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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
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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
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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
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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
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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
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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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Nivek4012
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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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gcrank1
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# Posted: 18 Dec 2025 12:44pm - Edited by: gcrank1
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The old, leaky cabins didn't have a problem with condensation, that's true. They were also more than hard to heat.
Fwiw, We are recreational cabineers, here in sc WI we had for a few years a poorly built, sawmill board cabin (more of a shack) that had scraps of 1/4" 'plywood' under the sawmill board siding, the walls stuffed with unfaced f-glass insulation and No VB under the inner boards. When we tore it down a year ago it was vermin infested/tunneled, damp and junk. There was no question it 'breathed' but it was NOT successful. I guess the previous owner just didn't pay attention to the 'best practices'...... So the cabin we paid for was trash (really it would have been better if he hadn't put insulation in the walls and cath ceiling at all. We had to tear it down and start over.
Pls PLS...avoid the regrets! Why waste your time, effort and money if you wont do it right.
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Steve_S
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# Posted: 18 Dec 2025 06:55pm
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@Nivek4012 Opinions are as common as arses walking down the street. STICK to your New Brunswick Provincial Building Codes, they exist for good reason. In CANADA (for those from outside) we have our Federal & Provincial/Territorial codes, which do vary somewhat. Codes, rules & Requirements from elsewhere are MOOT, and following them instead of the codes that apply to you can be costly and create other problems if they are not for "Your Climactic Region".
As for Tin or other similar siding, using a Rainscreen Method to allow for passive airflow is highly suggested and also helps resist heat gain or loss depending on the season. Rainscreen walls & Cool Roofs are a great way to improve the efficiency of the thermal envelope with the least cost & ease and highest return & CMHC has detailed info online covering that, Cool Roofs systems & Methodology and even how to build using Thermally broken walls & roof plus heaps more.
FYI: I'm up near Algonquin Park, Thermally broken walls with solid high density foam with Rainscreen Siding (Live Edge Cedar) and a high performance Cool Roof that uses convection as for venting the walls & roof out the ridge vent, it is extremely efficient. -30C outside, radiant heating runs 2x for 2 hours each cycle to maintain 25C inside. Summer it can be 40C outside, inside the house NEVER goes about 27C and I haven't owned an AC since I finished build my place. I am Full Time, Rural & Remote and 100% Off Gris solar powered.
Advice & Suggestions are free, you have freedom of choice and therefore you enjoy the benefits of choosing, regardless if the outcome is good or not. Follow the NB Codes which want 6mil poly vapour barrier.
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travellerw
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# Posted: 18 Dec 2025 08:20pm
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Quoting: Nivek4012 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.
I just have to say that your logic is flawed. Like I said, the vapor barrier keeps the warm moist air inside (it can't get in the wall from the inside). Now even if there is cold moist air in the tiny gap between the insulation and tin (very unlikely as cold air holds little moisture). The heat remains on the inside and that air stays cold (thus no condensation).
Anyway, it seems you are only asking so you hear the answer you want. To me its seems you mind is made up and you are just looking for validation. If that isn't true, then the fact that its code for permanent or temporary residential structures should be enough.
Your cabin, build it how you see fit (unless you are getting it inspected).
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DRP
Member
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# Posted: 19 Dec 2025 06:33am
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Code link?
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Steve_S
Member
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# Posted: 19 Dec 2025 09:21am
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NB-fed codes: https://mneca.ca/2025-national-build-code/
NB Building Code "TOP"
https://www.gnb.ca/en/topic/laws-safety/licensing-inspections/technical-inspections/b uilding-code.html
National Building Code of Canada 2020
https://nrc.canada.ca/en/certifications-evaluations-standards/codes-canada/codes-cana da-publications/national-building-code-canada-2020
CMHC Universal Design Guide (PDF - ENG)
https://publications.gc.ca/collections/collection_2023/schl-cmhc/NH21-19-2022-eng.pdf
* They have a very extensive library of publications related to all building tech/methods etc.
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travellerw
Member
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# Posted: 19 Dec 2025 10:22am
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One thing that hasn't been mentioned.
Vapour barriers aren't just for keeping hot moist are in, but also for keeping radon out. Probably not a huge deal for a raised cabin, but thought I would point it out since its actually mentioned in the code!
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DRP
Member
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# Posted: 19 Dec 2025 02:22pm
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Do you have a code citation for the vapor barrier?
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Steve_S
Member
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# Posted: 19 Dec 2025 04:10pm
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@DRP, seriously ? Do the legwork and learn. It's not cool to ask the Teacher for the answers to the test. You do have some personal responsibility.
ALL of it also depends on WHERE YOU ARE too...
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DRP
Member
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# Posted: 19 Dec 2025 08:43pm
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I'm sure we all get "code" regularly from dogwalkers, laborers, subs and sometimes inspectors who are out of their depth. One needs to be able to sort through the noise. Whenever you get busted by enforcement, or when someone is quoting "code", the first request is "citation please". It'll sort the wheat from the chaff real quick. With chapter and verse we can read through the text and commentaries and have an actual discussion.
I have read the codified law and the commentary that I could find. That is why we want the actual text to talk about. There is more there. I find it more accurate to start at the source, then work into opinion and interpretation.
I'm liking @lalan45's suggestion at the moment, there is drying potential. I'm also not saying anyone is wrong, teaching begins with knowing the material.
"Section 5.5. Vapour Diffusion
5.5.1.
5.5.1.1.
5.5.1.2.
Vapour Barriers
Required Resistance to Vapour Diffusion
(See Note A-5.5.1.1.)
1) Where a building component or assembly is subjected to differentials in
temperature and water vapour pressure, the properties and position of the materials
and components in those components or assemblies shall be such that they control
vapour diffusion or permit venting to the exterior so as to minimize the accumulation
of condensation in the building component or assembly.
2) Except as provided in Sentence (3), a vapour barrier shall be installed to provide
the principal resistance to water vapour diffusion.
3) Avapourbarrier is not required where it can be shown that uncontrolled vapour
diffusion will not adversely affect any of
a) the health or safety of building users,
b) the intended use of the building,or
c) the operation of building services.
Vapour Barrier Properties and Installation
(See Note A-5.3.1.2.)
1) Thevapour barrier shall have sufficiently low permeance and shall be positioned
in the building component or assembly so as to
a) minimize moisture transfer by diffusion, to surfaces within the assembly
that would be cold enough to cause condensation at the design temperature
and humidity conditions, or
b) reduce moisture transfer by diffusion, to surfaces within the assembly that
would be cold enough to cause condensation at the design temperature and
humidity conditions, to a rate that will not allow sufficient accumulation of
moisture to cause deterioration or otherwise adversely affect any of
i) the health or safety of building users,
ii) the intended use of the building,or
iii) the operation of building services.
(See Note A-5.5.1.2.(1).)
2) Coatings applied to gypsum board to provide required resistance to vapour
diffusion shall conform to the requirements of Sentence (1) when tested in accordance
with CAN/CGSB-1.501-M, “Method for Permeance of Coated Wallboard.”
3) Coatings applied to materials other than gypsum board to provide required
resistance to vapour diffusion shall conform to the requirements of Sentence (1) when
tested in accordance with ASTM E96/E96M, “Standard Test Methods for Gravimetric
Determination of Water Vapor Transmission Rate of Materials,” by the desiccant
method (dry cup)."
More reading;
https://buildingscience.com/documents/reports/rr-0004-air-barriers-vs-vapor-barriers/ view
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travellerw
Member
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# Posted: 19 Dec 2025 09:18pm
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You couldn't click on the link that Steve posted and hit ctrl-f to find it. On top of that, the document you posted is from 2000!
Thats all I will say.. Sorting wheat from the chaff.
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Steve_S
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# Posted: 20 Dec 2025 08:04am
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Hear Hear Travellerw, oivey. Building Science is good "in general" fro education & understanding but useless when it comes to actual Federal & Regional building code requirements for a specific locale.
Irony, when I was building, our local inspector was not up to speed on the usage of High Density Foam Sheets with regards to vapour barriers and sealing etc... So I provided a quick printout from the Fed & Prov codes and he took it really well and appreciated the enlightenment. At the time, our Governments were doing a major overhaul/update on the codes and there was a lot for a poor inspector to keep up with. That was also when they added in Solar Technology and Hyper Efficiency methodologies, lot's going on.
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