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Double vapor barrier?

user-7362852 | Posted in General Questions on

My wall stack is:

Vinyl siding
1/2″ strapping
Tyvek WRB/housewrap
1/2″ OSB
2″ XPS foam board *untaped*
1/2″ OSB
5″ Roxul in stud walls

The contractor forgot to tape the seams on the XPS foam board. Should I hang an interior poly vapor barrier? Or should I just use the drywall as air barrier with appropriate sealant?

Thanks! (By the way I’m in Toronto, Canada, which is definitely an Air Conditioning zone these days 🙂 )

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  1. Expert Member
    Dana Dorsett | | #1

    Counting on air leakage around the XPS AND exerior layer of OSB to improve drying toward the exterior is a dubious strategy at best.

    However, per the IRC's prescriptives 2" of XPS is adequate for dew point control on 2x6 ( 5.5" ) fiber insulated walls in Toronto's zone 5/6 climate. Standard interior latex paint on air tight gyprock is enough:

    If local inspectors are the types who want to parse paragraphs in the NBC with you all day & night you may be able to head it off by installing 2 mil nylon (Certainteed MemBrain) as the interior side vapor retarder, which meets the NBC's definition for "vapour barrier" when dry, but becomes more vapor open if it ever needs to, allowing the assmbly to dry toward the interior.

  2. user-7362852 | | #2

    Thanks. The inspectors are going to require poly VB on the interior regardless, but this is only a small addition so not much cost or effort to do so. I'm only wondering if I should slash/remove it after its been passed :) or if just leaving it won't cause any harm.


    1. Expert Member
      Dana Dorsett | | #3

      Slashing the polyethylene doesn't affect it's vapor retardency, only it's air tightness. A measurable fraction of the total area of the polyethylene needs to be removed to make it reasonably vapor open. A gazillion pin-pricks on 1/4" grid could raise it's permeance to something close to that of latex paint on wallboard, which is still a vapor retarder.

      If the assembly is air tight, the wood all measures <18% moisture content with a 2-pronged moisture meter, and the flashing details are PERFECT, the risk of leaving up the polyethylene is low. But when the exterior OSB is dry the 2" of XPS + exterior OSB combined has a vapor retardency of less than 0.3 perms, which is very low drying capacity indeed, but at least some- it's ~5x the permeance of 6 mil polyethylene.

      Putting it in other terms, the most-open NBC-legal vapour barrier would have about 3x the drying capacity of that exterior side XPS + OSB stackup. Even though it has some drying capacity, it's still pretty damned tight!

      MemBrain doesn't look too different from clear 4-mil polyethylene:

  3. Jon_R | | #4

    Attempt to avoid poly where AC is used (see below). I would use a Class II on the interior (which meets the < 1.05 US perm Canadian code requirement). Not Class III (because you don't conservatively exceed the R-value ≥ 11.25 over 2×6 wall in Zone 6 that US code would require).

  4. user-7362852 | | #5

    Sounds like the best thing is to remove the VB after inspection - which is what I will do :)


  5. Expert Member
    Akos | | #6

    The OBC does allow for the vapour barrier to be located in the middle of the wall (meaning the exterior foam) provided you meet the ratio in Table

    With 2" XPS and R24 batts you are well above the 0.2 ratio it requires.

    Either way, the 2" of XPS still has a perm rating, so you will get some drying through it, the presence of the interior vapour barrier doesn't really matter much.

    If it makes the inspector happy, I would put it in and loose no sleep over it. I would not spend the money on a smart VB either.

    P.S. My own home in Toronto with R23+2" polyiso walls has this, recently opened up a wall and after 6 years it looks as good as the day it was built.

  6. user-7362852 | | #7

    Thanks for the feedback. I did have a followup question. The walls are surrounded by CI foam, but the attic and roof space is traditional cold attic construction. There is a vapor barrier on the ceiling, but without the interior wall VB there is still technically a path for vapor from warm space to cold space through the wood of the top of the stud walls. Is this a concern?

    1. Expert Member
      Dana Dorsett | | #8

      It's not a concern.

      Being reasonably AIR tight matters, but there was never a real need for a vapor barrier in the ceiling of a vented attic in a zone 6 climate, back when Toronto was still zone 6. The more recent 40 years weather data puts Toronto solidly into zone 5 (which is even more forgiving), well UNDER the 4000HDD (base 18C) warm edge of zone 6. By 2050 Toronto it projected to be on the warm edge of zone 5:

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