerk used to think that the moisture in these attics was morning dew that was being driven through asphalt shingles by inward solar vapor drive, he now reports that the inward-solar-vapor-drive theory has been disproved.

    "If you have an unvented conditioned attic with high humidity problems, the best solution is to install a supply-air register connected to your forced-air HVAC system in the attic (and in some cases, a return air grille as well). If the attic is heated in the winter and air conditioned during the summer, humidity problems should disappear."

  2. charlie_sullivan | | #2

    I would be interested to see a comparison of the energy implications of taking care of the problem with a register on the HVAC system vs. a dehumidifier. Of course on day 1 the dehumidifier would be a huge energy hog, but if it's on a humidistat, it might be able to run very little after that first day, assuming the fan doesn't run when it doesn't need to. Without more careful analysis to show which is a more efficient option, you could opt to choose whichever is easier.

  3. Expert Member
    Dana Dorsett | | #3

    If it's simply a vapor buoyancy issue this problem would show up with ANY air-tight attic that had no active venting with conditioned air, but it seems to be more commonly found with open cell foamed roof decks that have no interior side vapor retarder.

    I suspect the foam itself and the roof deck are the moisture reservoirs, where the moisture gets driven into the attic air during the day when the sun is heating the roof deck, then drawn back in over night when the radiational cooling of the roof into the night sky brings it's temp down below the outdoor air temp, near the outdoor air's dew point temp. If this is the case, an interior side vapor retarder (even better, a smart vapor retarder) should relieve those symptoms quite a bit, though without actively exchanging that air with conditioned space air it could take a long time.

  4. user-509810 | | #4

    Thank you all. I will read the article your recommended Martin..

  5. user-509810 | | #5

    What would be optimum attic humidity with an indoor humidity of 54% and an outdoor of 75%?

  6. GBA Editor
    Martin Holladay | | #6

    If your attic is included in the conditioned volume of your home, it should be possible to maintain the indoor conditions in your attic at the same levels as the indoor conditions in your living room. If your house has a forced air system and central air conditioning, you can accomplish this by installing a supply air register and a return air grille in your attic.

  7. user-509810 | | #7

    Thank you all again.

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