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Community and Q&A

Vented I-Joist Rafters

Charon0739 | Posted in Green Building Techniques on

I am building a ranch in southern Michigan with 24 foot rafter spans over a cathedral ceiling (4/12 pitch). Originally I was going to place 3 inches of foam on top of an unvented roof, but to simplify the roof and reduce costs I am now planning on using 12 inches of cellulose with a 1.5 inch ventilation gap at the top 2x piece of the I-Joist with cellulose filling the cavity underneath and a vapor barrier between the I-joists and the purlins that the drywall attaches to. The assembly is shown in the attached image. Does any one see any issue with this plan. Is 1.5 inch enough gap to vent the roof?

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  1. Expert Member


    That's a pretty common roof assembly which seems to work well. The 1 1/2" air gap will be fine as long as you use rigid enough baffles not to reduce it by deforming.

  2. DirkGently | | #2

    Malcoms advice is always good and i would trust it, however a 2" to 2.5" gap would be way better. First off 4/12 is flat in my opinion and flatter needs more air gap.
    Second, as Malcom stated about the rigid baffles.....even they can deflect under dense pack which reduces your 1.5" air space....especially if the installer is not experienced.
    I plan on 2.5 inch gap for a much for a 8/12 roof and dense pack.

    1. Expert Member
      Akos | | #3

      I wouldn't call 4/12 low slope. The slope is low enough that you can get away with loose fill but there will be some settling. You can avoid the settling by semi dense packing which would limit the amount of bulging you get.

      What is sufficient roof ventilation is very hard to quantify. My general take is that a well sealed assembly needs very little venting and no amount of venting will save you if you have a leaky ceiling.

      So, build to the code min vent gap, make sure it is not blocked by insulation, include the right amount of free area for the intake/exhaust vents and most important, make sure your ceiling is tight. You do that and your roof will work great.

    2. Expert Member
      MALCOLM TAYLOR | | #4


      I agree that if possible deeper vent are always a good idea. The problem with I joists is as soon as you go to a different depth than the top flange, the attachment gets a lot more complex and time consuming.

      Our own preferences aside, Akos is right. Code compliant venting works unless something else has gone wrong, and generally more venting won't save a failing roof.

  3. Charon0739 | | #5

    Thanks for the feedback. I think I should be ok with the 1.5 inch because the OSB will maintain the space. I was thinking another inch would be nice, but I don't think it will be worth the extra hassle. I will tell the insulator to blow it in tight, but it will not exactly be dense packed. should still have an R40+ and a really rigid roof while making life easy on the framers and the elections.

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