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So I just had my roof done and wanted ridge vents. The roofer used VentSure® 4-Foot Strip Heat & Moisture Ridge Vents. I am trying however to check my math on my balance, and thing I have way to much exhaust venting.

The roofer used about 35 linear feet of ridge vents (the whole ridge). I have about 966 square feet of attic floor. And I have 40 soffit panels with a single strip of lanced holed venting, the area of the perforation is 24 by 5 inches roughly.

Now I know I have way more venting than I need (966/300 = 3.22 sq/ft), but I am worried about my balance.

By my calculations I have Free Net Area in the ridge of 700 sq inches (4.9 sq/ft). Using OC's 20 sq/in NFA per linear foot (20 x 35). Ridge Vent Stats

I could not find the exact NFA for my under eave soffit vents but I used the following logic to estimate. I have one strip of 24 " x 5" of lanced holes per soffit panel. So that is 120 sq inces of perforation area (or .83 sq ft). Looking at a similar product that is perforated all over I found it had a NFA of 5.87 sq in/sq ft. So I figured each of my soffit vents gave me a NFA of 4.5 sq inches (5.87 x .83). I have 40 soffit panels (with rafter channels so no blockage), so that should equate to 180 square inches of intake (1.25 sq/ft). So while I have enough total venting 6.15 sq ft... I have a ratio of 20% Intake to 80% exhaust.

Or are my calculations way off? How big of an issue is this?

I will say that because of the roof layout, one side of my house has 30 soffit vents and the other side has a length of 11 of them. The wind usually will hit the 30 soffit side first.

  • Ridge vents work best for the first few years then start to degrade. The best way to tell is what is the attic temps? I don't think I have ever seen two many vents, more often not enough. What is your concern? With good insulation the house will stay cooler in summer and your roofing will last longer. – Ed Beal Jul 28 '16 at 0:30
  • My concern is that with to much exhaust I could have more egress of my homes interior air getting into the attic to make up for the air that the soffits can not supply. So that would lead to more moister in the attic as well as costs for the hot or cold air (winter or summer) leaking into the attic from my livable space. – Warklaw Jul 28 '16 at 0:48
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You can never have too much attic vent. If you have proper insulation installed, with vapor barrier, there's no problem with heat or moisture loss from the house proper.
The roof itself is 'happiest,' i.e. will last longest, when the underside (in the attic) is as close in temperature as possible to the top, exterior, side. That's why attics are supposed to have full airflow from outside the house.

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