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I live in Denver, CO (relatively low humidity). I am trying to finish the basement which already has a vapor barrier on the walls stretching from floor to ceiling joists. Between the joists I can see and touch the fiberglass insulation. Should I extend the vapor barrier between the joists? I intend to finish the ceiling as well and the exposed part would be above the drywall level. Additional info:

  • basement has well windows, therefore joists are above ground level.
  • kb home built in 2017
  • Furnace/boiler located in the basement.

Thank you.

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The easiest thing is to get rigid insulation xps and cut it to squeeze fit the joist bays. Once you've squeeze fit the rigid pieces spray foam around the outside of the pieces to seal it.

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From what you describe, the answer is no, you would not want to put vapour barrier on the ceiling. Especially in dry climates, only the exterior walls, the attic floor, and the basement floor would have a vapour barrier. If you were to put a vapour barrier on the ceiling, you’d likely need to increase ventilation or add dehumidification to the basement — particularly if this is the entire basement.

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  • Also, if the upstairs gets a leak, it'll likely pool up so heavy until it blows out the ceiling below. Without it, it'll find the low spot and most likely show signs before causing major damage.
    – user19565
    Sep 11, 2022 at 19:23
  • Thank you for the reply. I realized my description wasn’t clear. What I meant was that the vapor barrier extends up the wall from floor to the lowest point at which ceiling joists come in contact with wall. Therefore, the top 12” of the wall, right before wall and ceiling join, has fiberglass but no vapor barrier. Idk if I should get some 6 mm foil and cover the wall between the joists. Thanks
    – Raz
    Sep 12, 2022 at 0:22
  • @Raz yes you want to have insulation and vapour barrier on the walls and up into the rim joist bays.
    – RibaldEddie
    Sep 12, 2022 at 0:42
  • Thank you for the clarification
    – Raz
    Sep 12, 2022 at 2:45

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