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Community and Q&A

Plastic Layer Between Drywall and Fiberglass Insulation

wayno_from_vt | Posted in General Questions on


When I took out the drywall on the upper portion of our west basement wall (2×6 framing) that’s above grade, there was plastic sheeting between it and the fiberglass insulation. It did not appear to be well sealed at the edges, so I was kinda wondering what it was there for.

I inspected the existing insulation and found evidence of a dead ant colony (I could see where they’d worn “tunnels” through the batt), but nothing else. The back of the OSB looked solid, no evidence of mold.

I added round boxes for wall light sconces on that wall, sealed the back of the boxes, and replaced most of the fiberglass (didn’t want to waste what was already there).

I’m getting prepped for the drywall install later this week (using 5/8″) and wonder, do I need to replace the plastic sheet? The house is wrapped in Tyvek (or something similar), the wall gets plenty of sun in the summer, not so much in the winter.

VT, climate zone 6
– Wayne

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  1. Jon_R | | #1
  2. Expert Member
    Akos | | #2

    Without exterior insulation, you need an interior vapor retarder in zone 6. Craft faced batts or standard poly is fine for this.

  3. wayno_from_vt | | #3


    When you say "exterior insulation", you mean rigid foam or something like that outside of the sheathing, correct?

    As a "recovering" engineer...I'm amazed at what goes into building a that I'm neck deep in finishing the basement.

    And, there are SO many opinions on the "right" way to do things when you have conversations locally.

    - Wayne

  4. Expert Member
    Peter Engle | | #4

    Yes, "exterior insulation" is some sort of generally rigid insulation outside of the sheathing. This insulation keeps the sheathing warm enough that you don't get condensation on the inside of the sheathing.

    Most of the details on this site are the "right" way to do things. Or more accurately, better than "right" as this site promotes better-than-code performance levels. Remember when having local conversations that the information being shared may have been passed down through generations and may not have been right at the beginning. Building technology advances with science, but folklore does not. Sometimes there are good reasons that we don't build things the way we used to.

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