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Community and Q&A

Thermo-Ply under drywall?

Ralph1962 | Posted in Energy Efficiency and Durability on

I am working on a ranch style house. We have pulled all plaster and insulation from the ceilings in the house. As we start to put the ceilings back together, can I use a product like Thermo-Ply on the underside of my ceiling joist and drywall over that? That gives me the advantage of being able to put the ceilings partly back together, re-insulate the attic with blown insulation, and then drywall at my leisure.

I am putting blown-in to R-36, and will have an air space above to vents. No storage in the attic, and no mechanical.


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I use a product like Thermo-Ply on the underside of my ceiling joist and drywall over that?"

    A. Yes. The only possible problem with this approach is that the Thermo-Ply may sag. If it does, it is going to complicate drywalling. It takes almost as much work to install Thermo-Ply as drywall -- so why not just hang the drywall and be done with it?

  2. Ralph1962 | | #2

    I was thinking that the thermo play would be a vapor barrier of sorts , and provide me some insulation value as well. I would use this instead of plastic or paper, and it would be easier to drywall over.

  3. GBA Editor
    Martin Holladay | | #3

    I had forgotten that most types of Thermo-Ply have a very low permeance. (The permeance of Thermo-Ply varies from 0.53 - 0.63 perm, which is quite low.) While Thermo-Ply in this location won't cause problems in a cold climate, it might cause problems in a hot climate, especially if the house is air-conditioned.

    Building codes don't require interior vapor barriers, although some building codes require interior vapor retarders. A requirement for a vapor retarder can be satisfied with vapor-retarder paint. In general, I don't recommend the use of interior vapor barriers.

    Since the most common type of Thermo-Ply is only 1/8-inch thick, I don't recommend it for this application. If you install cellulose on top of it, it is likely to sag.

  4. Ralph1962 | | #4

    Thanks for a prompt answer.

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