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Community and Q&A

Rigid foam on the inside of basement walls

y7PCYTKqFT | Posted in Energy Efficiency and Durability on

Per the details on the site, it shows rigid foam on the inside of the basement wall and then batts between the studs. Is there a moisture benefit of doing this? What is the real benefit of this vs batts? We do a handful of finished basements here in my area and everybody just does batts. Inspectors also require a vapor barrier inside but that is creating mold inside the wall and behind the barrier as we have seen that on jobs only a few months old that were insulated during the initial construction. I’m in northern Utah by park city so cold climate for sure.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't recommend the use of fiberglass batts to insulate basement walls.

    Fiberglass batts are air-permeable, so they allow relatively warm, humid interior air to contact the cold concrete wall. The result is condensation and mold.

    The situation can be prevented by attaching an airtight layer of continuous rigid foam to the interior side of the concrete wall. Once this is done, some experts say that it's OK to install a stud wall with fiberglass batts between the studs, but I advise anyone who asks that it makes more sense to avoid any fiberglass batts in basement walls. If you want more R-value, just install thicker rigid foam.

    Polyethylene sheeting should never be installed on the interior side of a basement wall. If a local inspector asks for poly on a basement wall, I would ask the inspector to cite the section of code where this is required.

  2. y7PCYTKqFT | | #2

    Thanks. Sounds like it has more to do with the air transfer. So is the thought that the rigid is not going to allow the warm air to contact the cold wall at all? If so, where does the moisture end up? Just drying to the inside side of the wood wall and the outside of the concrete wall? Just trying to make sure I understand this properly.

    Here, nearly all inspectors are making us put poly no matter what and stating a code that it must be on the inside of the warm wall regardless of above or below grade.

  3. jklingel | | #3

    Does the code specifically say "poly", or "vapor retarder"? I heard this same thing from an inspector here until I asked him to read me the code. The code (IRC?) actually said "vapor retarder" and he assumed it meant poly because "that is what everyone does". I'd ask to see the code, as Martin said.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Where does the moisture end up?"

    A. As you probably remember from high school, moisture never really goes away. On the exterior side of your basement wall, the soil in most of the country is damp. That means that your concrete wall is also likely to be damp, even if it has been "damp-proofed."

    On the interior side of the basement wall, the basement air is likely to be warmer that the soil outside for much of the year, and to be holding moisture. The indoor air will always hold moisture.

    So, to answer your question -- the outdoor moisture should stay put, on the outside. The inside moisture should stay put, on the inside. And if you install an airtight layer of rigid foam on the interior of the basement wall, you'll prevent the inside moisture from reaching a cold surface, where it might condense.

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