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Nivek4012
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# Posted: 17 Dec 2025 01:59pm - Edited by: Nivek4012
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Hi, long post but I would appreciate valid feedback.
My cabin is located in New Brunswick, Canada (climate zone 6). Cabin is newly built and I'm at insulation stage. My question is do I need a vapor barrier or not (something else?) to let wall breathe considering these:
Cabin will only be heated during weekends with wood stove.
Cabin is on screw piles with insulated floor
Wall construction from outside -> inside: sheet metal siding directly on taped tyvek, 7/16 OSB, 2x6 wall studs filled with r22 fiberglass batt insulation.
ceiling is vaulted with R40 fiberglass batts. Roof is vented through soffit and ridge vent.
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travellerw
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# Posted: 17 Dec 2025 04:17pm - Edited by: travellerw
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Absolutely.. No question.
Vapor barrier is an essential part of the insulation system in very cold climates. Not only does it provide an air seal to protect against convection heat loss, but protects the insulation against humidity.
The 2 enemies of any type of batt insulation is:
1. Moisture (prevents air from getting trapped between the fibers and providing insulation. Moisture also causes the insulation to slump exposing the top of the wall to no insulation).
2. Air movement (causes heat losses due to convection).
In fact, this is the one area you should take extra care. Take your time and seal all the gaps, use poly seals around electrical boxes, spray foam and tape where poly can't get. This will ensure the least amount of drafts when everything is finished.
Its not about expense or time. This is about how comfortable the building will be at -20C
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FishHog
Member
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# Posted: 17 Dec 2025 05:52pm
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yeah. exactly what he said
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Nivek4012
Member
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# Posted: 17 Dec 2025 06:53pm
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Even if it only get heated up during weekend? My concern is I will get condensation between the cold outside wall (metal) and warm side and will not breathed. In a house that's heated all winter, no question, a vapor barrier is needed but a cabin that get cold during week and heated on weekends I don't know. I appreciate the feedback.
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gcrank1
Member
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# Posted: 17 Dec 2025 07:18pm
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Build to 'Best Practices' as known and understood today and have no regrets later 
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Grizzlyman
Member
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# Posted: 17 Dec 2025 07:54pm
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Think of it this way. It’s cheap and easy. Just some poly and staples.
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travellerw
Member
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# Posted: 17 Dec 2025 10:28pm
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That is why you used Tyvek on the outside. Its breaths (even though it looks like it doesn't)
All I can say is we have a similar cabin here in Northern Alberta. It -40C in the winter and we are only there on weekends. I let my ocd kick in during that stage and plugged EVERY hole (I even taped the staples). Once the cabin is up to temp, it takes very little wood to keep it there. There is not a draft anywhere.
I will note, that if you seal it up that well, you will need air for your fireplace. Either a dedicated vent, or you have to crack a window near the fireplace. We can't even get ours started if the vent is closed.
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travellerw
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# Posted: 17 Dec 2025 10:29pm
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P.S. The reason you seal it up so well is so that you don't get condensation. All the moist warm air is kept INSIDE and can't get to that nice cold tin!
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DRP
Member
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# Posted: 18 Dec 2025 07:55am
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We set up drying conditions.
https://buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design/ view
Spend some time wandering around that site.
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Steve_S
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# Posted: 18 Dec 2025 09:38am
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Canuck here (Ontario), it's not only part of our National Code but Provincial codes as well and you won't pass inspection without it. 6mil poly is the default. It absolutely has to be sealed ! That also means taping all seams and using a Poly based caulking like Accoustiseal which will not harden or dry & remains flexible (and extremely sticky - hint) where the sheets overlap and at the top & bottom along the walls on the framing as well as around the door & window framing as well. Don't forget also around anything else going through the walls like wires/plumbing. In some regions they also want to see this caulking along every stud and a building inspector will direct as needed if you ask.
A GOTCHA ! because of foams now being used more frequently which are impermeable and therefore cannot pass moisture, vapour barriers are "optional" Codes have mostly all been updated to reflect these new materials. federal codes have just gone through a full revision this year, so look to the most recent guides & books. BTW, they also made a few more things simpler and addressed the latest building methods like thermally broken walls/roofs, and materials used etc. Avoid referencing the older codes, you could end up in a serious pickle.
Also, I am sorry to say this, you won't like it. Fibreglass insulation comes with a heap of issues and even worst with part time occupied builds. Bugs, rodents love to nest within, chipmucks & mice will also steal it to make nests. FGlass is also not hydrophobic and can hold moisture & water, which in turn results in moulds, freezing/thawing causing other issues. Rockwool (ROXUL) has none of those characteristics and critters HATE it and will not nest within or steal it. Rockwool is also virtually fireproof, takes 1500C before it is affected but will never burn.
Hope it helps, good luck.
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MtnDon
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# Posted: 18 Dec 2025 10:04am
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Even if at present you only used the place once a month, in the future your pattern of use can change; maybe become more like full time. Correcting faults later always costs more.
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lalan45
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# Posted: 18 Dec 2025 11:09am
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Quoting: Nivek4012 Hi, long post but I would appreciate valid feedback. My cabin is located in New Brunswick, Canada (climate zone 6). Cabin is newly built and I'm at insulation stage. My question is do I need a vapor barrier or not (something else?) to let wall breathe considering these: Cabin will only be heated during weekends with wood stove. Cabin is on screw piles with insulated floor Wall construction from outside -> inside: sheet metal siding directly on taped tyvek, 7/16 OSB, 2x6 wall studs filled with r22 fiberglass batt insulation. ceiling is vaulted with R40 fiberglass batts. Roof is vented through soffit and ridge vent.
In climate zone 6 you generally do need a vapor control layer on the warm side. For a weekend-heated cabin, a smart vapor retarder (Class II/III, like MemBrain or Intello) is a good choice—it limits vapor in winter but still allows drying when the cabin cools down. Avoid a full poly vapor barrier, as intermittent heating increases condensation risk.
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Nivek4012
Member
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# Posted: 18 Dec 2025 12:06pm
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Interesting, Membrain is exactly what I was recommended but never heard of it nor seems to be used around here per couple local hardware stores. Most people even all the great reply in this post still recommend a 6 mil vapor barrier. The more I read the more I'm questionning the metal on outside restricting the breathing and adding a vapor barrier that would restrict the breathability on the inside. That would be a bad situation for condensation between the walls with all the heating cooling cycles. There are thousands of cabins across Canada thus the reason I'm looking for inputs.
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