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Vapour barrier required?

Helpmewithmyhouse | Posted in General Questions on

Climate zone 7, Canada
A room on the main floor of my old 1.5 story house has been gutted and we are just about to start reassembling it.  After reading Martin’s article, I know it’s ok to put vapour barrier (poly) on the walls (no foam on the outside of the house).  Half of the ceiling of the room looks up to the attic kneewall.  Is it ok to put vapour barrier (poly) on the ceiling of the main floor room? Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Help me,
    I addressed this question in a recent article, "Martin’s Six Rules for Polyethylene."

    In that article, I wrote, "If you live in a cold climate (Zones 5 through 8), installing polyethylene between the ceiling drywall and the insulation on the floor of a vented unconditioned attic is harmless but unnecessary. Instead of worrying about a vapor barrier at this location, pay attention to airtightness. If you can make your ceiling airtight, you’ve solved 95% of any potential moisture issues."

    The main reason to install polyethylene in this location is to keep your building inspector happy.

    1. Jon_R | | #2

      Relevant to many in moderate cold climates:

      "Avoidance of the installation of vapor barriers such as polyethylene vapor barriers, ... on the interior of air-conditioned assemblies – a practice that has been linked with moldy buildings"

    2. Helpmewithmyhouse | | #4

      I don't have a vented attic. It's an unconditioned unvented attic.

  2. Peter Yost | | #3

    Air sealing is important for two reasons: both btus and moisture move with leaking air. In cold climates, up to 40% of total heat loss can be attributed to air leakage, and along with those btus, vapor is moving and happy as a clam to find a cold surface to have a liquid party on.


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