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Community and Q&A

Not sure what to do with basement insulation plan

Carfar96 | Posted in Green Building Techniques on

We had planned to use 1″ XPS over the slab of our walk out basement and then finishing. We will also be using 2″ XPS on the concrete walls. We had planned to seal all joints between the rigid foam, put 3/4″ TG plywood over the floor XPS and then frame and install laminate. However, we have run into some headroom issues and the fact that one wall is load bearing and we have the issue of the stairs when run in the middle of the basement (not sure if these should rest on rigid foam). So, now we are thinking of just doing XPS on the walls. However, I do still want a vapor barrier on the floor to prevent moisture issues. And I am worried about the wood framing being placed on the concrete slab with no thermal break. Any suggestions? We are thinking about maybe doing cork underlayment under the laminate to give a tiny bit of insulating help. Thank you!

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  1. Expert Member
    Dana Dorsett | | #1

    Your location / climate/ deep subsoil temperatures are...???

    You definitely want a vapor barrier between any finish floor and the subsoil. It can be on top of foam or on top of the slab, or even under the slab, but it has to be there.

    Putting foam under the sill plate of a load bearing wall would require an engineering analysis. If you must put it on the slab, at the very least slip some EPDM or 10 mil polyethylene under it as a capillary break. As long as the deep subsoil temperatures are above the average outdoor summertime dew points there's very little risk of moisture accumulation in the bottom plate, even if uninsulated. If that's a concern, use pressure treated lumber for the bottom sill.

    XPS comes with a heavy greenhouse gas footprint due to it's HFC134a blowing agent (about 1400x CO2 for 100 year warming potential.) As it loses it's blowing agent over the next 50 years or so it's performance drops to that of EPS. EPS is blown with pentane (7x CO2), most of which is recovered at the manufacturing plant, often burned for process heat. Polyiso is also blown with pentane. Using polyiso on the walls is a greener alternative to XPS without adding thickness, but should not be used under the subfloor, since in that application it can potentially take on ground moisture.

    Using EPS under the subfloor is fine, but is about 15% lower R value (initially) than the labeled R of XPS. ( But in 50 years they'll be about the same.)

    The amount of R-value needed under the subfloor to mitigate mold potential in the flooring is a function of both the average dew points of the ventilation air in summer, and the deep subsoil temperatures, and the R-value & composition of the flooring itself.

  2. GBA Editor
    Martin Holladay | | #2

    You can certainly install a strip of rigid foam under the bottom plate of any wall that isn't load-bearing.

  3. Expert Member
    Dana Dorsett | | #3

    What he said!

    If the only "load" it's bearing is the stairs (and not the house) you can go ahead and put 1" EPS under it with zero concern.

    Foam under the bottom plate of a studwall carrying the structure of the house itself would need to be analyzed/specified by an engineer.

  4. Carfar96 | | #4

    Thank you Dana and Martin! I am in Michigan. There is vapor barrier (6 mil poly) under the slab - taped at seams. What thickness rigid foam under the bottom plate would you recommend? Would you also recommend a 6 mil poly under the laminate and tile (small bathroom)? If so, how should this be adhered to wall XPS and XPS under bottom plate? Would rigid foam be preferable to any type of foam sill plate gasket? Thank you!

  5. Carfar96 | | #5

    Can you put xps under the stairs?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "What thickness rigid foam under the bottom plate would you recommend?"

    A. You're just interested in providing a capillary break, so the thickness of the foam doesn't really matter. Even 1/2 inch is fine, but a thicker piece of foam would also work.

    Q. "Would rigid foam be preferable to any type of foam sill plate gasket?"

    A. Foam sill seal under the bottom plates of your walls would also work fine as a capillary break.

    Q. "Would you also recommend a 6 mil poly under the laminate and tile?"

    A. If there is polyethylene under the slab, you don't need to install another layer of polyethylene on top of the slab. Make sure to follow the installation instructions provided by the manufacturer of the laminate flooring.

    Q. "Can you put XPS under the stairs?"

    A. The bottoms of the stair stringers for basement stairs usually bear on a pressure-treated 2x6 that is installed flat to the slab. If that's the detail you have (or intend to create), you can certainly install a thin layer of XPS between the slab and the PT 2x6 if you want.

  7. Dana1 | | #7

    Since there is a poly vapor barrier under the slab, the capillary break isn't all that important, but the thermal break might be if you're not using pressure treated lumber for the bottom plate. Deep subsoil temps in the zone 6 parts of MI are in the mid-40s:

    With subsoil temps in the 40s you'd want a minimum of R3 under a standard 3/4" subfloor to keep the subfloor & laminate flooring above the average summertime outdoor air dew points. With an insulating type finish floor such as rugs you'd want at least R4.

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