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I live on a small remote island and I'm building a cabin out of what once was a barn. My soffits are plywood with a 2.25" diameter hole in the center of each 16" OC roof truss. This has equated to a total of 26 holes on each side of the roof, as seen in the photo below. Since I made these soffits I don't have a NFA value given to me for each hole. I'm then having a difficult time balancing my exhaust venting since I'll be purchasing something to use for the exhaust. Since products are given with NFA values, such as roof static vents at 50" sq inch NFA, it doesn't seem to line up with simply calculating the area of all the circles for my intake.

In the picture below you can see I did that math, 52 holes with each hole being 10" area, that would equate to 520" square inches of open intake. But I do have rat wire and insect screen over the holes so they aren't pure unobstructed openings. I'm looking at static roof vents that have a 9"x9" square opening, wondering if I can do 9x9 = 81 square inches and be using that as a comparative value to my intake holes to find a balance.

The cabin is really quite small, as you can see, I kinda felt like 3 of these https://www.homedepot.ca/product/gaf-master-flow-50-sq-in-nfa-aluminum-square-top-roof-static-vent-in-black/1000405504# would be enough? But I don't want to have too much exhaust.... I'm stuck :( please help!

enter image description here

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    Math error - each hole is about 4 square inches. It's Pi * r * r not Pi * d * d (radius-squared, not diameter-squared)
    – Ecnerwal
    Sep 10, 2023 at 2:38
  • Sorry I skipped over some steps in my math explanations. Diameter of each hole being 2.25" or 5.7cm. The radius then being 2.85cm. Area of the circle being (2.85)(2.85)Pi = 25.5cm If I'm correct. That being approx 10"
    – Joel
    Sep 10, 2023 at 2:52
  • Length is not area. Why are you mixing units, anyway? it just invites such errors. 25 square centimeters is a bit under 4 square inches, converted correctly.
    – Ecnerwal
    Sep 10, 2023 at 2:53
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    Wow clearly my math is messed up in numerous ways, I'm sorry
    – Joel
    Sep 10, 2023 at 2:58
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    Just pick a system and work in it. If you're in a metric area (i.e. most of the planet), use the metric units consistently and the math will be consistent. Mixing units is how NASA lost a Mars mission...
    – Ecnerwal
    Sep 10, 2023 at 3:03

1 Answer 1

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Your hole area is actually 4 square inches per hole, if we can not quibble about 0.0239 square inches. That is indeed 25.5 square centimeters, with a similar non-quibbling policy.

If your insect screen is roughly 1/16" (or 1.5mm) mesh, it cuts the net free area in half, so 2 square inches NFA per hole (the "rat wire" is free, unless it 1/8" (3mm) or smaller, with 1/8" cutting the net free area to 80% of open, and 1/4" having no impact on NFA) according to a chart at this site: http://www.atticbreeze.net/ventilation.html (not affiliated)

So 104 square inches NFA with insect screen at approximately 1/16", or just over 80 square inches NFA if the added rat wire is 1/8" mesh.

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  • The insect screen is 1/16" mesh and the rat wire is 1/4" hardware cloth. Thank you for this answer, I don't know still how my math is so wrong with calculating my holes. Thank you for pointing this out. So my intake at 104 square inches NFA is barely meeting 1:300 rule, dang. If I keep intake as it is, I could then do 2 of the static roof vents I listed, both being rated 50 NFA... hmm
    – Joel
    Sep 10, 2023 at 3:06

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