Attic Ventilation with Vinyl Soffit

4 years ago

Im currently trying to figure out if my math for the attic ventilation is correct and if im understanding how it works correctly. Here are the numbers

Attic Sq Ft - 1550

Ventilation need for 1:150 - 744 sq in at eaves as well as the ridge vent

Vented Vinyl Soffit - 5.867 sq in / sq ft NFA

22 inch deep soffit - 10.75 sq in NFA per soffit

Need 55 vinyl soffits at front and 55 at the back of house.

372 sq in NFA for back of house divided between 10.75 sq in NFA per soffit is about 35 pieces.

Should i do 35 vented vinyl and 20 non vented? Every 2 vented to 1 solid. Is this the correct math or am i missing something.

There is old wood soffit board under the current vinyl soffit. My plan is to drill a 4 inch hole in the wood where each vented vinyl will go.

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Comments (5)

millworkman
4 years ago
I would use center vented vinyl the length of the house in conjunction with a full ridge vent. Soffit vents alone are no enough.
Stephen Vicole
Original Author
4 years ago
The house has a full ridge vent and a few hidden vent vinyl soffits surrounded by solid ones.

I wasnt able to source and center vent vinyl so i got T4 triple vent and T4 solid to replace whats up now.

Curious if my math is right above in that i should do 1 solid for every 2 vented panels.

There is solid wood board above the current vinyl soffit so i will be drilling (2)- 4 inch holes spaced a foot apart every other foot.
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3onthetree
4 years ago
Keeping the existing wood soffit negates the use of the vented vinyl, other than just preventing you from painting the wood. Your 4" holes will have about 8 perforations over it, so you've reduced the functionality of the vented vinyl.

No need to do 2-1-2-1- . . . . just use all vented soffit after you take down the wood. More intake is better.
Stephen Vicole
Original Author
4 years ago
Doing that would put my intake around 3000 sq in NFA. Im not exactly experienced in this topic but it seems you want intake and exhaust around the same number.

A 4 inch hole has 12.56 sq in area so even if the board is still there with the holes it should still pull in at least as much as the soffits rated NFA.
3onthetree
4 years ago
The 4" hole will not be completely open. Look at a piece of vented soffit. It only has perforations, so there is still a lot of material that would block the 4" hole you went to the trouble to drill out, instead of just removing the wood entirely. There will also probably be blocking/bridging that might interfere with some of the soffit panels (which would further reduce your plan of 2-1-2-1), not even getting into whether you have insulation baffles or anything.

Code calls out 150/300 as a minimum calculation, but this is not an exact science. The best situation for an attic is to have the air in the space be exactly the same as outside, with airflow to remove moisture, but obviously we can't ever achieve this because we need to enclose the space for weather protection. So more air is better. Code also references where you want more intake (at the eave) than exhaust (at the ridge). This helps insure the balance of cool/warm air and high/low air pressures remain to function low-in and high-out air movement.