Replies

1. GBA Editor
   Martin Holladay | Apr 30, 2014 05:39am | #1

   Ralph,
   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 | Apr 30, 2014 06:04am | #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 | Apr 30, 2014 06:24am | #3

   Ralph,
   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 | Apr 30, 2014 05:30pm | #4

   Thanks for a prompt answer.