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