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Do I need a moisture barrier on the inside of a below-grade foundation wall?

AveryDIY | Posted in General Questions on

I’m in Seattle (zone 4). I want to finish a mostly below-grade basement room. Should I use a moisture barrier on the inside of exterior basement foundation walls (between the foundation wall and the soon-to-be framing)?


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  1. user-5946022 | | #1

    Even better is an exterior waterproofing system plus an exterior foundation drain. I'm guessing this is an existing situation, and you don't want to dig up the exterior. Can you dig down deep enough to at least determine if the wall has any exterior waterproofing? Do you know if there is an exterior foundation drain?

    1. AveryDIY | | #2

      Yes, 1958 built house.
      I can't see any kind of waterproofing on the foundation that is above grade on the exterior and I am not aware of any exterior foundation drain.

      1. Expert Member
        BILL WICHERS | | #3

        You can’t always see exterior waterproofing above grade. Try digging down a foot or two and see what you see then. A house of that age would have had an asphaltic coating (if anything was applied), which is a black tar-like material that sets up hard. It will look like a smeared on thick coating of what will now be hard black material.

        If you don’t have anything on the exterior (which is the best place for foundation waterproofing), then an interior coating might help, but probably won’t completely solve the problem if you’re seeing water coming through the wall. Interior side coatings nearly always eventually separate from the wall and fail when there is a big water problem.


  2. Jon_R | | #4

    > Should I use a moisture barrier ...

    Per Joe Lstiburek, approximately 1 perm foam:

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