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Old 05-10-2012, 09:43 AM   #1
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Intake and Exhaust out of balance.


Hello.
I am having a roof installed as I type this and replacing the old cans with Cobra Snow Country Exhaust Ridge Vent (4' sticks that provide 18sq in of exhaust per linear foot). I have 42 linear feet of ridge venting for a total of 756sq inches of exhaust (I went into the attic and measured the cut they made at the ridge, from one end of the house to the other).

I'm concerned with the soffit venting and being out of balance.. There are 14 - 16"x8" vent panels around the perimeter. But the holes they cut are only 12" x 5".
12 x 5 / 2 = 30 x 14 = 420sq. in. of intake - out of balance by 336sq inches.

My attic is 1500sq feet so technically, I have way more ridge venting than I require, but I'm fine with that.

My question is: Do I need to have them cut more soffit vents? (at 30sq in per intake vent, I'd need a total of 25 soffit vents to balance it out).
Or, could I go into the attic and seal up roughly 18' (324sq in) to bring the soffit and ridge into balance?

Thank you for taking the time to read this and for your help.
Due
SE Michigan

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Old 05-10-2012, 12:54 PM   #2
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Intake and Exhaust out of balance.


I know my local code says 50/50 is preferable, but it can vary up to 25/75. Having more at the top is probably the best practice. I'd say you are fine!

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Old 05-10-2012, 05:23 PM   #3
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Intake and Exhaust out of balance.


It's possible that having more outlet than inlet can de-pressurize the attic slightly, causing conditioned air to be pulled from the living space if the ceiling plane hasn't been tightly air sealed. I'd add more inlet and make sure the attic is air sealed.
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Old 05-11-2012, 06:35 AM   #4
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Intake and Exhaust out of balance.


Quote:
Originally Posted by shazapple View Post
I know my local code says 50/50 is preferable, but it can vary up to 25/75. Having more at the top is probably the best practice. I'd say you are fine!
The exact opposite unfortunately.

Quote:
Originally Posted by seeyou View Post
It's possible that having more outlet than inlet can de-pressurize the attic slightly, causing conditioned air to be pulled from the living space if the ceiling plane hasn't been tightly air sealed. I'd add more inlet and make sure the attic is air sealed.
Seeyou...that is exactly right. If anything, it should usually be 60% intake and 40% exhaust.

Well said.
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