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Unvented attic, are baffles needed if insulation is below sheeting on raised heel trusses?

mhenson | Posted in General Questions on

I have searched and searched and can’t find an answer to what I thought would be a basic question.
Vented attic, new construction, Zone 5a, 8500ft altitude in high desert Colorado, foundation and slab complete, framing to start and finish in May 2024.
I have 24″ raised heel scissor trusses, 4/12 pitch, and will be blowing in 18 inches of cellulose with the nozzle pointed down to reduce settling.  The eves are 24″ and the air-sealed sheeting will extend to the bottom of the top chord.  Raised heels are also 24″.   This leaves the top of the cellulose two inches below the top of the sheeting and five and a half inches below the roof deck so three and a half inches of perimeter vent. 
Do I need to install insulation/soffit baffles in this case?  I am more concerned if air traveling from the soffits will wind wash the cellulose.  But also if 3.5 inches of vent is too much.  I would expect that the air flow would be more controlled by the drill holes in the soffits, but?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Henson,

    Baffles are there to maintain clear path for the air, and to reduce wind-washing. If you don't need them for the first reason, and are willing to accept a minor amount of degradation in the R-value at the eaves, they aren't necessary. My own take is that with the amount of insulation you have at the eaves, wind-washing isn't worth worrying about.

    There are very few circumstances where a large volume of air-flow into a vented roof is a problem. If the inlet gives an unimpeded path for the air it can scour the cellulose so that you end up with lower levels where it has been moved. Again, with a 5 1/2" gap, I don't see that as a problem.

  2. mhenson | | #2

    Thank you Malcolm, great answer as usual.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      mhenson,

      Thank you for the kind words.

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