Help calculating soffit vent cuts

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Old 07-29-18, 02:33 PM
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Help calculating soffit vent cuts

We had a new roof installed last fall. It now has a continuous ridge vent. The roofer said we should install more ventilation in the soffits to improve the balance and we would like to do so. The attic is about 1,000 square feet (38x26). The ridge vent goes almost all the way across. We also had them install baffles between all the joists so at least that is done. It is a typical roof with the soffits on each of the two 38' lengths.

The current soffits overhang about 12 inches and about 1 of every 6 pieces covering the soffit is vented vinyl with the rest solid vinyl. We pulled back some of the vinyl soffit and low and behold it is almost all plywood underneath with an occasional rectangular metal slotted vent. The vented vinyl soffit pieces don't even line up with the slotted vents in the plywood - it looks like the siding installer just put it up in a nice pattern regardless of where the vents are.

So we are planning on pulling down the existing vinyl soffits pieces, cutting away some of the plywood soffit, and then reinstalling 100% vented vinyl across the entire length. But I need help calculating how much plywood we need to cut away and do we actually need to install/reinstall metal vents only to cover them with the vented vinyl? That seems to defeat some of the purpose by adding barrier to air flow barrier.

Thanks much!
 
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Old 07-29-18, 03:28 PM
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Your ridge likely has a 1" slot cut into the sheathing across the length of the house. That would mean a 1" continuous slot across the soffit would give you 50-50 venting. They make continuous vent that goes into a 2" wide cut... The louvers cut down the net free area by probably 50%. Youd have to check product info if you want to be exact. Keep in mind you want that if you just cut holes in the soffit that you will want bug screen over them.
 
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Old 07-29-18, 05:20 PM
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Thanks XSleeper. That math makes sense to me and so does the rest of your advice. But since we aren't going to use a continuous vent, if I understand correctly, we should cut away the equivalent amount of wood as the continuous vent, maybe in sections, cover those openings with bug screen and then install the perforated vinyl over top. So do you think we need to cut away a little more than the equivalent to compensate for the reduced flow from the bug screen? I imagine when we are done, it will look like this.

 
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Old 07-29-18, 05:23 PM
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Picture

If only I could figure out how to add a picture before hitting enter. Here it is. Name:  Soffit.jpg
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Size:  19.5 KB
 
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Old 07-29-18, 05:31 PM
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I don't think you could go wrong with a 2" gap covered in bug screen on each side.

your "solid" soffit is not really that big of a deal, it's not air tight so you still get air flow no matter what. There would be a little more turbulence which might slow air flow a negligable percentage.
 
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