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What is recommended basement wall foam board thickness when insulating stud cavities?

dtbingle | Posted in General Questions on

Located in climate zone 5 (southeast Michigan) with house being built in 1954.  Planning to insulate, frame, drywall, and paint our basement.

Was looking to use unfaced EPS foam board attached to the poured concrete foundation walls in basement and then build walls using typical framing methods (studs 16″ OC with top/bottom plates).  I will also be filling these stud cavities with either fiberglass or mineral wool.  Part of the reason is that empty stud cavities allow for a lot of noise to travel between rooms and between the basement and main floor.

Given that the framed wall will have insulation, what is the minimum thickness of EPS foam board to use on the foundation wall?

I see that R-10 is the minimum for foam board only requirement in my climate zone, but what about when you add in fiberglass in the stud cavity in front of it?  For example, if I did 1″ EPS (R-4) + fiberglass (R-13) in studs = R-17 total, would that be fine?

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Replies

  1. agm413 | | #1

    You'll want at least 2 or 3 inches of rigid foam in order for the ratio of rigid foam to fiberglass to not cause any issues. Too much fiberglass and not enough rigid can cause issues.

    1. Deleted | | #3

      Deleted

  2. kyle_r | | #2

    According to Martin’s article on how to insulate a basement: (https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall)
    He suggests using the same ratio as above grade walls which would be R5 for a 2x4 wall in zone 5. However his preference is to use thicker rigid foam and leave the stud bays empty.

  3. johngfc | | #4

    Are you asking about insulation on the inside or outside of the foundation? If inside, then a key role of the foam is as a vapor barrier and unfaced EPS (perm ~ 1) may be marginal.

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