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Poly and Canadian building codes

Malcolm Taylor | Posted in Building Code Questions on

This comes up quite frequently and I thought it might be useful to lay out what is actually required by the code. My description relies on the BC Building Code, but I am fairly confident it applies to all provinces that have based their codes on the NBC.

The code requires all building assemblies to include an air barrier and a vapour barrier.

The air barrier can consist of a variety of sheet or panels materials. It can be located anywhere in the assembly. That is the inner, intermediate or outside surfaces of the assembly (9.25.5.1).

The vapour barrier can be any material less than 1 perm. It has to be located “sufficiently close to the warm side of the assembly to prevent condensation at design conditions (9.25.5.2). The requirements further explain it must be “at a location where the ratio between the total thermal resistance of all materials outboard of its innermost impermeable surface and the total thermal resistance of all materials inboard of that surface is not less than that required by table 9.25.5.2.” This table has columns for degree days, minimum RSI ratio, minimum outboard thermal resistance, and sheathing thermal resistance.

The code anticipates the use of exterior foam as a vapour barrier and requires that the inner-surface of the foam meet the requirements of maintaining a condensation free temperature or the assembly is required to include another vapour barrier closer to the interior. ((9.25.5.1 Appendix A (thermal Insulation).

So in summary: Poly is only one of a variety of materials described in the code accepted as both air and vapour barriers. Other sheet goods, panel materials such as plywood and OSB, paints and coating, and foam are all explicitly mentioned. Exterior foam may be used without an interior vapour barrier as long as it is thick enough.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Malcolm,
    Thanks for posting this information clearly and completely. In the future, I will refer Canadian readers who are confused by the code to your useful summary here.

  2. D Dorsett | | #2

    Using exterior foam as a vapour barrier serves no useful function in most Canadian locations & wall stack ups, but rather only impedes moisture from leaving the susceptible sheathing material toward the exterior. The interior temperatures and the sheathing temperatures rarely dwell below the outdoor air's dew point long enough to matter, which is why in most wall stack ups it's safe to install interior polyethylene. (The exception is in some masonry or stucco stack ups, where sun driven moisture coming out of the cladding brings the dew point of the entrained air inside the wall cavity above the temperature of the interior, causing liquid moisture to condense on the polyethylene.)

    When dry both half inch CDX and OSB both perform as vapour barriers at the Canadian code definition thereof, but become more vapour open than the definition when wet- they are "smart" vapour retarders. If the code really intended for there to be a vapour barrier on the exterior, it wouldn't matter what the vapor retardency of the exterior insulation was, only the RSI value.

    Table 9.25.5.2 in the NBC is not the equivalent to the IRC's TABLE R702.7.1, since it presumes a Class-II or tighter vapour retarder, whereas the TABLE R702.7.1 presumes Class-III vapour retardency. Under the NBC the outboard insulation material is ALLOWED to be low permeance, as long as it meets the minimum ratio in the table, but it is not REQUIRED to be a vapour barrier, and in fact the assembly will be more moisture resilient if the outboard insulation is high permeance (such as rigid rock wool) allowing moisture to easily leave the sheathing toward the exterior.

    A quick internet search comes up with that table, found on page 39 (PDF pagination) here:

    http://www.nrc-cnrc.gc.ca/obj/doc/solutions-solutions/advisory-consultatifs/codes_centre-centre_codes/revisions_errata-revisions_errata/2010_nbc_revisions_errata_november2013_2nd_printing.pdf

  3. Malcolm Taylor | | #3

    Dana,
    Thanks for the useful commentary. Whether or the code requirements make sense from a building science point of view is much better left to you.

    Two points that need clarifying:
    - The table you have linked to from my post (9.25.5.2) was not the one I should have cited. It is found in 9.25.5.2 Appendix A, Sorry I can't find it online.
    - Here is what the code says in relation to foam (9.25.5.1 Appendix A):
    " Thermal Insulation
    Where low-permeance foamed plastic is the sole thermal insulation in the assembly, the temperature of the inner surface of this element will be close to the interior temperature. If the foamed plastic insulation has a permeance below (1 perm)... it can fulfill the function of a vapour barrier to control condensation within the assembly. However, where low permeance is installed on the outside of an insulated frame wall, the temperature of the inner surface of the insulating sheathing may fall below the dew point. in this case the function of vapour barrier has to be provided by a separate building element installed on the warm side of the assembly."

  4. User avatar
    Rob Myers | | #4

    Malcolm,
    OK, I'm a little confused now.
    How would the statement " rigid foam can only be used without an additional vapour barrier "where it is the sole thermal insulation in a building assembly" apply to a wall with interior batt insulation and exterior foam (where the sheathing is the air barrier and the wall is designed so that the sheathing temperature remains above the dew point)? Does this wall assembly then technically not meet code?

  5. Charlie Sullivan | | #5

    Dana, good point that with insulation outboard of the sheathing, from a building science point of view, it is OK for it to be a vapor barrier, but it is not necessary for it to be one, assuming the R-value ratio is adequate. In a typical design with mineral wool exterior insulation, requiring the vapor barrier is silly. I can think of one scenario in which the lack of that vapor barrier might cause a problem, but it's a pretty unlikely scenario: If you used exterior mineral wool board, and then used T1-11 siding over that, with no rainscreen gap and with the top, bottom and seams well sealed. Maybe the code is designed to avoid that problem? More likely they just didn't think it through as carefully as you did.

  6. Malcolm Taylor | | #6

    Rob,
    Apologies, the confusion was all mine. I stumbled downstairs in the middle of the night for a snack, looked at the code appendix, thought I had mis-read the intent of the section and posted that caveat. Once I fully awoke i edited my post.

  7. Malcolm Taylor | | #7

    Charlie and Dana,
    It does seem like the situation is only halfway towards reconciling the old, expected interior poly vapour barrier to a solution that that deals with impermeable exterior insulation. Coincidentally, I just received a package of six hundred pages of recent code revisions. I'll wade through them in the next couple of days and see if they have addressed the problem further.

  8. Alec Shalinsky | | #8

    Very opportune comment. Until today, I had never seen table 9.25.5.2. (and I've only read the Ontario code a hundred times!) I am currently being taken to task by my building officials, because I am against putting a second vapour barrier in the wall. My configuration is 8" brick, 1" air gap, 3" polyiso (foil and sealed to the interior), 2x4 stud wall with batt insulation and 1/2 drywall with primer and latex paint (no poly). It seems that the official policy in my town is "when in doubt, put poly in", and well, they are always in doubt it seems. Aside from table 9.25.5.2, we did a lot of dewpoint simulations, and found our assembly to be fine. We presented this to the staff, and received our permit. Now that we are executing the plan, they have doubts. But the table seems clear. If I read (and interpret) it correctly, they are discussing the ratio of insulation outboard (to the exterior) of the vapour barrier (impermeable surface) to inside. If this interpretation is correct, and as an example, a wall with R-6 to the outside and R-19 to the inside would still pass muster (according to the code). So R-20 outside and R-20 inside would also pass (= 0.5). As I have R-19.5 outside (3 inches polyiso), it appears that a truly absurd amount of interior insulation (up to R-64) would still pass. Am I missing something? And I guess, why are they busting my chops??

  9. Ryan Magladry Ottawa, Ontario | | #9

    Alec, I am having the same discussion with a code official here. He insists on the 2/3 exterior to 1/3 interior. I referenced that same table, and haven't gotten a comment back. I have followed up.
    My preferred stack up includes the following, from outstide-in

    Cladding (stone skirt, fiber cement)
    1/2" rainscreen
    1/2" exterior gypsum (26perms)
    2x6 stud wall with Roxul R22
    1/2" plywood (vapour control layer, 1 perm)
    3.5" dense pack cellulose R12
    2x4 interior stud wall with dense pack cellulose R12
    1/2" interior gypsum (26perms)
    Primer + paint

    Exterior ratio is 0.47, which exceeds the 0.20 required by table 9.25.5.2. I feel like this is a very robust, resilient, and easy to build wall, but can't seem to get anywhere with it.

  10. Malcolm Taylor | | #10

    Reply to Alec and Ryan,
    I'd urge you to read the appendix A-9.25.5.2, especially the table A 9.25.5.2 which spells out in much greater detail how to calculate minimum thicknesses of low-permeance materials and also gives examples with ratios. It really clears things up.

    The 1/3rd -2/3rd ratio seems to be folk wisdom and certainly isn't in the code.

  11. Alec Shalinsky | | #11

    Thanks for all the responses. At the end of the day, the city accepted the proposed profile we developed with the engineer. We are good to go.
    Now all I have to convince them of, is to let me use Comfotube (semi-rigid ventilation duct). Sigh!

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