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I have a wall on the first floor of my house that is very close to another building, but they are not connected. This wall has an exterior black vapor barrier in the outside and the sheathing is treated plywood on that wall. I live in Michigan, and my question is if I should leave a vapor barrier off the inside on that wall. I was planning to use unfaced rock wool (mineral wool) insulation. I know that generally double vapor barriers are bad, so do I do an unfaced insulation? Looking for ideas. Taking the exterior vapor barrier off that first floor wall is not an option.

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    Is it a real vapour barrier(like a black poly garbage bag) or a house wrap? My house wrap is white on the outside and dark grey on the inside.
    – crip659
    Mar 29 at 12:46
  • It is a black vapor barrier.
    – Jason M
    Mar 29 at 12:51
  • Odd for Michigan. I know in hot humid climates the vapour barrier is placed on the outside, but in cold climates a breathable wrap on the outside and the barrier on the inside/warm side.
    – crip659
    Mar 29 at 12:55
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    Please take the tour. What you posted below is not an answer. No, omitting insulation is not a good plan. The fact that your wall is in the vicinity of another wall doesn't change anything.
    – isherwood
    Mar 29 at 14:39
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    I revised your description. "Butts against" implies an end-to-end arrangement in direct contact. That's apparently not what you have.
    – isherwood
    Mar 29 at 14:41

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Dual vapor barriers are bad, but so is a dew point inside the insulation. Much more moisture will derive from human activity inside the home than will from the outdoor climate during the cold season, and it will condense and soak your insulation. I would still install a plastic vapor barrier on the inside face of the framing. If possible, open the outside of the wall to ventilation in a protected area, such as under a roof overhang.

If you have any means of replacing the outer plastic with a proper breathable membrane, even in part, do it.

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