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Community and Q&A

Attic condensation

Firehorse5 | Posted in General Questions on

I live in zone 4a. I have a saltbox design home which is 1 story in the front, 2 stories in the back with ridge vents. At the highest point I get wet spots on the ceiling over my bedroom—even when there hasn’t been rain for days. There are no leaks and when I had the roof replaced I was told it was moisture condensation due to having no baffles installed in the soffits. I have lots of blown insulation and they just filled the soffits up with it when the house was built.

Do you agree with this assessment?

Also, the upstairs gets really hot in the summer and I was thinking of installing a solar gable vent at the highest peak on the side of the house. Do you think this will help with the condensation?


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  1. GBA Editor
    Martin Holladay | | #1

    Condensation in your attic is almost always a sign of a major air leak in your ceiling or the tops of your partition walls. Warm interior air is rising into your attic, and the moisture is condensing on cold roof sheathing.

    You (or an experienced contractor) need to enter the attic and seal the air leaks. Here is a link to an article with more information: Air Sealing an Attic.

    I don't recommend that you install an attic fan. For more information, see Fans in the Attic: Do They Help or Do They Hurt?

  2. Firehorse5 | | #2

    Ok, thank you for the fast response!

  3. Firehorse5 | | #3

    Thanks for your help! The house is only 1500sf, with probably 2/3 of that on the first floor. There are 3 bedrooms and a bathroom on the 2nd floor.

  4. GBA Editor
    Martin Holladay | | #4

    I don't share Jon R's belief in the efficacy of powered attic ventilation. I stand by the advice in my article, Fans in the Attic.

  5. Jon_R | | #5

    > Do you agree with this assessment?

    Blocking airflow from the soffits reduces attic ventilation which does increase condensation accumulation.

    > Do you think this will help with the condensation?

    In the Winter, an attic fan blowing inward (increasing dilution and reducing or reversing the pressure imbalance that causes moist indoor air to flow into the attic) will certainly help. The typical attic exhaust fan (blowing outward) will increase this ex-filtration but also provide more dilution - probably with a net result of making attic condensation worse.

    Regarding Summer heat, a small attic exhaust fan (as most solar ones are) can reduce or stop the infiltration to the interior caused by reverse stack effect and provide a little bit more attic cooling via dilution. A larger one (probably not solar powered) can go too far and cause significant ex-filtration. This latter case is the one that most write ups conclude is harmful. Without pressure measurements, I wouldn't guess which one you will get.

    Air sealing the attic floor always helps but might be significantly more expensive than a solar fan.

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