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I have a large crawl space under my first floor that I can access from my basement. The ceiling of the crawlspace is insulated with fiberglass (my library/den is above it on the first floor). The floor of the crawl space is compacted clay soil with crushed rock, and there's already an old, thick vapor barrier over it. The walls are the concrete foundation on all four sides, minus the access opening. There is no radon issue as I had that mitigated already to acceptable levels. The crawlspace is NOT vented as far as I can tell.

Question: the Google says it's a good idea to insulate the walls of the crawlspace with foamboard, and then apply vapor barrier to all the walls, overlapping it with the floor (which already has that vapor barrier that seems to be in good condition). Is insulating the walls overkill/totally necessary? Also, some people hang storage shelves from the ceiling joists so that plastic storage boxes/whatnot dont sit on the vapor barrier, but is that necessary?

Basically, just wondering if I'm good to just start throwing storage boxes in there, or if I need to invest some time/money to make any improvements.

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  • Foamboard (if you don't use the super cheap styrofoam beadboard) is already a vapor barrier. You wouldn't then apply poly over that or you'd have a moisture trap. That said, this is entirely a matter of local conditions and your prerogative. Do you have a more specific question?
    – isherwood
    Jun 29 '21 at 13:56
  • What local conditions would require adding a vapor barrier to walls, in addition the pre-existing floor vapor barrier?
    – Chris
    Jun 29 '21 at 14:38
  • Well... moisture.
    – isherwood
    Jun 29 '21 at 14:38
  • Haha. How much? If I'm at 30%, is that ok?
    – Chris
    Jun 29 '21 at 14:51
  • 30% what? If you're asking about relative humidity, that depends entirely on outside temperature. Is it a problem? Do you see or smell mold? Will you be storing sensitive things not in sealed containers? This is why it's a subjective question whether you should improve your crawl space or not.
    – isherwood
    Jun 29 '21 at 14:53

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