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Help with soffit vent calculation/sizing

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  • Help with soffit vent calculation/sizing

    I'm having my house siding redone, and having soffit vents installed in the existing, unvented soffits. The contractor is planning on cutting a 2" continuous vent in the existing soffit, and I'll then have Certainteed 3 1/3" Invisivent installed on top so it looks much nicer. I have a 1,260 sq. ft. attic, so need 605 sq. in of intake and exhaust venting according to the 1/150 rule. Per the Invisivent literature, there is approximately 10 sq. in of net free area/sq. ft. I'm having about 80 feet of Invisivent installed, soffit is about a foot wide, so approx. 80 sq. ft. That would give me 800 sq. in of net free area for the intake, more than enough. My concern is that I won't get anywhere near this number with only a 2" continuous vent cut into the existing soffit. We will not be installing anything in the continuous soffit, it will just be cut into the existing and covered with the Invisivent. Is there any calculation to try to determine how much intake I may get? The 2" continuous would be a little over 2,000 sq. in of intake (80' of soffit * 2 in). I have a 40' ridge vent already installed, although it's the cheaper mesh type. I'm concerned the ventilation won't be balanced or work efficiently. My attic is typically 40-50 degrees hotter than outside, and I was really hoping adding the soffit vents would help to lower this considerably. Thanks for any and all input and advice.

  • #2
    not understanding your question. you are cutting a 2in wide slot in your soffits 80ft long.
    80 * 2 * 12 = 1920 sq in which is more than the net free area the soffit provides. The Invisivent will have a baffling effect. Generally, you need to be 50/50 intake to exhaust ratio, Some say that 60/40 is acceptable but let's stay with 50/50.
    Keep in mind you are exceeding the intake on 1/150 calculation. You need to know what your exhaust area is. At this point, with 800 intake you need to be at 800 exhaust.
    How much exhaust does your existing ridge vent provide?

    Plenty of info at the links below.

    https://www.lomanco.com/vent-resources/vent-calculator

    http://www.support.lomanco.com/kb/faq.php?id=11

    Mark Parlee
    BESI(building envelope science institute) Envelope Inspector
    EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
    EDI Seminar Instructor
    Level one thermographer (Snell)
    www.thebuildingconsultant.com
    You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

    Comment


    • #3
      Thanks for responding. I was concerned the combination of invisivent with only a 2" cut out would yield significantly less than the 800 sq in of net free area the invisivent would provide if installed by itself, without the existing soffit remaining underneath. If I'm understanding your post that may not be true. I have 40' of the rolled mesh type ridge vent, which from checking online seem to provide 12-14 sq. in per lineal ft, so I think I should have in the neighborhood of 500 sq in of exhaust.
      Last edited by tdidona; 07-20-2020, 08:34 PM.

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      • #4
        Given you measurements, you should be in the range of acceptability.
        Yor venting will exceed the minimum requirements so you should see a difference in attic temps when finished. I t will always be hotter in the attic when the sun is on the roof so do not expect it to be the same temp as outside air. your difference of 40° to 50° is really not all that bad.
        Mark Parlee
        BESI(building envelope science institute) Envelope Inspector
        EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
        EDI Seminar Instructor
        Level one thermographer (Snell)
        www.thebuildingconsultant.com
        You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

        Comment

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