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Interior air/vapour barrier alternatives

Dentside | Posted in General Questions on

Hello All,

My question concerns the application/installation of an air barrier on a new construction cabin.  I’m in climate zone 5 in the dry, southern interior region of BC in Canada.  Summers are very hot and dry (often one of the hottest spots in Canada) and winters are cold and usually dry.

The basic structure is already in place.  The walls are staggered-stud 2×4 construction (on 2×8 plates).  Exterior sheathing is 1/2 plywood.  No vapour barrier/house wrap or exterior siding installed yet…just plywood.  Insulation is two, staggered layers of R14 Roxul.  The roof structure is 2×4 trusses sheathed with 1/2″ plywood, which is covered in roofing paper and metal roofing.  Ceiling insulation is two layers of R22 Roxul.

The interior ceiling (pine t&g) is already in place with 6 mil poly underneath.  I’m now beginning the process of finishing the interior walls and have only just now discovered that 6 mil poly may not be the best option for the walls (too late to change the ceiling!).

I was not planning on using drywall on the interior walls.  I’m leaning towards 1/2″ or 5/8″ plywood (not OSB) for the finished walls, which I would either paint or wallpaper over.

So, it seems that poly is being deemed unnecessary or overkill for most climate zones.  Can I simply nail up plywood to the interior studs (without poly underneath), seal the seams somehow and then paint with a vapour retardant primer/paint?  My plan for the exterior is to use 15# paper over the plywood followed by corrugated metal roofing oriented vertically (no stand-off).  Options or suggestions welcome.

By the way, building inspectors/codes are not a concern in this situation.  The only concern is good practice.


Lillooet, BC

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  1. Expert Member


    Yes you can use plywood with a vapour retarder paint. That will work as your required vapour barrier, but with no way to effectively seal the joints, you would be better off moving the air-barrier to the exterior sheathing and taping the joints on it.

    I say "required" Vapour barrier because there is no where in BC that isn't governed by the provincial building code. Both civil liability and insurance coverage depend on meeting code, whether the local authorities enforce it or not.

  2. Dentside | | #3

    Jon R,

    Thank you for the link...much appreciated.

    Malcolm Taylor,

    Thanks for your input.

    If I interpret your reply correctly, as well as painting the exterior plywood with vapour retarder primer/paint, you suggest that I tape the joints on the exterior sheathing (and otherwise seal all penetrations in the sheathing, I imagine) versus doing the same on the interior sheathing, with the rationale that there is "no way to effectively seal the joints...".

    Sounds like a simple solution...move the retarder application to the outside. However, I'm a little lost. If the joints cannot be sealed effectively on the interior, how will there be more success on the exterior? Or is it because there will now be tape to somehow cover up or deal with aesthetically on the interior side (which would not be an issue on the exterior)?

    And, because I value all input, I'll come about the question of air/vapour/moisture barrier from this angle:

    Given the climate zone that I live in, does the following wall assembly seem appropriate for a non-air conditioned cabin...working from inside to outside:

    1/2" or 5/8" plywood ("regular" interior paint or papered) / Roxul insulation on 2"x8" plates with staggered 2"x4" studs / 1/2" plywood sheathing with sealed seams and painted with vapour retarder paint/primer / two layers of 15# roofing paper / corrugated metal siding oriented vertically.

    I understand that modern practice is to create a rainscreen through a variety of materials and techniques. I've been told that in a nearby town which receives far more precipitation per year that vertically-oriented corrugated metal siding meets rainscreen requirements, so I think it's safe here in the dry belt.

    As an aside, I cannot get home insurance where I live.



    1. Expert Member
      MALCOLM TAYLOR | | #5


      Yes - it's only easier to seal the exterior because you can use tape without having to look at it. Separating the location of the air and vapour-barriers in your situation makes sense. If you are not concerned with meeting code, I wouldn't worry that much about using a vapour-barrier paint. Any finish you decide to use will make the plywood less permeable than the exterior layer, promoting drying to the outside.

      Your wall assembly sounds like it will perform very well. Corrugated metal siding creates a pretty good rain-screen without strapping. I use it a lot. I've got some on my house.

  3. Jon_R | | #4

    While an exterior side air barrier (and an interior side vapour barrier) may be your best bet, note that interior and exterior air barriers are not completely equivalent - interior side has some advantages in a cold climate. Both sides is even better.

  4. GBA Editor
    Martin Holladay | | #6

    Note also that your ceiling air barrier is somewhat flawed -- it sounds like you have 6-mil polyethylene with tongue-and-groove boards on the interior side of the poly. That approach isn't ideal -- there are usually lots of air leaks at the polyethylene seams, and fasteners may leave holes in the poly that allow air leakage. Whether or not this approach works depends in part on how carefully the polyethylene was treated when you installed your ceiling boards.

  5. Expert Member
    Dana Dorsett | | #7

    The vapor permeance of most plywood even WITHOUT paint meets then NBC's definition of "vapour barrier" ( a hair over 1 US perm) if your indoor humidity levels are in a normal range:

    Unless the plywood is made from southern pine (not very likely if sourced from a mill in B.C. rather than Georgia) it has sufficiently low vapor permeance on it's own.

  6. Dentside | | #8

    Thanks to all for your input.

    It appears that Heating Degree Days for Lillooet is 4444 and dropping. I believe I'm in Climate Zone 5, but I'm not entirely sure. Again, the typical pattern is for cold and relatively dry winters and hot, dry summers. Very little humidity to be concerned with.

    Martin, yes, there is 6 mil poly under my t&g ceiling. This was done before I understood all the details of air/vapour barriers/retarders. I do know that the poly seams were all taped, the chimney penetration was taped, and the lights and ceiling fan box were also taped. As well, all staples were backed by tar paper washers. Where the poly meets the top plates of the walls is still unfinished. I plan to use acoustical sealant where the poly meets the plates. Of course, the brad nails used to hold the t&g boards would have made minuscule penetrations.

    Malcolm, thanks for your vote of confidence. So, as it stands, I believe I can do without poly on the warm side of my insulation. This is a cabin (under 750 sq. ft.), although it will be my primary residence. Nothing fancy is required in terms of aesthetics. Open concept interior...basically just one big room. The only interior walls will be for a bathroom. No air conditioning, just a ceiling fan. Wood heating only (supplied by a Fisher Mama Bear stove).

    Again, my exterior wall assembly plan is as follows: interior plywood siding (likely painted, but might be left alone if I splurge on nice plywood) nailed directly to staggered-stud framing. Insulation is Roxul batts...two layers of R14. Exterior sheathing is 1/2 plywood, and I will seal the plywood joints and any other penetrations with air barrier tape (or silicone?). Over the exterior plywood will be house wrap or 15# tar paper, followed by vertically-oriented corrugated steel cladding.

    Alternatively, I could seal the interior plywood to the framing and make this the air barrier, with the house wrap or tar paper as vapour barrier. Correct?

    Am I missing something in this assembly? Something vital or important that I haven't thought of? I'm trying to achieve the objectives of keeping the wall assembly free of any errant moisture without surrounding myself in plastic or buying any unnecessary materials just because "code" says so.



    1. Expert Member
      MALCOLM TAYLOR | | #9


      I think you have everything covered. To aid drying on your walls I'd use a perforated L flashing at the base of the corrugated siding, rather than a solid one.

      I think the finished wall assembly you are proposing is going to end up meeting code anyway.

  7. Dentside | | #10

    Thanks, Malcolm.

    I was intending on using some kind of plastic-based "profile" drainage strip designed to do what the perforated L flashing does. I used a similar thing for my ridge adhesive strip that follows the roofing profile and allows breathing while blocking insects and wind-driven rain and snow. I'll check out the perforated L...never heard of it before. Looks like gluing a strip of bug screen to the flashing would bring it up to snuff!

    I hope I didn't screw up the ceiling assembly with the poly, but it's up there now.

    I've been pondering the idea of where to place the air barrier. Jon R's comment about having it in the interior of the wall in a colder climate...well, it can and does dip into the minus 20s Celsius around here. So, can the air barrier be accomplished without taped seams? How about using acoustical sealant on the studs and plates when I put up the interior plywood, then silicone in the joints? The joints would need to be trimmed out anyway, so I may be able to hide tape, too? I'm guessing this would help a fair bit in terms of reducing the heat and moisture loss into the wall enclosure. Then, with unsealed sheathing on the exterior, house wrap as vapour barrier and the vertical roofing panels any moisture that does get in the enclosure should vent/dry to the outside.

    What I haven't told you yet is that the house is built on piers...sitting a few feet off the ground. I've skirted this crawl space by digging a 6" deep and 12" wide trench below the perimeter which I filled with concrete, then poured a 6" wide concrete sill on top of that, placed 2"x6" plate on top of that and framed up to bottom of my floor joists. The exterior house sheathing goes over this framing (1/2" plywood) and I've insulated this skirt wall with R26 Roxul. The floor joists for the cabin are NOT insulated.

    The crawl space is NOT vented. I plan on placing 6 mil poly over the ground and sealing it to the skirt wall once I've completed all the plumbing. It's a work in progress!

    So, my new question is how to seal the crawl space so that although it's not a heated space (at least not directly) it is a "conditioned" space. Can I just do what I'm doing with the cabin's main walls? I was considering attaching 2" foam board to the interior of the framing...including the rim joist voids. Thus, my skirt walls would consist of (from outside to inside): corrugated metal roofing, house wrap/tar paper, 1/2" plywood, 2"x6" framing with Roxul batts, 2" foam panel. The rim joist voids would be the same. The foam panel seams would be taped.

    I know I might have colder floors with this arrangement, but I could always insulate the joists later if I find that to be the case. My main concern is moisture in the crawl space. My research on venting suggests that I either use no vents at all (and seal the ground) or use humidity-controlled vents.



    1. Expert Member
      MALCOLM TAYLOR | | #11


      I was recommending using the exterior sheathing for the air-barrier because I assumed you were putting the plywood on the interior without trim - as it is very much the thing to do in all the architectural magazines right now. It's also incredibly difficult to get tight joints at the intersection of walls, as framing is never exactly square. If you are planning on trimming the sheets your method will work, as would applying gaskets to the framing before hanging the plywood. Check around specialty suppliers. The last time I used interior plywood I was able t9 get 5/8" maple for less that $5 more a sheet than standard exterior grade in the lumberyard.

      As to your crawlspace: My own preference would be to avoid using foam that will be close to grade and hidden from inspection, because of the possibility of carpenter ant infestations. If you are diligent in keeping them out them maybe the foam is worth it. I've had too many bad experienced to chance it.

      Once you have completely enclose the crawlspace you have a few options to keep it conditioned. You can place a series of vents in the floor or walls to connect the crawlspace with the house above. The downside is you are essentially creating one space which will share the air and equalize the humidity. The other is to install a vent from above and an exhaust fan on the crawlspace skirt to remove humidity and stale air. If you are planning mechanical ventilation anyway this could be a good solution. i wouldn't recommend humidity controlled vents. Once you have opted for a closed crawlspace, intermittently opening it causes more problems than it solves.

  8. Dentside | | #12


    Yes, I have been inspired by the birch and maple plywood that is available for reasonable prices. There's a place in Richmond, in particular...1/2" Birch, smooth-sanded both sides, $34 a sheet. However, I have not been inspired by the images I see of how it's used in modern architecture. I prefer a more rustic look, and I may just buy some good-one-side and paint it...hence the assumption of trim on my part. Still, the nice, smooth Birch panels do look good and they would really make the place bright. As it is, the ceiling is bare Pine t&g panels, as previously mentioned, and there are four glass skylights and several nearly floor-to-ceiling windows...there's a lot of light inside!

    Regarding tape as a joint sealant, I just stumbled upon two rolls of Resisto "Red Zone 25 All Weather" vapour and air adhesive membrane. I used it as part of the flashing system for my skylights, and it seems I have a lot left over. I'm certain this would be very effective to seal any joints.

    I appreciate your comments regarding foam panels on the interior of the crawlspace enclosure. I guess the alternatives are similar to the cabin's main walls...plastic or plywood or drywall? As you might guess, I'd rather not drywall. Exterior grade plywood could work, but would be a bit awkward in places. I suppose I could use poly...good for constant inspection, effective for air and moisture, easy to handle in a crawlspace, and it wouldn't be in the living space.

    I have long considered joining the main living space and the crawlspace with vented openings in the floor between. I've seen some ideas (particularly from '70s classic, Ken Kern's "The Owner Built Home") which seem to make sense to me...for example, having a chute/chase close to the middle of the home that is open near the ceiling (where the heat hangs out) and goes down through the floor joists and into the crawlspace. There is a fan inside the chase near the top. It blows warm air down into the crawlspace, and there are vents in the floor to accept the rising air from the crawlspace for circulation. It seems to me that equalizing the humidity is desirable...spread the moisture out into a greater air space.

    Given that my plumbing and battery bank/inverter assembly is in the crawlspace, I'd like it to be as warm as possible in there without direct heating. Makes sense to share some of the "up stairs" heat. Even with just vents in the floor (no fan), there would be air movement. They could easily be shut/blocked-off if desired. Another consideration for air and air movement is that I depend on propane appliances and wood heat, all of which need air for combustion and proper venting. I plan to have vents for these appliances that draw from the outside through ducting in the skirt wall.

    Your second alternative to venting sounds interesting. What do you mean by "vent from above"? Do you mean have an exhaust fan pulling air out of an opening at the top of the skirt wall? As well as poly on the ground or in lieu of it? I suppose this fan would have to run continuously? I'd have to consider the constant power load on my inverter.

    If you ever get off the Island and want to come up to the beautiful Southern Interior, let me know! I appreciate your thoughtful assistance.



    1. Expert Member
      MALCOLM TAYLOR | | #13


      If you connect the two spaces with a vent, the air movement will almost invariably be from the crawlspace up into the house, due to the stack effect. To move air down from the house you would need an exhaust fan on the skirt to de-pressurize the space.

      Last year I got as far as Sydney (60kms). The year before that I was tricked into going to Nanaimo (150kms). Barring funerals, the chances of me leaving the Island in the near future aren't great - but thanks for the offer!

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