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Community and Q&A

What to use on a concrete floor in finishing a walk-out basement?

PhylisFinEastLansing | Posted in Energy Efficiency and Durability on

I recently purchase a condo that had a walk-out basement space ready for finishing–cement half walls on 2 sides, studded wall on another and all concrete wall on the third. The studs were installed and fiberglass batts installed in rim joists and between studs with a plastic barrier over the wall surfaces. I live in Michigan (Zone 5). As I prepared to finish the basement and did lots of research here, also discovered leaks and cracks in the foundation walls–since repaired.
Questions. There is evidence of mold on the back side of the fiberglass batts so my plan is to remove all fiberglass. I’ would like to level the one 1/2 cement wall for a continuous plane with polyiso insulation covering the concrete and over the studded space above –this is above grade.
1) What’s the best way to insulate/fill the space between the existing studs on this 1/2 cement wall? is it necessary to fill if it will be covered with foam sheets to 3 inches? and if so will something like rockwool or denim batt be okay–given the previous moisture issue, I’m leery of it.
2) The BuildingScience recommendations on finishing a basement suggest rigid foam on the concrete floor (1 inch). Is it possible to use polyiso on the cement instead of XPS rigid foam?

Current plan (money of course a concern): foam the exterior wall cavities and possibly the rim joists, (or fill rim joists with polyiso and foam any gaps).

Thank you all for the great reads. Phylis

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Phylis,
    A 3-inch layer of polyiso has an R-value of about R-18. That's OK, but it's not great. If I were you, I would beef up the R-value of the stud walls by insulating between the studs with fiberglass, mineral wool, cellulose, or denim batts.

    If you add interior polyiso, you don't have to worry about the mold returning, because the polyiso (if it is installed with attention to air sealing) will prevent the flow of warm, humid interior air into your stud cavities.

    The best types of rigid foam to use above a concrete slab are EPS and XPS, because they have more compressive strength than polyiso. I would choose EPS over XPS, because it is more environmentally friendly.

    For more information in insulating a concrete slab from above, see:

    Finishing a Basement Floor.

    Green Basement Renovation.

    The Stay-Dry, No-Mold Finished Basement.

    The High Cost of Deep-Energy Retrofits.

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