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Community and Q&A

Basement wall insulation

gz99 | Posted in Green Building Techniques on

I am considering putting up walls in my basement. The home is 25 years old, and has a french drain. It has been a very dry basement, there is 1 small 6″ spot of condensation at the bottom of the block wall.
If I read you correctly, I should install 2″ of rigid foam insulation directly on the block walls, then stud them with no FB batts, and then add green sheetrock.
and no other vapor barrier?
What about my sump pump, (it almost never runs). should I place the walls on the outside of the pump, making the finished room a little smaller, or somehow build around the pump?

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  1. Expert Member
    BILL WICHERS | | #1

    You don’t just want “2” of rigid foam on the walls”, you want the code mandated R value worth of insulation on your walls. 2” of rigid foam will range between about R8 and R13 depending on what kind you use (EPS is on the low end, polyiso is on the high end). You didn’t mention what climate zone you’re in so we can’t give you a recommendation for a minimum amount of insulation to install.

    You don’t necassarily have to build a regular stud wall, or even any stud wall at all. Dow thermax polyiso is rated to be left exposed, so it can be installed over your basement foundation walls and left that way. If you want a more finished appearance, many people put studs flat against the wall to create a service channel for electric wiring, but they don’t build a full stud wall. You can build a stud wall if you want to.

    You are correct about not using batts in this application.


  2. gz99 | | #2

    I am in zone 6.. so the 2" foam sold locally says R-10... if I then stud the walls and sheetrock would that = to R 13 which I think is required?

  3. Expert Member
    BILL WICHERS | | #3

    R10 rated 2” foam is probably XPS. That will work, but isn’t the most popular with the green building people due to the blowing agents used.

    In climate zone 6, you need R15 basement wall insulation. That’s 3” of XPS, or 2-1/2” of polyiso. I’d use polyiso myself. You might be able to find some reclaimed polyiso in your area (check Craig’s list) to save some money.


  4. Expert Member
    Dana Dorsett | | #4

    2" XPS may be labeled R10, but is only warranteed to R9 (read the fine print), and would not meet IRC code for zone 6. From a design point of view assuming anything more than the warrantteed R9 isn't legit. When fully depleted of it's climate damaging HFC blowing agents (in 50+ years) it'll be about R8.4, barely more than half the current code minimum.

    3"of cheap reclaimed fiber faced roofing polyiso would run R16.5-R17. Polyiso of any facer type works fine as long as the bottom edge of the polyiso is above the high tide mark of any prior flooding, and not resting on concrete. (The 3" roofing polyiso in my basement stops 4" from the slab in some areas, 8" in others.)

  5. Expert Member
    Peter Yost | | #5

    That little 6-inch spot of "condensation" is interesting: how do you know the moisture is from condensation?


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