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What is a cost-effective wall assembly?

Erik Olofsson | Posted in General Questions on

it seems a variation of this building question gets asked a lot so forgive me… i am building a 600 sq. ft. home in the Rocky Mountains of British Columbia where the building code calls for poly on the warm side of the insulation.

i would like to get as close to R-40 for the walls as economically as possible. Seeing that the received opinion around the GBA is the tandem of polyethylene sheeting and exterior rigid foam is not ideal, what do the builders on this site recommend? Larsen truss seems fairly labour-intensive and rigid foam is expensive… Is a double stud wall the answer?

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  1. John Klingel | | #1

    " a double stud wall the answer?" •• That is the conclusion to which I came, after looking at a variety of walls. Search for some of the variations; Sunrise House, Riversong Truss, etc. I built a double-stud in 1980 and have never regretted it. New house will be the same, but thicker, and with dense packed cellulose instead of fiberglass. Good luck.

  2. Albert Rooks | | #2


    A double stud with a plywood exterior and interior poly and ADA will work. Another good quality wall is a 2x6 standard frame with taped Ply/OSB exterior+WRB+ 4 to 6" of high density mineral wool. It will eliminate cold sheathing and rim joist bridging while allowing the use of a service cavity in the stud bays.

    The poly in this case is redundant since the air/vapor control layer is the OSB/Ply sheeting.

    Use a rainsceen detail, good air sealing and ventilation.

    I'm a fan of the mineral wool because it doesn't settle, doesn't rot if continually wetted, is fire proof, won't support mold or bugs. It's like a little piece if the Canadian Rockies (since it's made of Canadian Basalt) covering your house and near as durable.

  3. GBA Editor
    Martin Holladay | | #3

    1. You might want to negotiate with your local building inspector; many inspectors will accept MemBrain or vapor-barrier paint as an alternative to interior poly.

    2. There are lots of articles on GBA about double-stud walls. Here are a few to get you started:

    Double-Stud Walls

    Is Double Stud-Wall Construction the Path to Efficiency on a Budget?

    Choosing a High-Performance Wall Assembly

    Building an Energy-Efficient Home on a Budget

    A Superinsulated House in Rural Minnesota

    A Thick Cocoon of Cellulose Protects This Superinsulated House

    Six Proven Ways to Build Energy-Smart Walls

  4. Erik Olofsson | | #4

    @john, one of my concerns with the double stud wall is i cannot use a dense pack cellulose for insulation. while i have used the DIY loose fill cellulose machines with success, unfortunately, there are no local insulation contractors who can install dense pack cellulose.
    @albert, i had not heard of using mineral wool (when i think of mineral wool, roxul "safe'n'nsound" comes to mind. i have read elsewhere on this site there are denser varieties of the same product) what product should i ask my supplier for? how do you install rain screen and/or cladding on top of it? i am familiar with typical rain screen details when used adjacent to exterior foam or plywood sheathing...
    @martin thanks for the links. as you know, the construction industry is remarkably resilient to change. i get just a liiiitle bit nervous when going against the grain of standard construction practices. would you know of any links to literature extolling the virtues of poly-less airsealing for my inspector to look at?

  5. GBA Editor
    Martin Holladay | | #5

    Very few builders use polyethylene as an air barrier. The most common materials used for air barriers include wall sheathing and roof sheathing (for an exterior air barrier) and drywall (for an interior air barrier).

    For more information on using wall and roof sheathing as your air barrier, see Airtight Wall and Roof Sheathing.

    For more information on the Airtight Drywall Approach, see

    For more information on air barriers, see Questions and Answers About Air Barriers.

  6. Erik Olofsson | | #6

    thanks, martin. i misspoke, i meant to say poly-less vapour barrier..

  7. GBA Editor
    Martin Holladay | | #7
  8. Expert Member
    Malcolm Taylor | | #8

    The BC building code allows an Airtight drywall Air barrier to be used instead of Poly. Illustrated details covering all aspects of the approach can be found in the "Building Envelope Guide for Houses" published by the Homeowner Protection Office, which is a Provincial Government department.

  9. Erik Olofsson | | #9

    wow, thanks malcolm. i was actually wondering how the HPO would feel about a deviation from the polyethylene vapour barrier detail.

  10. GBA Editor
    Martin Holladay | | #10

    Thanks for the useful information you provided.

    However, I can't help but shake my head at the inconsistency in the BC code. Do code officials think that polyethylene is a vapor retarder or an air barrier?

    If they think it is a vapor retarder, then the Airtight Drywall Approach is no substitute. To retard the flow of vapor, you need vapor-retarder paint. The airtightness of the drywall is irrelevant.

    If they think it is an air barrier, I wonder whether they require airtight installation details for all installations of poly in the province? For example, do they verify that the poly seams are sealed over framing members with Tremco acoustical sealant? Do they verify that all electrical boxes are airtight boxes?

    If they do, bless them. However, I doubt that they do. I think it's far more likely that the building inspectors have no idea whether the poly they insist on is supposed to be a vapor retarder or an air barrier.

  11. Lucas Durand - 7A | | #11

    Do code officials think that polyethylene is a vapor retarder or an air barrier?

    I'm not that familiar with the BC building code, but here in Ontario there are plenty of inspectors that Martin's comment would apply to.

    For the house that I am building for myself, I have gone with taped 1/2" plywood for both air barrier and vapour retarder (and racking resistance).

    There was some initial skepticism from my inspector but it didn't take that long to convince her - I am lucky she has been open-minded.

    In terms of an argument, I started by making the case for plywood as a vapour retarder.
    Then moved on to getting agreement that 1/2" plywood is a sufficient vapour retarder for this climate.
    Once that was established it was not difficult to get her to imagine that taping the plywood seams makes for a top-notch air barrier.

    Not sure if this will help any if you go with something other than an "airtight" sheathing approach...
    But good luck and may the force be with you.

  12. Albert Rooks | | #12

    Reply to Erik:

    @albert, i had not heard of using mineral wool (when i think of mineral wool, roxul "safe'n'nsound" comes to mind. i have read elsewhere on this site there are denser varieties of the same product) what product should i ask my supplier for? how do you install rain screen and/or cladding on top of it?


    Here are a couple of points:

    Product: Roxul itself has many products. For exterior insulation any higher density board works when it's above 6lb density. That is the range where the roxul can be applied without compressing and causing a siding problem. You can look at Rockboard 80, cavity rock, curtain rock. All of these will work at the higher density. For this product range, you want to work with a commercial supplier, not residential.

    We solved the application problem buy using the right fasteners. In the thickness of 4 to 6 inches we stock a special screw from Heco that works great. While we don't sell Roxul, we do resell the screws to builders to apply it. It's gotten rid of a host of application problems. You can see a rough video of how it works at:

    For the rainscreen detail: It's the same detail that you would use for 4 to 6" of foam. Place a WRB over the wood sheathing. Use window and door bucks. Use the WRB to flash outward at the tops of the bucks. Place a "core-A-vent" type product at the bottom of each cavity.

    Your air and vapor control is done by the sheathing just as Lucas is doing, yet slightly different since the service cavity is your stud bays rather than an added chase. Having that air and vapor "control layer" as I call it, well back from the finish layers is really great from the standpoint of having it not degrade or punctured over the life of the envelope.

    For a closer look at the Heco screws we've got them in our online store at: . If you decide to go this route, give us a call, we'd be happy to help. We focus on products and methods for Passivhaus builders so we generally live on the exterme edge on insulation and airsealing. Out of that comes some pretty good tricks for just plain old "really good" envelopes.

  13. Expert Member
    Malcolm Taylor | | #13

    Martin, I can't speak to how the code in enforced throughout the province but on Vancouver island and the Lower Mainland they insist on meticulously sealed poly. All seams, the join between poly and lower plate and plate and subfloor all must have generous amounts of acoustic sealant. Gasketted electrical boxes are also required. The publication I referred to by the ominously named "Homeowners Protection Branch" is a fantastic resource with illustrations of both poly and airtight drywall approaches to airtightness and rain screen sealing of the building envelope. Well worth a look if you get the chance.

  14. Erik Olofsson | | #14

    @martin, i can only speak of my interaction with the building inspectors in the city of vancouver and in another community 800 kms away in the rocky mountains: they must view poly as both an air and vapour barrier. while they insist on insulated pot light cans and gasketed electrical boxes on exterior walls and ceilings i have never heard of any of them viewing the sheathing as an air barrier. furthermore, i have never seen any projects anywhere around here with taped seams in the sheathing. in fact, i've seen many framers leave up to 2" gaps between the long edges of the sheathing panels. the mantra (in vancouver, at least) seems to be "you don't want the building too tight."
    that said, i am going to review the bc building code again and see what is said of air and vapour barriers....

  15. Erik Olofsson | | #15

    @martin, p.s., from the BCBC appendix A- : AIR BARRIER SYSTEMS

    "in residential construction, the airtight element in the air barrier system often provides the required resistance to vapour diffusion and thereby also serves as the vapour barrier. in this case, the combined air/vapour barrier must be positioned sufficiently close to the warm side of the assembly to remain above the dew point temperature of the indoor air.
    "any moisture from the indoor air which diffuses through the inner layers of the assembly or is carried by air leakage through those layers is likely to be trapped at such an air barrier. this will not cause a problem if the air/vapour barrier is located where the temperature is above the dew point of the indoor air; the trapped water vapour will remain as vapour and no harm will be done. but if the air/vapour barrier is located where the temperature is below the dew point of the indoor air, the trapped water vapour will condense or freeze. if the temperatire remains below the dew point for any length of time, significant moisture could accumulate. moisture that remains in a building assembly into warmer weather can allowthe growth of decay organisms"

  16. GBA Editor
    Martin Holladay | | #16

    It sounds as if the inspectors on Vancouver Island are aware of the need for an air barrier. That's good news.

  17. GBA Editor
    Martin Holladay | | #17

    Most of the building science information included in the passage you quoted from the BCBC Appendix is correct. However, I would disagree with the implication that exterior air barriers are problematic. The passage seems to imply that wall assemblies that keep the stud bays warm by installing exterior rigid foam are somehow problematic -- and they aren't.

    As is often the case, this is an example of government bureaucrats enforcing regulations that are based on an incomplete understanding of the relevant scientific principles.

  18. Lucas Durand - 7A | | #18

    in this case, the combined air/vapour barrier must be positioned sufficiently close to the warm side of the assembly to remain above the dew point temperature of the indoor air.

    It is very much possible to locate an "airtight" sheathing layer within a wall assembly so that it stays warm enough in winter to avoid condensation issues.

    For example, on my house I located the sheathing on the interior face of the innermost stud wall.
    As Albert mentioned this requires building out a service cavity - which is not to everyone's taste.

    The exterior rockwool detail Albert described earlier will also keep the sheathing plenty warm and doesn't require an additional service cavity - but apparently requires some special screws.

    The Larsen truss ARCTIC detail used in SunRise Home is another good example of how to insulate the exterior side of an "airtight" sheathing layer.
    Larsen trusses may involve some extra labour, but my question is "compared to what"?
    Even hanging exterior rigid foam is labour intensive in its own way - and unpleasant too (I'd rather be cutting wood than plastic any day).

    Maybe another way you could approach the problem of resistant code officials could be to find a good architect that understands the building science involved...

  19. Albert Rooks | | #19


    The nice thing about your questions is that it is exposing that you can move the envelope quality in your area forward: The code seems to allow enough room for well planned modification. Lucas points out a few more methods towards improved assemblies. They are all great when executed correctly.

    There is no short cut to a "quality envelope". Plan on it being more cost and work. I think it's the only sensible thing to do.

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