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Total Net Free Vent Area Question

stephenr | Posted in GBA Pro Help on

So, i am trying to better understand this calculation as it applies to this client’s house for the purposes of sizing ridge vents and soffit vents.  Its a 12/12 single gable cathedral roof over a 250 square foot, 2 floor vaulted space.  The top floor is a loft that partially covers the first.  I have read several articles on calculating the NFVA and have come up with this logic.  Feedback please.

The total roof covers a floor area of 10×20 for 200 square feet.  There is a 35 square foot bathroom off the first floor that the roof covers, so that is a vented area of 235 square feet.  I then divide this number by (and this is where I am a little confused) 300 to get the nfva.  Then I put it into inches and divide by a ratio of 60 (soffit)/ 40 (ridge) to get the number for each end of the vent.  I then divide it by the number of rafter bays to see how big a venting gap i need on each end, for each bay.

But wait, don’t they say that one should actually divide by 95 to get a more correct number for a steep cathedral roof (12/12) like this one. ?? 

I don’t want to create a situation where i pull moisture through my roof assembly by miscalculating the venting needs and ratios for this assembly. Any insight would be helpful.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    stephenr,

    This is how our code puts it:
    "... the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area."

    So you need more venting because the ceiling follows the slope than you would if that same 12/12 pitched roof had an attic.

    Maybe it's different in hot/humid climates, but I don't think you can over ventilate a roof space.

  2. rockies63 | | #2

    Notice that he said "unobstructed" vent area - which means the area of the holes in the vent mesh, not the vent size itself. In other words, you have to deduct about 20% from the vents size (due to the presence of the metal mesh) to properly size the vent. Thus a two foot square vent actually has 1.6 square feet of open, unobstructed air flow area.

  3. stephenr | | #3

    ok. So the roof area I am working with is 8' x20'. So, to double it would be 320 square feet. So its 320/300 so basically 1 square foot. That's 144 square inches divided by 30 (top and bottom of 15 bays) and I get almost 5 inches per bay. That's a 10 inch 1/2 inch vent area, maybe a little bigger on the soffit vent. That's not much. If I had a 1 inch strip at the soffit and the ridge, that would be more than enough. and over venting is never a problem. Sound about right?

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