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I am in the middle of a complete main floor bathroom gutting and re-finish job in my house in CANADA.. and WINTER IS COMING!!!!

THE BACKGROUND (pics below):

My bathroom is 5'x 13' in a bungalow with 2x6' outside construction walls with Fiberglass batt insulation in the walls, and blown cellulose insulation in the attic. My house has been a very energy efficient and almost perfectly sealed home built in 1985.

I consider myself to be well versed with applying the proper rated vapour barrier to the outside walls using proper vapour-barrier/acoustic sealant (black tar of death) and stapling properly to have a PERFECT air-tight vapour barrier against our crazy -40C/F winters.. I even know about the Tuct-Tape work required to seal joins and overlap edges of separate sheets of poly, etc.

... However.. when I took down the old popcorn drywall ceiling (after i moved over all the cellulose insulation so it wouldn't fall in).. I noticed the original vapour barrier was ONLY stapled up to the rafters with the 5/8" drywall screwed/nailed through it... the bathroom ceiling vapour barrier had NO BLACK sealant along the edges, so it was not 100% air/vapour sealed to the attic.

THE QUESTION:

Since i'm NOT doing popcorn ceiling again, should I apply black-sealant as well to the ceiling sheets of vapour barrier just like I will do to the outside wall to create a perfect seal??

I have read some stories that because proper attics (like mine is) has natural air-flow its OK for a small amount of warm-air to get into the attic because the natural airflow will vent the moisture!! Is this TRUE? or would i be creating a condensation disaster in the making?

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Pic of outside wall with new vapour barrier ONLY sealed at top for now

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Another close-up pic of the opposite corner showing exposed attic

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Most of the time in a house in Canada you would have 6-10mil plastic over the drywall in the ceiling.

However the answer isn't that easy and the vapor barrier isn't needed for sure.

It also depends on the type of insulation that you have (faced or not), what other types of barriers are installed in your attic, and really what is on the rest of your house.

So from this picture I would get a ladder and look at what was in the next room on the ceiling. I would do the same thing.

My personal opinion... is that I don't like a vapor barrier in the attic above a bathroom. Unless you paint the hell out of your ceiling (which I do for bathrooms including two coats of Killz oil based primer) water will get into the drywall. It might after 4 coats too... So I want that water to be able to escape, not be trapped in a vapor barrier.

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I keep seeing your question has not been answered. I do not build in the type of climate you describe but here are my thoughts: Do not trap moisture within the wall/ceiling structure- so one side only. Keep moisture from entering - barrier on room side of wall only. Around here I have had inspectors insist on removing moisture board on ceilings - they have claimed it will hold water if there is a leak in the roof. I have seen a couple of manufactured homes with plastic sandwiched between drywall and insulation board - one moldy mess. Sorry I can't add anything more.

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There should be a vapor barrier to all exterior/cold areas. Seal and tape all wall/ceiling interfaces. The plastic should be 6 mil thick.

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