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Final unvented cathedral ceiling insulation questions

user-6816910 | Posted in Green Building Techniques on

I posted in my previous thread about my roofline being problematic (which was pointed out quite often!);
PLEASE KEEP IN MIND THIS HOUSE IS ALREADY FRAMED! I cannot change the roofline unfortunately (had I known then what I know now)……..with that in mind please see my questions below:
https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/101524/insulating-new-construction-home-no-exterior-rigid-

I have come to the conclusion that I need to do Closed Cell Spray Foam in the main areas where the roof is not a simple plane but interrupted with valleys, etc. (?)
1) With that said is 4″ of CCSPF enough in Zone 5? With no fluffy stuff? Or do I need to add fluffy stuff in conjunction?
2) If I need fluffy stuff, what is best in conjunction with CCSPF? Blown Cellulose or blown Fiberglass?
3) Is it problematic to use Sharkskin Underlayment under the Owens Corning 35Yr architectural Shingles? Do I need to do anything special with the OSB like tape the seams?

In the bedroom areas that have a simple plane(above garage on far left and then the far right area), I would like to do a vented roof system with Baffles. I would prefer site built battles with 2×2’s in rafter and then adhere plywood to those. Then air seal with caulk?????? (I prefer a vented assembly in bedroom areas because I really do not want spray foam….)
1) In this case, would blown cellulose or fiberglass be a better option against the baffle?
2) Will I need a smart vapor retarder like Membrain?
3) Is it problematic to use Sharkskin Underlayment under the Owens Corning 35Yr architectural Shingles? Do I need to do anything special with the OSB like tape the seams?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is 4 inches of closed-cell spray foam enough in Zone 5? With no fluffy stuff?"

    A. No. The 4 inches of closed-cell spray foam will give you an R-value of about R-25 or R-26. Building codes require a minimum of R-49 for roofs in Climate Zone 5.

    Q. "Or do I need to add fluffy stuff in conjunction?"

    A. Yes, you need fluffy insulation.

    Q. "If I need fluffy stuff, what is best in conjunction with CCSPF? Blown cellulose or blown fiberglass?"

    A. Either one will work.

    Q. "Is it problematic to use Sharkskin Underlayment under the Owens Corning 35Yr architectural Shingles?"

    A. The manufacturer is a little vague. According to the installation instructions, "SHARKSKIN COMP™ roofing underlayment should generally be installed above ventilated attic spaces in most cases." Asphalt felt would be better; most manufacturers of synthetic roofing underlayment specify that their products are only designed for vented roofs.

    Q. "Do I need to do anything special with the OSB like tape the seams?"

    A. No.

    Q. "In the bedroom areas that have a simple plane(above garage on far left and then the far right area), I would like to do a vented roof system with Baffles. I would prefer site built battles with 2x2's in rafter and then adhere plywood to those. Then air seal with caulk. In this case, would blown cellulose or fiberglass be a better option against the baffle?"

    A. As long as you achieve at least R-49, a wide variety of insulation materials will work.

    Q. "Will I need a smart vapor retarder like Membrain?"

    A. Not necessarily. Vapor-retarder paint on the drywall will also work. In any case, pay attention to airtightness.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    For unvented roofs in zone 5 where ccSPF is combined with fluff, make sure that the fluff is no more than 60% of the total R. Stated another way, the ccSPF layer has to be at least 40% of the total R.

    At 40/60 it's still worth considering "vapor barrier latex", but you make it ~50/50 you'll have enough margin that any standard latex is fine, and minor air leakage wouldn't become a mold problem, and the drying rate for the fluff-cavity will be 8-10x faster than with special vapor retardent paints- it's more resilient.

  3. user-6816910 | | #3

    Thank you again! I was asking about the 4" of CCSPF being enough because the insulation contractor keeps telling me CCSPF is held to another standard and you don't actually have to reach R49 with it (I think he said 5.5" would be plenty). He did not rec'd fluffy stuff in conjunction though. Argh...

  4. GBA Editor
    Martin Holladay | | #4

    You wrote, "The insulation contractor keeps telling me CCSPF is held to another standard and you don't actually have to reach R49 with it."

    The insulation contractor is misleading you. Regardless of which type of insulation you install, you still need to comply with minimum building code requirements. For a full discussion of this issue, see It’s OK to Skimp On Insulation, Icynene Says.

    -- Martin Holladay

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