# Attic Ventelation Calculation?

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Currently our code says:

R806.2Minimum vent area.

The minimum net free ventilating area shall be 1/150 of the area of the vented space.

I think at one time it said floor of the attic.

So if you had cantilevered trusses would you include soffits in that area?

I think I've used the exterior face of the exterior wall for years.

I seem to remember the ceiling delimiting the attic & I used the interior face of the exterior wall. Maybe 20 years ago.

Question is what defines "area of the vented space"?

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I would suggest to use the plan view area of conditioned space at the ceiling level. I can't see this being a problem with any code reviewer. Perhaps the only problem with using the footprint of the entire roof is added cost in roof vents; a minor issue but one that might bug a contractor.

And, when in doubt, consult with the plan reviewer.

In Canada it is more clearly defined. Our building code term is "the insulated ceiling area". Otherwise, our 2 codes are very similar re.: roof venting.

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1 SF of net free area for every 150 SF of attic floor space is defined as Length X Width of the floor of the attic.

Usually, if you have an 1800 SF attic floor area you /2 = 900 SQ IN for intake and 900 SQ IN for exhaust ventilation, that goes for about up to 6/12 pitch from there you add about 20% for up to 10/12 pitch anything after that you add about 30%.

This is a basic calculation for the soffit/ridge balanced vent system to meet 1/150 req. If you adding fans, etc you will have difftent calculations.

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In my area, if you have eve and gable vents, you can cut the vent area to 1/300th.

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I always spec "ohagins vents . They go on the roofing and gppd for fire areas--no eave vents

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14 hours ago, DRAWZILLA said:

I always spec "ohagins vents . They go on the roofing and gppd for fire areas--no eave vents

Perry,  do you use 1/300 or 1/150?

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3 hours ago, dshall said:

Perry,  do you use 1/300 or 1/150?

1/150

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• 1 year later...

I found this handy guide awhile back! Hope it helps anyone following this topic

Kristina

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I have this little library piece that I made for myself as well. Please peer review it to make sure that my work is correct however it hasn't gotten any kickback during plan reviews so far! Hope it helps and all, please feel free to improve it or redline if it needs correcting!

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Required attic ventilation is 1/150 or 1/300 depending on ......

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It would be great if there were more attic vents in the library.  The only ones I've found are "Gable End" triangular.  It should be possible to create schedules with the areas to verify the provided vents are adequate.

In order to make that work for the code section Scott referenced there would need to custom fields to identify the positions.

Scott;  Note that the 1/150 requirement is the most stringent so it will always work, regardless to the positions.

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12 hours ago, Joe_Carrick said:

It would be great if there were more attic vents in the library.

There are a handful of others in the core catalogs.

I was looking at them just the other day. Are these

not what you are looking for?

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7 hours ago, Rich_Winsor said:

Are these not what you are looking for?

They can work, but they are essentially "windows" and their properties are not correct.  ie: energy values, glazing, etc.  In addition, there doesn't appear to be a single mesh "Eave Vent".  There also doesn't appear to be an "Eye Brow" roof vent.

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On 1/8/2022 at 6:17 AM, Joe_Carrick said:

There also doesn't appear to be an "Eye Brow" roof vent.

Appearances can be deceptive. There are at least two: one in the "Roof Attachments" Bonus Library, and one in the standard MEP Ventilation library. Search under "Eyebrow"  (single word).