attic venting – oft-missed code requirement?
This question is regarding ventilated attics. Assume you’re calculating the required NFVA using the 1:300 ratio. Both IRC and IBC limit upper ventilation to 40–50% of the total required NFVA. For example, if your attic requires 1,000 square inches of NFVA, you must locate 400–500 square inches of NFVA — no more, no less — within 3’ of the highest point of the attic, regardless of the total amount of NFVA you’re providing. On most roofs, if you ventilate the entire ridge with a typical ridge vent, you’re probably providing way too much upper ventilation (unless you have a hip roof with a really short ridge). What’s the most feasible way to keep your upper ventilation within the code-required limits? Do you selectively ventilate only a portion of the ridge? Do you reduce the width of the sheathing gap at the ridge? Do you just ventilate the entire ridge without worrying about the numbers? Do you avoid the problem by using the 1:150 ratio exclusively? There’s so much discussion about attic ventilation, but I haven’t read anything about the need to limit ventilation at the ridge.
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Replies
Don,
Can you post a link to the relevant code section? It would be very useful in this discussion.
Thanks Malcolm, the relevant code sections are 2018 IRC 806.2 and IBC 1202.2.1.
Don,
Codes aside, I think it's good idea to have more of the ventilation located at the soffits. As Jon said: a bit of positive pressure in the attic is useful.
I also don't see any downside to providing more ventilation that the 1:300 ratio, so maybe it's as simple as seeing what the ride vent yields and making sure the same or more is present at the eaves.
If you wanted to stick to the 1:300 ratio, probably the easiest way is as you said to cut the appropriate sized gap in the sheathing.
What they're looking for is the same area on the ridge and the soffit.
Thanks, DC — I agree, ideally the eave/soffit ventilation and ridge ventilation will be equal. What I'm getting at is if you ventilate the entire ridge on a typical roof with a standard ridge vent, you'll probably exceed the maximum allowable ridge ventilation. And so I'm wondering how others avoid that problem — I've never seen it addressed in any attic ventilation article.
My read is that it's not a maximum allowable ridge vent, it's that anything over 50% doesn't count toward the required ventilation area.
That's what I thought at first. But I contacted the ICC, and they confirmed that the ridge ventilation cannot exceed 50% of the total required ventilation. These requirements are stricter than most people realize, I think.
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A 1:150 ratio is a good idea anyway - and then you can stop worrying about the roofers ignoring your "make the gap exactly x" instructions.
Also interesting to discuss the purpose behind the restriction. Supposedly to produce a little positive attic pressure - a good idea in cold weather. But in hot AC weather, a few pascals of *negative* attic pressure would offset stack effect (ie, be beneficial). But since wind effect (the primary driver of attic ventilation) varies so much, I expect that attic pressure is all over the place, positive and negative. So air seal well.
'regardless of the total nfva.'
Can you post the exact language where you think this is said?
From IRC R806.2 Exception item #2: "Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space."
IBC 1202.2.1 Exception item #2 has the same requirement.
The key word here is "required". This tells you that the upper ventilation is capped at 50% of the required ventilating area (i.e. the number you get when you divide the attic area by 300), regardless of how much eave ventilation you're providing. I actually called the ICC to get a technical opinion, and they confirmed that this interpretation is correct. It would be more logical to cap it at 50% of the total ventilation provided (eave + ridge), but that's not how the code is written.