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Community and Q&A

Retrofit Vapor Retarder/Barrier in Ceiling Plane

pjmeg | Posted in General Questions on

I finally crawled up into my attic of the house I purchased four months ago. I knew that I needed to add significant insulation (currently has about 6″-8” of loose cellulose) as I want to get to R60. In advance of adding the additional insulation I am air sealing the attic. I was digging around in the insulation to find the various wiring penetrations, to foam them closed, and I was immediately struck that I didn’t find poly (or any other vapor inhibitor) on top of the ceiling drywall, as is the typical scenario in homes in my area. The ceilings are textured with sprayed drywall compound, and are original to the home (house was built in 1980) so I am confident that the drywall ceiling does not have paint that can act as a vapor retarder. The attic is vented at the soffit and at the ridge, and there doesn’t appear to be any signs of mold or moisture damage on the underside of the roof sheathing. I haven’t been up in the attic during the colder months to see how much, if any, condensation there is on the underside of the roof deck.
Given that I have no vapor retarder, and I wish to keep a vented attic, I need to add a vapor retarder/barrier to the ceiling plane (Right?). The first and easiest way that immediately comes to my mind is to remove the cellulose and flash fill the truss bays with ccSPF at the ceiling plane, as it would accomplish both air sealing and installation of a vapor retarder/barrier in one shot. I would then fill over with blown in cellulose to the necessary thickness. Another idea I have would be to cut and foam-in board insulation, but that would be very arduous as the attic has limited room to maneuver the boards without cutting them into 3’ pieces, and I have about 1000 sf to do this to. What other suggestions are there for retrofitting a vapor retarder/barrier into the ceiling assembly?

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  1. GBA Editor
    Martin Holladay | | #1

    You don't mention where you live, but your signature line indicates that you are located in Climate Zone 7, which is cold.

    The short answer to your question is that everything sounds fine. Even ordinary paint helps retard vapor diffusion. If your current situation worries you, you can always repaint your ceilings with vapor retarder paint -- but if I were you, I wouldn't bother.

    Your inspection reveals no current moisture problems.

    Almost all moisture problems in attics arise from air leakage, not vapor diffusion. So if you are sealing air leaks, you're doing the right thing.

    For more information on this issue, see:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  2. pjmeg | | #2

    Thanks Martin. Yes, I live in zone 7, 10 miles from Lake Superior in NW Wisconsin. Thanks for the links too!

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