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1:300 net free vent area ratio vs. continuous soffit vent

user-5355320 | Posted in Energy Efficiency and Durability on

In the GBA article Lstiburek’s Rules for Venting Roofs (07-24-2011), he notes the importance of having continuous soffit vent so the entire under side of the roof gets air flow. Although he does not mention the 1:300 rule (in that particular article) he does talk about splitting venting 60% to soffit and 40% to ridge so the space stays at a higher pressure than the inside of the house.
I assume that we are suppose to still follow the 1:300 guidance for determining NFVA. BUT, what if running a continuous soffit vent means the ratio is going to be more like 1:150?
In parts of California we have to use vulcanized soffit vents, which leaves limited options, and we can’t simply put in a lower value NFVA product to stay close to 1:300.
What’s more important, not over-venting or having continuous soffit vent?
What ratio is too much over-venting?

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  1. GBA Editor
    Martin Holladay | | #1

    Joe Lstiburek and I have slightly different opinions on this issue. You should read several articles on attic venting and then make your own decision. Of course, you need to make sure that your local code official approves of your plan.

    Here's a link to an article I wrote on the topic: All About Attic Venting.

    Q. "What's more important, not over-venting or having continuous soffit vent?"

    A. Having a continuous soffit vent.

    Q. "What ratio is too much over-venting?"

    A. Stop worrying about the ratio. I don't think the ratio matters very much. Remember, there is no code requirement for ridge vents; only soffit vents are required by code.

    That said, here's what researchers have found: to keep moisture out of an attic, the most important thing you can do is to make your ceiling as airtight as possible. If you are able to limit air leakage through your ceiling to an absolute minimum, attic venting really doesn't matter very much.

  2. charlie_sullivan | | #2

    No ratio of ridge to soffit venting can make the attic pressure higher than the pressure inside the house, just below the attic, during the winter. It will always be lower. That's why, like Martin says, good air sealing is more important. But bigger soffit vents relative to the ridge vent will help reduce the pressure differential between the attic and the interior of the house. So there's nothing wrong with making the soffit vents bigger than would be indicated by the 1:300 ratio or the 60:40 ratio. Neither of those is a hard and fast rule, but to the extent that they are rules, they should be considered minimums for the soffit vent sizes.

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