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Attic insulation: open cell or cut and cobble rigid foam?

_matt_p | Posted in General Questions on

Hi, I live in climate zone 4a and have a conditioned attic to insulate between rafters. Reading about moisture condensation issues, I looked into closed cell spray foam. Turns out that no spray foam company wants to install a type of foam that has low global warming potential such Icynene EcoSeal Pro. So that leaves me with open cell or rigid foam. I understand that both may let moisture laden air condense against the underside of the rafters, for slightly different reasons. Which method is less risky in that respect? Also would it help if I left a inch deep ventilation channel between the underside of the roof sheathing and the cut and cobble rigid foam? I have no ridge vents, I am not sure that any moist air has anywhere to escape to. In either case, (open cell or rigid foam) I was planning on following with existing R 19 fiberglass. Thank you !

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Matthias,
    You are correct that both open-cell spray foam and the cut-and-cobble approach (without a ventilation channel) have been associated with damp roof sheathing and sheathing rot.

    Of those two choices, I guess I would choose cut-and-cobble -- but only if you make your roof assembly vented. That means you will have to install a ridge vent. But installing a ridge vent isn't particularly expensive or complicated.

    Here is a link to an article with more information on ventilation baffles: Site-Built Ventilation Baffles for Roofs.

  2. _matt_p | | #2

    Thank you Martin. I have dormers on my roof. Does that mean it will be impossible to create a proper vented cathedralized roof and I would be better off with a non-vented roof and open cell foam? Two dormers are really small maybe 60 by 80 inches, but a third one is much bigger.

  3. GBA Editor
    Martin Holladay | | #3

    Matthias,
    As you have correctly guessed, if you are planning to insulate a sloped roof assembly, then a roof with dormers is a poor candidate for a vented approach. The dormers will interfere with venting.

    That means that you are restricted to one of the unvented approaches. For more information on all of your options, see this article: How to Build an Insulated Cathedral Ceiling.

  4. Dana1 | | #4

    Water blown Icynene isn't the only lower-impact closed cell foam game in town. LaPolla makes a 2lb foam blown with an HFO1234_ _ blowing agent, which has an extremely low GWP.

    In Zone 4A you can get away with a flash-inch of closed cell against the roof deck, with the remainder being fiber or open cell, provided you install an interior side smart vapor retarder (or use vapor barrier latex on the ceiling gypsum. That's a safer bet than cut'n'cobble. The ~1 perm ccSPF protects the roof deck, the variable permeance smart vapor retarder keeps the rest sufficiently dry.

    http://buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems/view

    http://buildingscience.com/file/5809/download?token=18Y6NJQ8

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