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Old 01-21-2011, 12:59 PM   #16
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Vinyl Perf Soffit over Wood - No Vents

Originally Posted by jmiller View Post
Did you calculate the needed ventilation for an attic your size? The ridge vent may not need to be continuous to get the minimum exhaust, which solves the problem of lack of room for intake if you can leave some uncut. It's better to have more intake than exhaust, than it is to have 'continuous' ridge vent, so long as you satisfy codes.

The attached image is the suggested (50/50 NFA @ 150) ventilation for my attic space. So this would work. But more is better if available, right? What if I wanted to maximize my full soffit intake area... i.e., run it the full 37' on each side of the house instead of just the 26'. So, instead of 52' of intake I go 72' (almost the entire length). That would of course up the ridge vent length too. Again I have approx. 37' of ridge to work with. So a 30' ridge vent seems to work well...?

Also I figure if I go with the 60/40 vs 50/50...

72' (INTAKE) x 9" NFA = 648" NFA
30' (EXHAUST) x 18" NFA = 540" NFA

648" NFA (approx. 60% soffit)
+ 540" NFA (approx. 40% ridge)
= 1188" NFA

That's 9 total pieces of Continuous Soffit Intake Vents - 9 NFA
(4.5 each side) - OR - 648" NFA / 9 NFA = 72' / 8' pieces = 9 total vents

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Vinyl Perf Soffit over Wood - No Vents-vent_cal.jpg  


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Old 01-21-2011, 01:01 PM   #17
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Vinyl Perf Soffit over Wood - No Vents

There are many many threads on attic ventilation. Based on pro commentsit sounds like it is very common for roofers and estimators to not be knowledgeable about ventilation.
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Old 01-21-2011, 02:40 PM   #18
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Vinyl Perf Soffit over Wood - No Vents

NS, you need to have soffit vents across every rafter bay to move the heated air with moisture from the rooms below. Otherwise there will be dead air spots that will get condensation on the roof, framing or insulation. The 150/300 rule is an older one, if you ventilate on a 1/100, you are still fine.
Working backward;

37 x 9 int. = 333 / 144" = 2.31 x 2 sides = 4.63 total int. + 4.63 exh. = 9.26 total

37 x 18 exh. = 666 / 144" = 4.625 exh.

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nickelshine (01-21-2011)
Old 05-29-2012, 10:12 AM   #19
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Vinyl Perf Soffit over Wood - No Vents

Over the weekend I discovered that our house, built in 1986 and just purchased the week before, has exactly the same problem that nickelshine described: vinyl siding covering solid wood soffit, so no ventilation.

I'm curious to know: did you solve your problem?


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