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Old 03-12-2011, 12:19 PM   #1
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Soffit vents - math check


Let's say I put in a 16 x 4 rectangular vent with NFA of 26 sq in. Now, what happens if instead of a rectangle cut the size of the vent, I cut three 3 in holes for the vent. How does that impact my nfa? Obviously it goes down, but by how much? The area of three 3 in circles is roughly 21, so would that be my new NFA or it be less?

Not looking for a debate on the best way to vent soffits, just curious how the math works in this exact scenario.

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Old 03-12-2011, 12:53 PM   #2
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Soffit vents - math check


The best way I can think to answer your question is by giving you more info than what your asking for, but hopfully there will be something in it that will help you.

I take the square footage of the building, or area that I want to ventilate and divide it by 300. That will give you the square footage of ventilation you will need over all. 60% of that needs to be in the soffit. So if you have a 1500 square foot area to vent it would look like this. 1500/300=5 sqft, and the soffit would figure 5x.6=3 sqft=432 sqin. If your using a vent with an area of 21 sqin you would need 21 vents to cover your needs. Hope I helped....Clint

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Old 03-12-2011, 01:52 PM   #3
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Soffit vents - math check


Sounds like you have most of it figured out. 3-3" circle under the 4x16 screened louvered metal vent would give you total NFVA of 7.71 sq.in. Because 3" holes for a round vent give 2.57NFVA each; http://www.lomanco.com/ProductPAGES/CirkVents.html You would need them end to end, continuous, and still be short of the 9sq.in. recommended per foot of soffit venting each rafter/truss cavity. Install them close to the fascia board, away from the wind pressure blowing up the house wall.

Only divide attic by 300 if a vapor barrier is present on ceiling below, otherwise by 150 or 100 (industry standard).

Gary
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Old 03-12-2011, 04:05 PM   #4
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Soffit vents - math check


Thx guys. Gary- that was precisely the info I was looking for.

I've attached a picture of a slot I've cut for a 16x4 rectangle vent. Is this adequate to achieve the full 26 nfa? You can see the screw holes to get an idea of where the vent edges are relative to the hole.

Essentially, I'm trying to determine how much NFA I'm getting from one of these things so I can then do the math to balance my ventilation.
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Old 03-12-2011, 10:40 PM   #5
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Soffit vents - math check


I could only guess from here.....

With a clear space behind a new vent, about 25 NFVA, I would guess yours is about 20? http://www.lomanco.com/intakemain.html

Problem is you should have 9" per lineal foot of soffit, a continuous vent. Which, with the other side of the gable roof would equal the 18 NFVA of the ridge vent. With your 18-20 every 2' apart, the spaces skipped would not get optimum venting, especially if attic wasn't air sealed (ice dams) first. Many times the wall cavity insulation is fiberglass batt of a low density inherent with convective loops. These deposit the heat to the wall top plates where the insulation is weakest with only 2-3 inches from the warm top plate to the underside roof sheathing. The "biggest loser" in fiberglass insulation....

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Old 03-14-2011, 04:03 PM   #6
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Soffit vents - math check


Thanks Gary. Needed someone else's guess. Think I get what your saying.

How does a hip roof impact all this. My home does not have gables so do I then need to vents on all 4 sides?
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Old 03-14-2011, 08:57 PM   #7
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Soffit vents - math check


Based on attic floor space; http://www.airvent.com/homeowner/pro...it-specs.shtml

Could use the hip vents, haven't heard good or bad yet; http://www.airvent.com/professional/...hipridge.shtml
Maybe the pro roofers will chime in.....

Static (turtle) vents work for high, also. Yes, need to move air over every exterior wall, every bay to prevent ice dams, along with air sealing; http://bct.nrc.umass.edu/index.php/p...ting-ice-dams/

Gary

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