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Old 09-18-2009, 04:33 PM   #1
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Figuring out soffit ventilation amount


Ive researched this topic and have 2 questions question:

I have an 1100 sq foot attic. Ive divided it by 300 and get 3.7 sq ft or 528 sq inches. I divide that by 2 and get 264 sq inches. So that means 264 for roof venting and 264 for soffit to be balanced. My question is this... For the soffit 264 sq inches, do I divide that by 2 to get the amount for each side of my ranch,132 sq inches per side, OR, is that 264 sq inches per side?

Then... Im looking at vinyl soffit venting at the big box stores, the 12 in wide by 12 ft vented lengths and I cant find a NFVA listing. What would be the approx NFVA for this type of venting?



Last edited by creamaster; 09-18-2009 at 04:35 PM.
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Old 09-18-2009, 05:11 PM   #2
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not sure which one.


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Old 09-18-2009, 05:24 PM   #3
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If you are in a severe cold climate with a vapor barrier on the ceiling, divide by 300. Otherwise you should divide by 150 or even 100. So you need, minimum of 113 lin. ft. of that soffit venting of which mics 54 said. Divide that total by how many sides of rafter bays you have.
1100/150 = 7.34 total / 2 (50/50 intake/exhaust) = 3.67 x 144 = 529 sq. inches / 4.68"per ft. = 113 lin. ft. of soffit full venting.

Be safe, Gary
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