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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14 hours ago, DRAWZILLA said:


Perry,  do you use 1/300 or 1/150?  


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