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My 120-year-old house has no soffit vents and 2 passive roof vents a couple feet below the ridge line (no ridge vents). I want to add soffit vents in the wood soffits to provide more airflow because the attic gets HOT (I'm in Los Angeles, CA).

The roof is 12:12 pitched on 4 sides (no gable ends) so soffit vents are my only option other than cutting new holes through the roof deck. my question:

Should I opt for more, smaller vents (3.5"x14") or fewer, larger vents (5.5" or 7.5" x14") to get to the same amount of net free vent area?

Larger vents would be less work to cut and seal the vents against the soffit surface (the soffits are NOT flat) but I was thinking that more, smaller vents would provide more air movement across the whole inside surface of the roof deck, removing more heat from the surface. Or, maybe it makes no difference as long as the total NFVA is the same, and I'm overthinking this.

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    Good question. More smaller should limit any dead air sections, but larger might increase air flow. Increasing the outlet(top) flow will help also. Any insulation needs to be moved out of the way of soffit vents and/or added to the area above the living space. Don't want attic heat fighting with the AC.
    – crip659
    Sep 9, 2023 at 22:16
  • The AC unit & ducting are in the attic; it's absolutely fighting the attic heat which is why I want to add more ventilation. My house is a rectangle;, the attic is wide open & covers the entire living space. I was planning on distributing vents evenly around the perimeter including the corners so I think dead air won't be a problem. The airflow should be about the same, assuming the total net free vent area (NFVA) is the same. IE 20 smaller vents with NFVA of 0.5 vs 10 larger vents with NFVA of 1.0. Using larger vents would definitely be a big labor saver but I want them to be effective!
    – Victor R.
    Sep 9, 2023 at 23:35
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    Don't miss the "don't block the new vents with loose, fluffy insulation forced all the way down to the soffits" part of @crip659's comment. You'll want to pull all the insulation back from the soffit, install soffit baffles (creating an air gap on the under side of the roof), then push the insulation back into place. Otherwise, your new venting efforts will be wasted because air won't be able to flow through the insulation to get up to the roof vents. You'll want a soffit baffle in every rafter bay you install a vent into.
    – FreeMan
    Oct 10, 2023 at 13:40

2 Answers 2

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More holes spread over a larger distance matters most where moisture control over the entire width of the subroof, in all corners and pockets, is a concern.

For heat management, however, the total air mass movement matters most, and missing a few corners or pockets is less of a concern.

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Ideally, a continuous vent that allows air to enter each rafter bay is best. Most homes do not have this; rather individual cut vents. The importance is to have intake square footage greater than exhaust square footage. This reduces the amount of air that the system is starved and thus pulling more conditioned air from the interior of the home, through penetrations. I have helped several neighbors eliminate ice damns over overhangs simply by adding soffit venting.

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