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Unvented cellulose in attic rafters: How to let it breathe inwards?

Matthew Fleck | Posted in General Questions on

I’ve been tormenting over how to insulate my attic ceiling (I recently posted my best proposal about how to do it with venting–see link at bottom).  Today I met with a third insulation contractor.  He, like all the others, wants to just blow in dense packed cellulose and not worry about venting.  Meanwhile, so many insist that there must be venting.  After reading a lot of articles here, it seems like this is a contentious point, especially here in the Northeast (my home is in the Bronx, NYC).

At this point, I’m leaning towards pumping in dense packed cellulose without venting.  I would be putting up a 2-inch polyiso rigid foam layer on the undersides of my 2×6 rafters, and then blowing in dense-packed cellulose into the rafter cavities.  As I’ve read on this forum, this can work as long as I also do two things:

1.  Ensure that my insulation can breath moisture back into the house…
My question:  I’ve had a lot of trouble finding a 2-inch polyiso material that isn’t faced.  The article “Can Cellulose be Used in an Unvented Roof?” on this forum says to “avoid using a polyethelene vapor barrier (and) foil-face polyisos board.”  All the 2-inch polyiso boards I’ve found online are faced.  Can anyone recommend a 2-inch polyiso board that isn’t faced or allows the insulation to breath back into the home?

2.  The article also stresses that I make sure there is an air barrier around the cellulose.  Should I improve the underside of the roof deck (it is asphalt shingles on wood sheathing” as an air barrier, or is that enough of an air barrier already?  The roof is set up with ridge venting and soffit venting.  I realize that I should definitely close up the soffit venting since I’ll be dense packing cellulose right up to the underside of the roof.  Should I also try to create an air barrier at the top of the roof (closing off air flow up and out of the ridge vent)?

Thanks again, everyone.

Here is my other approach that I am considering, though likely more costly…

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  1. GBA Editor
    Martin Holladay | | #1

    The approach you are considering violates the building code and can lead to damp roof sheathing.

    Nevertheless, some contractors continue to recommend this risky and illegal approach. The entire controversy is thoroughly discussed in the following article: "Smart Vapor Retarders for Walls and Roofs."

    1. Matthew Fleck | | #2

      Hi Martin,

      Thanks for your reply. Can you share some feedback on my alternate approach, described here?

      I really want to do this right.

      Thanks, Matt

      1. GBA Editor
        Martin Holladay | | #3

        As my article ("How to Build an Insulated Cathedral Ceiling") makes clear, it's permissible to install a continuous layer of polyiso insulation between the bottom of your rafters and your drywall ceiling, as long as you have a ventilation channel between the top of your fiberglass batts and the underside of the roof sheathing.

        I agree with Zephyr / Bill and Brian Pontolilo -- commenters on the other Q&A thread -- that you should choose one location for your insulation: either following your sloped roofline, or on your attic floor. Insulating both areas makes no sense.

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