Some last miscellaneous basement insulation questions

Discussion in 'Remodel Forum & Blog' started by dsmith060, Mar 18, 2021.

  1. dsmith060

    dsmith060 New Member

    Joined:
    Mar 13, 2020
    Location:
    SE Michigan
    I'm wrapping up the framing process in my basement finishing project and have a couple quick questions about insulation that I've struggled to find answers for anywhere else -- any help would be appreciated! As some brief context: located in Michigan (climate zone 5) and I've got 2" rigid foam on the concrete walls, a ~1" gap to account for uneven walls, then my framed wall. I have R-15 mineral wool batts to place between the studs.

    1. Due to the 1" gap between the foam and the studs, when I insert the 3.5" mineral wool batts I can either: A. push it right up against the rigid foam, leaving a gap between the mineral wool and the drywall, or B. push it flush with the back of the studs so it's right against the drywall but leaves a gap between the mineral wool and the foam. My inclination is to go with A, as an air gap between the insulation doesn't seem like a great idea, but the 2" foam acts as a vapor barrier so I'm not sure that it matters in that case?

    2. I also have a fair number of fiberglass batts available that I had to take down in certain places before putting up the rigid foam that I was curious about reusing. Instead of filling the stud bays top to bottom with the mineral wool batts, is there any problem with a "mix and match" approach? I would prefer to use the mineral wool batts with a higher R-value on the upper half of the wall since it gets colder than the lower half of the wall, since that should sit around 65 degrees. Then I could put the fiberglass batts on the lower half of the wall. Just wondering if this is a sound approach to save some money reusing the fiberglass, or if I should just go mineral wool top to bottom? I'm also hesitant to put fiberglass closer to foundation level since I've read that mineral wool will fare better when there is "inevitably" water damage (as I've heard repeated multiple times, it's not "if" it's "when")

    Thank you!
     
  2. fcrick

    fcrick New Member

    Joined:
    Dec 11, 2020
    Location:
    Redmond, WA
    1. I don't think it would make much difference.

    2. Mixing and matching should work fine. In Michigan, unless this is a really deep basement, it'll get pretty cold out there even quite far down the wall. Make sure if you use the fiberglass that you remove the vapor barrier, as it'll trap condensation in the wall and accumulate over time. My guess is you'd want the mineral wool towards the outside of the building, as if there are thinner parts of the foam, you'll likely get some condensation in the wall cavity at those points.

    Take advice with a grain of salt I'm just a DIYer.
     
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