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Community and Q&A

Basement Insulation

Jaems Sweepstakes | Posted in Green Building Techniques on

Hello. i have a basement in zone 5B. we are getting ready to finish it and I have a couple questions.
Can i use 1/2″ xps against the interior foundation, then frame up to that and use mineral wool batts in the stud wall to get to the reqd R value?
—the rim joists would also have xps and sealed with great stuff.
—wall seams will be taped and the bottom sealed with great stuff.

Should radon mitigation be done before or after insulating the foundation?

we have a crawlspace on one side of the basement. the foundation wall that separates the crawlspace from the basement is appx 42″ high. does this need foam board?
–the builder did not have blanket insulation on this portion of the foundation
–it looks like where the stairwell meets this part of the foundation the build glued the drywall right to that foundation knee wall.

if i am planning on having a bathroom with shower on the wall between the basement and crawlspace will there be moisture issues in the crawlspace?

thanks. great site.

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  1. User avatar
    Dana Dorsett | | #1

    Half inch XPS is insufficient R-value for dew point control at the foam/fiber boundary of a 2x4 studwall in zone 5B.

    You'll need at least R5-R6 foam plus the fiber insulated studwall to hit code-min performance (equivalent to R15 continuous insulation.) An inch of foil faced polyiso is easier to air seal (with foil tape on the seams), and it's greener (due to the much lower impact blowing agent.)

    Put an inch of EPS or XPS (but not polyiso) between the bottom plate and the slab as a thermal & capillary break to guarantee the bottom plate always stays above the dew point of the room air.

    It's better to convert the crawlspace into a unvented sealed-insualted space, in which case you wouldn't need to insulate the foundation wall on that side. But if you're not going to convert it into a conditioned crawl space, insulate that wall the same as if it were a fully exterior wall.

    If you want to save an inch or more of interior space, use 2.5" of foil faced polyiso strapped to the wall with 1x4 furring through screwed to the foundation with 5" masonry screws and skip the studs, hang the finish gypsum on the furring.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    1. Radon mitigation work needs to happen first. Here is a link to an article with more information: All About Radon.

    2. Here is a link to an article with more information on insulating basement walls: How to Insulate a Basement Wall.

  3. Jaems Sweepstakes | | #3

    Dana. right now the crawlspace is vented and has insulation on the foundation walls. there is also a heating/cooling register in there. the vents that are installed are the one that open and close automatically. would it make more sense to frame an inch from the foundation and use 2" of spray foam?

  4. User avatar
    Dana Dorsett | | #4

    It makes more sense to put down a ground vapor retarder sealed to your wall insulation, and seal & insulate over the vent hatches in the crawl space.

    Depending on your radon mitigation requirements you may need to put a vapor barrier over the concrete at that wall, but if the crawl is insulated you won't need to fully insulate it. An inch of polyiso would be more than enough R, and the foil facers would be a radon barrier if air tight.

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