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Roof vent size for vaulted ceiling

nepal | Posted in General Questions on

Hi
I’m building a garage with a vaulted ceiling and at the point where i need to vent the roof. I see there are formulas for calculating attic ventilation but i can’t find a formula for calculating a vaulted ceiling.
Should I measure sq. ft. of each roof side and add both sides to get the total sq ft? For example, my roof is 25′ long and 10′ from wall plate to ridge. This is 250 sq ft per side bringing the total to 500 sq ft to ventilate.
So i’m thinking i can now use the formula which says i need 240 sq in at the soffit and 240 sq in at the ridge???
i hope i’ve made sense of my question and appreciate comments. thank you Jim

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nepal,
    The venting rules for a cathedral ceiling are a little different than for an open attic. In short, you need continuous soffit venting and continuous ridge venting (using any widely available commercial product sold as a soffit vent or a ridge vent). You also need a continuous air gap in every single rafter bay, connecting the soffit vent to the ridge vent. This air gap (directly under the roof sheathing) must have a minimum depth of 1 inch, but 2 inches is better.

    For full instructions on implementing these guidelines, see these articles:

    "All About Attic Venting"

    "Site-Built Ventilation Baffles for Roofs"

  2. nepal | | #2

    hi Martin, Thank you for the quick response. Maybe vaulted is the wrong word or phrase. I 'm aware that I would need baffles between each truss and I'm aware it would be best to use screen soffit material. The roof trusses have roughly 26" of space between the under side of the roof plywood to the bottom of the truss cord at the ridge (plenty of air cap there). There is roughly 6" between the top wall plate and the roof plywood ...so yes, i would definitely need baffles at this end. My plans call for 1/150 with 25% min at eave and 25% min at ridge...uniformly....
    I was hoping to use gable end wall vents as well as vented soffits. i'll make a call to my inspector for clarification ....Thank you again

  3. GBA Editor
    Martin Holladay | | #3

    Nepal,
    I think you are describing a scissors truss, but it's hard to know without a sketch.

    In any case, your biggest challenge is figuring out what to do near the eaves, where you have only 6 inches of height to work with. That is sub-optimal.

    If you leave a 1-inch high ventilation gap, and your ventilation baffle is 1/2-inch thick, you are left with only 4.5 inches for insulation. That's not enough. You might get R-29 in that space if you use closed-cell spray foam, but that's not enough to meet minimum code requirements anywhere (except maybe south Florida).

  4. Joshua Salinger | | #4

    Hi Martin. I happened upon this Q&A when searching for approaches to venting cathedral ceilings. There's a lot of information online about the vent space needed between roof insulation and roof sheathing for a cathedral ceiling; 1" is code, 1.5"-2" is recommended. What's difficult to determine is how much NFA venting is needed at the intake of this vent space and at the exhaust. I can't seem to find specific info on this. Is there a general rule of thumb? I'm guessing that it would depend on the length of the rafter bay (distance between intake and exhaust) and perhaps also how step the roof is. I'd Love to know your thoughts.

  5. Jon R | | #5

    2018 IRC R806.2 contains the code requirements for "net free ventilation area". How much depends on the vapor retarder and the high/low ratio. The 1" is in R806.3.

    Do air seal the interior side well, test the air sealing and don't use a baffle that would significantly block moisture that makes it into the insulation from continuing to the vent channel.

  6. Joshua Salinger | | #6

    Thanks for your answer, Jon.

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