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Attic ventilation with covered soffits

dug8498 | Posted in General Questions on

I’m sure this has been asked before, but I couldn’t find this exact scenario.

Our house is a 2 story contemporary and we are in the process of having it completely re-roofed and re-sided. It currently has soffit, ridge, and gable venting. I came home the other day and the siding guy had completely wrapped the soffit vents in metal, turning them almost into a trim piece.

I expressed my concern about sufficient attic ventilation and he insisted that there will be no issues in the attic as i have the gable and ridge vents. I called the owner of the company (they have been in business a long time and are a well respected company) and he said the same thing. Told me he guaranteed it would be fine.

My attic has a pretty steep pitch. house is 35 years old, no issues with mold or moisture in the attic so far. last spring I air-sealed the attic floor (rim joists etc) and blew in 16 inches of cellulose.

What do people here think? Are they correct? Or is this a dumb idea that i should put a stop to?

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  1. Jon R | | #1

    > sufficient attic ventilation

    It's easy to verify/explain the 1:150 ventilation area that code requires for adequate ventilation in your new configuration. Harder to explain to others is that there is a small heat loss advantage (caused by less attic pressure and so less leakage) to 50-60% of the ventilation being low (eg, in the soffits). How much heat loss determines if it is worth reopening the soffits, but I'm not aware of anyone who has quantified this with a well sealed attic floor.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Dug.

    Over the years, I read and heard a lot of opinions on attic venting. While it may not be as important as some folks make it out to be, I've never heard of problems due to well-executed attic venting. Air flow promotes drying. That's a good thing. While I don't think you'll have a problem with the closed off soffit vents, if it were my house, I'd have kept them.

    Have you read this: All About Attic Venting? You may find it helpful. This is relevant too: Should I Close Off My Gable Vents?

  3. Expert Member
    Dana Dorsett | | #3

    >"It currently has soffit, ridge, and gable venting."

    Gable venting undermines the performance of soffit-to-ridge venting by providing a lower impedance short circuit path for the air being drawn out the ridge vent. So most of the soffits weren't getting any air flow to speak of in the first place. If it didn't have moisture problems before, closing off the soffit vents isn't likely to change that.

    Ideally it would be either soffit to ridge venting (with more than 50% of the free area at the soffits), OR gable vents of adequate size, not both.

    1. Expert Member
      Malcolm Taylor | | #4

      The most common venting arrangement for trussed-roof houses in Canada has always been soffit vents and roof vents located near the ridge spaced along the length of the roof. Where does that fall in the range of effectiveness? Do you think they draw from one another like the ridge/gables, or do they act more like an intermittent ridge-vent?

  4. Jon R | | #5

    US code suggests that either using 1:150 with vents anywhere or using a vapor retarder (in Z6-8) and 1:300 with high/low vents (at 40:60-50:50) is effective. High is defined as within 3 feet of the ridge. Low is defined as in the bottom third.

    I've seen no indications that, with good air sealing, anyone needs to worry about optimizing this. But if someone wanted to be conservative, a simple, sure-fire way is to increase vent area.

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