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Community and Q&A

Mastic to seal Thermopan to wood?

cord12 | Posted in Green Products and Materials on


Most of our return air ducts are joist or stud cavity bays covered with a product called Thermopan (foil-faced cardboard). I’m looking for a mastic that will seal the Thermopan to wood. I have one area that is hard to reach but I think I can get at it (with a putty knife on a pole) with a good, sticky, peanut butter-like textured mastic. I called Thermopan and they said any UL-rated mastic will do. However, I can’t seem to find anything locally. I would like a product with little VOC outgassing as it will be in our ductwork.

Thanks, David

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  1. GBA Editor
    Martin Holladay | | #1

    If you can't find duct mastic locally, you can order it on the Web.

    That said, I don't recommend your approach. Panned joists are no longer recommended or permitted in most building codes. Even if you managed to seal the Thermopan material to the bottom of the joists, there are lots of other potential air leaks (for example, between the top of the joists and the subfloor).

    The best approach is to install real galvanized ductwork with sealed seams.

  2. TomBalderston | | #2

    Years ago testing ENERGY STAR homes, we found that panned cavites sealed with mastic would pass the duct test at first. For some reason we had to re test a house two months later and the same house failed. Turned out the joists had shrunk almost 1/4" as they dried out, so all the metal pans had oil canned and split at the joints. Using I joists this might not happen, but still best to avoid panned cavities.

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