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Do I use a vapor barrier?

thegiz | Posted in General Questions on

Building a basement wall.  Using 1 inch rigid foam, sealing and taping seems, then building a 2×4 wall and filling stud bays with fiberglass batts.  Do I then use a vapor barrier before applying drywall?  What insulation batt should I use in this situation?

Joe DIYer

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  1. Expert Member
    Peter Engle | | #1

    No vapor barrier. That will trap moisture inside the wall. Fiberglass or rockwool batt is OK, but rockwool is somewhat denser and more tolerant of minor flooding. Either batt should be unfaced. I would recommend foil faced foam insulation, with foil tape. If using polyiso foam, keep it a few inches off the floor, and fill in the gap at the bottom with EPS (best) or XPS foam. Both are more water tolerant.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Joe.

    Just to add emphasis to what Peter said, do an excellent job air sealing the rigid foam. Some sealant around the panels' perimeter and diligent taping will help keep warm summer or heated air from reaching the cool foundation walls where it can condense.

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