Determining existing ridge vent net free air

Old 02-29-16, 06:02 AM
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Determining existing ridge vent net free air

As I prepared to add insulation to my daughter's home I noticed it has a ridge vent but doesn't have soffit vents so I'm going to add them. When adding soffit vents I want their net free area (NFA) to at least equal the NFA of the ridge vents.

My problem is I don't know how to determine the NFA of the existing ridge vent. All I know is the length of the ridge vent. How do I determine its NFA?

BTW, for now I'm not going to deal with overall ventilation requirements. I'll get to that once I know the ridge vent NFA.
Old 02-29-16, 06:19 AM
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What's there now in the soffit area?
Here's a chart that may help.
There's no way you can have to much venting.
Old 02-29-16, 06:24 AM
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Is it the shingle over style or standalone aluminum ridge cap with vents? Most of the shingle over styles seem to be about 18 Sq in NFA per foot.
Old 02-29-16, 07:20 AM
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When my dad added aluminum soffits to his house a couple years ago, the contractors just cut a 6" wide channel through the wood soffit first - plenty of air flow and no math required.
Old 02-29-16, 08:15 AM
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Since there is no testing laboratory to determine the actual NFA each mfg comes up with their own number. As CT says, 18 in² is a common advertised number. In reality, that is the 3/4" opening in each side but it ignores any bug screening and assumes the plywood and underlayment were sufficiently cut back.

Since the calculations are very simple the target number is good to know.
Floor area divided by 150 times 144 (for sq inches) and then divided by 2 for half high and half low.
A 1,500 ft² attic floor would yield 10 ft² total NFA with 5 for high and 5 for low. 5 times 144 + 720 in². Using the 18 in² per foot you would want 40' of ridge vent.

If the attic floor is very well air sealed and the ceiling below has a vapor barrier you can use 300 in place of 150.

The guide for attic venting is very lose yet codes and requirements seem rigid and they shouldn't be. Factoring in attic height, wind protection at soffits, prevailing winds, and air sealing are far more important.

But 18 in² per linear foot of ridge vent is an acceptable starting point.

Old 02-29-16, 03:33 PM
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Thanks to all for the information. I'm going to go with 18 sq. in. NFA.

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