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Community and Q&A

Fiber faced polyiso

Bonusroom1 | Posted in Green Building Techniques on

Ive got a opportunity to purchase reclaimed polyiso with the black fiber facing I plan to cut and cobble this and use it in my bonus room I will be stacking (3) 2.5 inch pieces in the catederal part of ceiling and leaving a 3.5 inch gap between the roof deck and foam as I have 11 inch rafters. for ventilation from soffit to ridge. Im in north Carolina and am required a r38 so my question is should i add another layer and reduce ventilation gap. And also which way should I face the black fiber facing. Thanks in advance for answers.

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  1. user-2310254 | | #1


    Do the rafters in your cathedral roof allow uninterrupted ventilation from the soffit to the peak? In this article (, Martin indicates you can use a one to two inch ventilation gap.

  2. Jon_R | | #2

    I'd add your last layer of foam underneath the rafters - this will reduce thermal bridging and provide better air sealing.

    Can't think of any reason the facing direction would matter.

  3. Bonusroom1 | | #3

    Steve I was actually planning on using rigid insulation almost like normal and insulating be hind kneewall instead of going entire length. As for a layer over rafters jon I don't want to sacrifice anymore headroom.

  4. Expert Member
    Michael Maines | | #4

    Eric, insulating behind the kneewall usually results in some under-insulated and difficult-to-airseal spaces. It can be done, but all things considered it's usually easier to just insulate the rafter bays from the plate to the attic or ridge.

    As a rule,the bigger the ventilation gap the better. Lstiburek says 1" absolute minimum, 2" is better. 3.5" would be generous. In your climate zone going from R-42 to R-56 or so would have a long payback period, so I would probably err on the side of extra ventilation space. On the other hand, if the foam doesn't cost much and you do an excellent job with air sealing, the vent space won't be as important and you might see a reasonable ROI on the additional insulation.

  5. Expert Member
    Dana Dorsett | | #5

    What Jon-R said. An all cut'n'cobbled R38 will dramatically under-perform an R38 that has a continuous 14+ of thermal break on the rafters. Even if it's only done that way in the roof section behind the kneewalls it will be worth it.

    Air sealing kneewalls is something of a fools-errand in a retrofit situation, with myriad air leaks to the joist bays under the floor. It's easier and more reliable to do it at the roof.

  6. Bonusroom1 | | #6

    Dana one difference is my bonus room is over first story of home not garage so the entire floor has cellulose in it already.

  7. Expert Member
    Dana Dorsett | | #7

    So, when the cellulose under the bonus room floor settles a half-inch, what's slowing down the wind through the joists from passing under the floor?

    Only if the cellulose dense-packed would the potential thermal bypass be of low concern.

  8. Bonusroom1 | | #8

    Nothing I guess, but the problem with my house as many is I don't have a clear shot from soffit to ridge on every bay,So ill figure something out.

  9. Bonusroom1 | | #9

    What no answer cant someone come with a real world solution,instead of "use closed cell foam" which no one down here does . Come on get real .

  10. Expert Member
    MALCOLM TAYLOR | | #10


    This is a board where people come for advice, and other posters are kind enough to offer it. If you don't like the advice being offered...

  11. Bonusroom1 | | #11

    Sorry im just looking for real world advise, you have to admit some of the solutions are not realistic or financially feasible.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Fair enough. Sometimes the discussion is pretty theoretical. Unfortunately that's the nature of the beast at a site about high performance buildings. In among the wilder speculation where is often a practical solution lurking somewhere.

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