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My carpenter installed a metal bulkhead to box in some plumbing. My question is, how can I effectively install a vapor barrier here? I was able to insulate behind the bulkhead, but now the top plate is inaccessible so I can't start my vapor barrier there.

Any advice? Should I wrap the barrier (poly) around the bulk head?

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    Why are you trying to install a vapor barrier on the inside?!
    – ThreePhaseEel
    Jul 9, 2016 at 16:29
  • I was thinking the same thing. The vapor barrier is usually on the exterior wall. +
    – Ed Beal
    Jul 9, 2016 at 16:40
  • I've been told that Ontario building code requires a vapor barrier between drywall and studs.
    – Parv
    Jul 9, 2016 at 21:18
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    Vapour barriers are installed don the inside in Canada, because the interior is hotter and more humid than the outside in winter, the season where the insulation is most ciritcal. I realize this is probably the opposite of what folks in Florida do, where is hotter and more humid on the outside, but Canada ain't Florida.
    – AndyW
    Jul 10, 2016 at 1:30

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It's highly likely you don't need or want an actual vapor barrier here. These horrible things are still popular in the frigid north climes like Canada, but they truly aren't needed in a well-insulated wall like the one you have. Even up there, on average they cause more problems than they solve and substantially reduce a wall's ability to dry. They can also become mold machines if the space is ever air conditioned and you have stucco or stone wall cladding on the outside. Good choice using mineral wool.

Bottom line: skip the vapor barrier and don't worry about it. Just install your drywall.

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  • I understand what you're saying but in Ontario I believe it's required by code to add a vapour / air barrier between board and insulation... see below 4.2. SHEET VAPOUR BARRIER SHALL BE INSTALLED ON THE INTERIOR SIDE OF INSULATION PRIOR TO INSTALLATION OF GYPSUM BOARD.
    – Parv
    Jul 9, 2016 at 21:21
  • This is terrible advice for anyone living in a cold clime like Canada. Anytime the inside of the house is warmer and more humid than the outside, vapour barriers are necessary to prevent condensation inside the wall/insulation where the temperature drops to the dew point of the inside air. Warm weather guys, and that's almost anyone in the USA, just doesn't get cold weather realities of Canada.
    – AndyW
    Jul 10, 2016 at 1:25
  • Parv: I would install the vapour barrier on the bottom portion, then cut a vertical slit on the top portion of the vapour barrier inline with each stud. Feed the plastic between the metal studs and insulation up into the top plate cavity as you would if there were not bulkhead. It's harder with the bulkhead there, but should work.
    – AndyW
    Jul 10, 2016 at 1:31
  • @AndyW - thanks! that's probably my best option.
    – Parv
    Jul 10, 2016 at 14:10
  • @AndyW - one more question, carpenters usually leave a vapour barrier (piece of poly) on the top header of the framed exterior wall.. what exactly is this used for? Should it be sealed to the vapour barrier on the wall? Or should it be used is if the ceiling requires a barrier?
    – Parv
    Jul 15, 2016 at 2:14

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