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Do I need a vapor barrier for blow-in fiberglass in vented attic 4a

mikeysp | Posted in General Questions on

Hi, I am in zone 4a.

Do I need a vapor barrier in new constrction between cealing and blow-in fiberglass on a vented attic?

I have a bunch of blow-in fiberglass I got for a pittance, micx matched companies, and at appears to be an option in my mixed humid 4a Nashville area; but, I am not sure. 

I have recycled 1.5″ polyiso foam board all along eve serving as baffles with an inch+ air gap 4 ft up the roof slop to allow an air channel to the vented ridge, keep the blow in out of the eve to ridge air flow, and to prevent any blow in from getting into soffit area. 

Thank you.


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  1. Expert Member
    BILL WICHERS | | #1

    Nope. In a vented attic, any moisture that sneaks through gets safely carried away by the ventilation system, assuming it's installed correctly (soffit and ridge vents). It doesn't hurt to have a vapor retarder directly above the ceiling under the insulation, but it's not required.


    1. mikeysp | | #2

      Thank you Bill!

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