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Can I get away with a moisture barrier and unfaced batts in an attic?

HarmanRestorationLLC | Posted in Building Code Questions on

I’m in zone 2, and I’m getting mixed information from my local building supply. Does code allow me to install plastic sheeting between attic rafters, before unfaced insulation batts? Wouldn’t this help prevent condensation by restricting air flow to the roof sheathing?

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  1. user-2310254 | | #1


    Generally you don't want to use plastic sheeting anywhere. Is this a cathedral ceiling or attic? Is it vented or unvented.

  2. GBA Editor
    Martin Holladay | | #2

    Your description is unclear. I'm guessing that you want to do the following:
    1. You want to insulate the sloped roof assembly above your attic.
    2. You want to install polyethylene sheeting against the underside of your roof sheathing before installing fiberglass batts between the rafters.

    Is that a correct description?

    If so, it's a bad idea. The polyethylene would be what's called a wrong-side vapor barrier. It would get cold in winter and would become a condensing surface for moisture.

    Before I can give you advice, I need to know whether you are trying to create a vented assembly (with soffit vents and a ridge vent) or an unvented assembly. In neither case, by the way, do you want polyethylene against the roof sheathing.

    For information on the correct way to proceed, see these two articles:

    How to Build an Insulated Cathedral Ceiling

    Site-Built Ventilation Baffles for Roofs

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