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How high above my attic insulation should my ventilation be?

AJ__ | Posted in General Questions on

New build, mid construction, I have raised heel trusses which measure around 18 inches to the underside of the sheathing and the plan is to have 14 inches of cellulose when settled, so around 16 inches when fresh. I’m leaving a 1.5′ opening for ventilation right now, with some bug screen stapled over to prevent material loss during installation. Martin’s articles suggest I’m fine with a couple of inches of clearance above the insulation prevent wind washing, but Hammer and Hand’s best practice manual suggests a minimum of 12 inches, and many of the site built baffles I’ve seen pictures of terminate much higher above the insulation. If it matters, my overhangs are 3ft. Is it worth building baffles to terminate the ventilation higher, or a waste of time?

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  1. Expert Member


    Wind-washing isn't an issue with cellulose. It's an open question whether It's much of an issue at all.

  2. AJ__ | | #2

    Perhaps I'm using the wrong term, my concern is the cellulose being displaced

  3. CMObuilds | | #3

    I fix this a lot when my insulation comes in to do air seal retrofits, and Ive also always referred to it as wind washing.
    From what I've seen, as long as there is a solid block to the top of the cellulose or higher it *generally, doesn't move. On raised heel we simply install exterior sheathing up to the bottom of the top chords when building with high raised heel trusses, we use a baffle with a fold down exterior flap that gets stapled to the top plate with shorter heels.

  4. Expert Member


    As I recall, you have ventilated soffits, and this gap is at the top of the sheathing on the raised heels - so twice removed from direct wind gusts. I'd be very surprised if the air velocity would be sufficient to displace the cellulose if the gap was two inches above the top of the insulation.

    1. AJ__ | | #5

      Yes that's correct Malcolm, I too would be very surprised but wanted to check after I saw Hammer & Hand's recommendation!

  5. creativedestruction | | #6

    For loose-blown cellulose, you can use the inexpensive box store baffles to maintain the air gap. It should be much quicker, cheaper and more consistent than using bug screen. Also puts a damper on the wind washing, if it matters.

  6. rockies63 | | #7

    A good article on roofs and venting by Joe Lstiburek.

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