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Gable fan vs. gable vent

kevinjm4 | Posted in General Questions on

Climate 4c. Wondering if I could get some opinions on this. After reading some articles on here I may be changing my mind.

My current set up is soffit intake with a gable vent on one side, and a gable vent/fan on the other. The fan however is a 10 speed variable fan, dependent upon level of temp, and/or humidity.

Im considering removing the fan altogether, but it rarely gets up to much speed anyways since our temperatures aren’t all that extreme out here and humidity isn’t either. So it usually sits at around speed 1-4 (lower end speeds). Then shuts off after a little, and turns off and on intermittently throughout the day.

eventually I will be re-roofing and either putting a ridge vent in, or some roof vents near the ridge, and closing off the gable vents. I believe those two are my best options for unfinished attic venting. A ridge vent being optimal.

but for me as of now, what is best? My current set up, with my “smart” fan on one end and gable vent on other with soffit intake, or remove the fan and allow the gable vents to exhaust “naturally”.

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  1. Expert Member
    Dana Dorsett | | #1

    A soffit + gable solution takes most of the intake flow near the gable, with almost no flow from the rest of the soffit area.

    Attic ventilation is primarily about removing humidity, not heat. If the attic floor is adequately insulated the impact on cooling energy use often goes up, since the fan used power, and depressurizes the attic, drawing conditioned space air up into the attic through any leaks.

    When you add the ridge vent be sure it's no more than 40% of the free area as the soffit vents to prevent it from depressurizing the attic, and close up the gable vent.

    1. kevinjm4 | | #4

      Dana, I will be able to meet that 40% mark but will be way under it even. Wondering if this will be a problem.

      When the fan was installed, because it had a max output of 2800cfm, The soffits were also adjusted to allow for more airflow should the fan run at full speed. So currently my soffit intake is 1440 sq. in. of nfva. While most ridge vents I see (given my 36’ ridge) are around 500 sq. In. Is this a problem... too wide a disparity? It’s close to 75% - 25%... if it’s a problem maybe I could just make the ridge opening wider in the framing possibly allowing for more nfva. Or cover some soffit, which wouldn’t be ideal for me with a low sloping roof.

  2. GBA Editor
    Martin Holladay | | #2

    You don't want to use a fan to ventilate your attic. To learn why, see this article: "Fans in the Attic: Do They Help or Do They Hurt?"

    The most important step you can perform to prevent attic moisture problems is to seal air leaks in your ceiling. For more information on that issue, see these two articles:

    "All About Attic Venting"

    "Air Sealing an Attic"

  3. Expert Member
    BILL WICHERS | | #3

    If the fan rarely ramps up to very high speeds, that’s an indication that it isn’t really doing very much and probably could be removed with little if any impact. A ridge vent is a much better way to go, and very easy to install while a new roof is going on (it’s pretty easy to install even without a new roof going on).

    Roof ventilation is much less of a concern if you have the attic floor well air sealed and insulated. I’d probably just leave the fan in place myself, and remove it once the ridge vent goes in. The thinking is maybe this will avoid a trip into the attic. I loathe attic work. You can always shut the fan off until you remove it.


  4. kevinjm4 | | #5

    I posed this question to Dana after he gave me some advice. Might as well ask everyone else too...

    My system will be about 75-25 intake to exhaust (1440 nfva soffit intake/442 nfva ridge-depending on the product). I’ve heard you want more intake than exhaust for sure, and Dana mentioned 60/40 intake/exhaust. But I’ve never heard any recommendations go further than that in terms of more intake than 60%. Wondering if mine will be ok.

    If that’s too wide a desparity between intake and exhaust, the one think I think I could do would be that I could cut one half of one side of the houses soffit intake in half reducing it to 1080 total intake, but that would mean 360 nfva from one side of house, while the other side of the house had 720 intake. Don’t know if THAT would be a problem either...

    Thanks again.

    1. GBA Editor
      Martin Holladay | | #6

      My advice: don't sweat the ratio between the soffits and the ridge. It doesn't matter very much.

      Here's what's really important: seal the air leaks in your ceiling, so that no conditioned air enters your attic from below. If you can manage that, the venting details really don't matter much.

    2. Expert Member
      BILL WICHERS | | #7

      The only issue that much intake venting might make is for a bit higher chance of windwashing with the insulation. If you use baffles in the recommended way it won’t matter though so you’re safe.

      I can’t think of any other reason way more intake venting would pose a problem.

      I agree with Martin: don’t worry too much about it. It might be worth putting in some baffles in the soffit area if you don’t already have them, and those baffles have other benefits too like keeping insulation from blocking the vents.


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