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Community and Q&A

Can R-10 XPS foam against concrete and R-15 mineral wool batts between framing be combined in a basement?

David Pfeffer | Posted in Plans Review on

I’m in the process of finalizing the plans for my basement. Our current plan is to do R-10 XPS attached to the concrete blocks walls, followed by R-15 mineral wool batts between the studs. I live in central NJ, and the foundation wall is more than 60% above grade.

I’m now concerned about condensation between the mineral wool and foam. Allowing (extremely generously) R-5 for the concrete block, the foam + block is R-15 total. In a scenario with 70 degree interior temperature and 0 degree exterior temperature, the temperature at the boundary between the foam and batts would theoretically be 35 degrees. Since mineral wool batts are not a vapor barrier, would I have an issue where vapor from the warm interior of the house condenses on the foam layer and drips down in the wall?

I’ve heard there may also be code issues with this due to a larger percentage of the R-value occurring closer to the warm side of the wall than the cold side.

I’m aware the R-15 batts are probably unneeded, and having done a rough calculation the extra insulation will probably only save me a little bit every year with my geothermal HVAC. However, we’re also interested in them for their fireproofing characteristics.

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  1. Nate G | | #1

    The temperature of your soil isn't going to be zero degrees. More like 40. So the delta T will be smaller and the XPS foam will perform fine and stay above the dew point. Furthermore, once you put drywall up over your stud wall that's protecting the foam, if you air seal it right, humid air isn't going to be able to touch the foam or the block. You should be fine.

  2. David Pfeffer | | #2

    My percentage was an average cross the wall. Large sections of the wall are 100% unburried (i.e. no soil contact to warm it.)

    Drywall (even with latex paint) is a vapor retarder, not barrier; won't moisture be able to migrate through it over time?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    You are either in Climate Zone 4 or Climate Zone 5, depending on your exact location in New Jersey.

    Let's design your wall for the worst-case scenario -- that is, the assumption that your concrete block wall is fully exposed to the outdoor air. We'll design it like an above-grade wall.

    To do that, we'll use the guidelines presented in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In Zone 4, you're fine. In Zone 5, you need a minimum of R-5 foam -- so your plan to install R-10 will work, with a healthy safety margin. Don't worry.

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