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Community and Q&A

Adding Ventilation to A-Frame Roof

derekr | Posted in General Questions on

Large A frame house, impossible to meet the required soffit ventilation sqft because of large roof?

posting this because of another question I asked not long ago that made me think I can’t get enough ventilation for my roof

the roof is 3,300 sqft, no attic just 1 to 2 inches of space between the roof and insulation

my understanding with the 1:150 rule means I would need 22 sqft of airflow, this is impossible to meet with any vented soffit on the market, I can’t even get half of that unless I’m not doing the calculations right

what am I supposed to do here or am I not calculating this correctly?

There’s no attic house is all open, roof is 58 degrees steep, 32 feet tall on each side, 28 feet tall at the center and 50 feet long, trusses are 24 inch on center

I’m going to put the styrofoam vent baffles in each channel from the soffit all the way to ridge vent and then put r38 rockwool batts over those then drywall

I’m in the southeast

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  1. Expert Member
    ARMANDO COBO | | #1

    It would be easier if provide roof frame detail, insulation plan, elevation, length of ridge and climate zone just in case.

    1. derekr | | #2

      There’s no attic house is all open, roof is 58 degrees steep, 32 feet tall on each side, 28 feet tall at the center and 50 feet long, trusses are 24 inch on center

      I’m going to put the styrofoam vent baffles in each channel from the soffit all the way to ridge vent and then put r38 rockwool batts over those then drywall

      I’m in the southeast

      The best vented soffit I could find is the certainteed invisivent that provides 10 inches of airflow, I’m not sure if this is per piece or per foot or what it means, I’m not sure I fully understand it, but if it’s what I think it means it’s not even close to enough for my size roof and this is supposedly the best on the market

  2. Expert Member


    I don't understand the dimensions you gave for the roof (how can an A frame have more than one height?), so I'll take your total as being right.

    The 22 sq ft of vented area is provided in three areas: the two soffits, and the ridge. So each of those only needs 7.3 sq ft.

    Did you check whether you need to meet the 1:150 ratio, or only 1:300? You can find the relevant section (R 806.2) in the link I posted in your other thread.

    1. derekr | | #4

      The height at the center is 28 feet where both sides meet at the top from the subfloor, I was just giving the total length of each side from soffit to ridge

      Ok thanks does the 10 inches of airflow in certainteed spefications mean only 4 sqft of air flow on each side of the roof? 10 inches of airflow multipled by 50 feet?

      I haven’t looked into wether I need to do 1:300 or 1:150

  3. Expert Member
    ARMANDO COBO | | #5

    Again, pictures or drawings will help more than your explanation. You have a 19.2/12 pitch. On an Isosceles triangle where the side is 38’ at 58°, the height to the ridge is 27’2”.
    The trusses, are they parallel trusses? Channel in trusses? Are they TJIs instead of trusses? What is the ridge made of?
    50’ long ridge sounds like a lot of room for ventilation, but depends how is framed.
    If you are in CZ 1-3 in the Southeast, his maybe be a good candidate for a vapor diffusion port instead of a vented channel.

    1. derekr | | #6

      They are channels from bottom to top

      It’s a shingle roof, shingle roof ridge vent

    2. derekr | | #7

      Best pic I could find atm, don’t know why the picture turned sideways after I posted it

  4. Expert Member
    ARMANDO COBO | | #8

    Well, I guess neither Malcolm or I understand your explanations. I don't know of any trusses with channels. On ventilation, read 2021 IRC R806.2 Minimum vent area.

    1. derekr | | #9

      I didn’t use the right wording didn’t mean to say trusses

      It’s 2x12’s from bottom to top

  5. Expert Member
    ARMANDO COBO | | #10

    OK, those are RAFTERS, not trusses. You may want to double check that 2x12-38' are not too long per code, and that a single 2x12 ridge board be enough per code too.
    To add a channel, you would need to install 1x2 or 2x2 rails on each side of the rafter from top to bottom, plus you need a vented soffit at the bottom, and cut back the sheathing at least 3" from the ridge board to allow for ventilation. Cut, fit and seal all the Styrofoam vent baffles. That is a lot of work! Structurally, you also need to strap both sides rafters over the ridge.
    I think a better way to do it is with a vapor diffusion port. No vented soffit, no channels. Install dense-packed cellulose or rockwool on rafter bays, at the ridge, cut sheathing 3" back from the ridge, install a smart membrane, with a permeance greater than 20 perms, over the opening and then the roof vent. On a 50' ridge with 6" clear opening on both sides, you'll have 25 sq. ft. of vents.

    1. derekr | | #11

      Its not 38 feet it’s 32

      I’ve already bought my baffle vents and insulation so I’m not taking all of that back and changing the plans I already have

      I’m just trying to figure out if I have enough net free area of airflow

      Certainteed invisivent says it has 10 inches of net free air flow so how much of that would I need to achieve 7 sqft of net free air flow on one side of the roof? I’m not sure how to calculate it or if it’s even possible to get the free net air flow that I need

      1. Expert Member
        MALCOLM TAYLOR | | #12


        Unfortunately a lot of this is confusing due to your description. Is it an A frame, which is when the roof extends down to grade with no walls at all, or a gable roof? If it is an A frame, where are the soffits?

        It's also hard to tell how the roof is framed. Are you sure it's continuous 2"x12"s? I've never seen them anywhere near 32 ft long. Without understanding what you have built, I'm not sure we can give you much useful advice.

        1. derekr | | #13

          Yes it’s a full A frame
          It’s like 10 foot 2x12’s on the first floor then 22 foot 2x12s on the second floor

          The soffits are on each side of the roof at the bottom close to the ground not the front and back

          I read if the peak of the roof is more than 3 feet high from the soffits then I use the 1:300 scale, the peak of my roof is 28 feet above the soffits which means I have enough venting I think

          I am worried about the workers squishing my insulation when they put up the drywall though and closing my 2 inch air gap between the roof and insulation and I would have no way to know if they did, I’m putting those foam baffles between my rafters but those aren’t strong enough if someone fell forward into the insulation for example when they put up drywall

      2. brendanalbano | | #16

        The Certainteed Invisivent specs ( ) say that they have 10 square inches of net free air space per 1 square foot of soffit panel.

        If you need 7 square feet of vent area, converted to square inches, that means you need 7 sq ft * 144 sq in/sq ft = 1008 sq in of net free vent area.

        1008 sq in vent / (10 sq in vent / 1 sq ft soffit panel) = 100.8 sq ft of soffit panel. Depending on how big your soffits are, that might be tough to fit!

        I hope that helps clarify how to do the calculations to translate between the net free area specs of a soffit vent product and the net free area you need.

        You'll of course want to double check that 7 square feet is really the amount of vent area you need (as mentioned by others, you may only need 1:300 vent area, rather than 1:150).

        For more specific advice about how to achieve the ventilation, as others have said, you'll probably need to upload some drawings.

  6. the74impala | | #14

    I have a solution for avoiding squishing your vent space, but it ain't cheap, or easy, or light.

    Using 1.5 inch eps foam, make 1.5 inch wide strips. Using fast grab, stick them to the 2x12 on either side of the rafter space. Then put 1/2 plywood in cut 22 1/4 inches wide secured with thin strips of off cut from the 4x8 sheets. Not the ideal 2inches, but you may need that extra room. Rockwool doesn't do r38, just r30, 23 and 15.

    1. brendanalbano | | #17

      For what it's worth, Rockwool Comfortbatt says it comes in a 9.5" thick R38 batt on their website:

      1. the74impala | | #18

        Guess you are right. Never seen it in a store. I wish they made an R45. Would make my life a lot simpler.

  7. the74impala | | #15

    In your area, if appropriate, the Vapor Diffusion Port would be easiest.

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