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Community and Q&A

Vapour barrier for above-grade walls and ceiling in Canada?

Regin8r | Posted in Building Code Questions on

So I’ve read a bunch of Q & A on the vapour barrier and how it’s not necessary for certain climates, but if we are stepping out our insulation above the dew points on our colder climate (Pickering, ON), would it hurt to have the vapour barrier?

We have omitted the poly with the 2 layers of rigid foam for the below grade insulation, but it’s still there in the above grade walls and ceiling. Our designer has included the poly vapour barrier in the design drawings for the permit application, but when we actually build and if we omit it, I just don’t want to have the building inspector come down on us because it was in the plans.

My questions is can we just leave it with the poly vapour barrier in the above grade walls and the ceiling?

I have attached the cross section of the wall in the plans.

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  1. GBA Editor
    Martin Holladay | | #1

    If you have exterior rigid foam, you don't want interior polyethylene. This type of wall is designed to dry to the interior. The concepts behind this approach are explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In Canada (as in the U.S.), there are two types of building inspectors: those who understand the building science behind walls with exterior rigid foam, and those who don't. I hope you have one of the good ones.

    If you have a stubborn old building inspector who doesn't understand the building science, it's sometimes quicker (and ultimately cheaper) to install a layer of interior MemBrain (a so-called "smart" vapor retarder with variable vapor permeance) than it is to educate the inspector. The MemBrain is unnecessary but harmless, and it looks a little bit like polyethylene, so it usually keeps the inspector happy.

  2. Expert Member
    Dana Dorsett | | #2

    What HE said. Interior polyethylene would only raise the risk of moisture problems by creating a moisture trap between the foam and polyethylene.

    At ~3750 HDD-C (about 6750 HDD-F) Pickering is a cool edge of zone 5A or warm edge of zone 6A type of climate in terms of annual HDD and moisture. (The 5/6 boundary is at 4000 HDD-C, which would be a colder than average year in your location.) With 2" of polyiso on the exterior you have more than adequate dew point control for a 2x6 dense-pack wall, and with cellulose in the cavities the sheathing safely shares what minimal wintertime moisture burden there is with the cellulose. The is a GREAT stackup, very moisture resilient as-is. MemBrain would probably be the cheapest way to solve a code-inspector problem without increasing moisture risk.

  3. Regin8r | | #3

    Ok thanks.

    Our wall is constructed from outside to inside as follows:
    Brick and Stone Masonry on 1" Air Space on 2.5" Rigid Insulation R12.5, Tape and Seal All Joints, Tyvek on 1/2" Plywood Sheathing on 2x6 SPF No2 Studs @16" Filled w/ Dense Pack Cellulose Insulation R24 on Cont. 6 mil Poly VB on 1/2" Drywall.

    So I'm good to skip on the 6 mil Poly VB, or replace it with MemBrain if the inspector insists.

    For the ceiling, we have no rigid foam, so we just have 1/2" ceiling drwall, 6 mil Poly VB and R60 insulation. Should we leave out the VB in the celing also?

  4. GBA Editor
    Martin Holladay | | #4

    In your climate zone, the ceiling polyethylene isn't necessary. It also isn't doing any harm. You can keep it or eliminate it -- as your prefer.

    Old-time builders don't like poly on the ceilings because a roof leak can create a large puddle in the poly before anyone notices. I don't think this is a big problem, but I've heard people talk about it.

    The most important point to remember about ceilings is that you need to pay attention to airtightness. You don't have to worry much about vapor permeance, though.

  5. Expert Member

    From a practical standpoint Martin and Dana have as usual given you excellent advice. From a building code standpoint what the inspector wants is a vapour barrier of less than one perm "sufficiently close to the interior..." That can be a number of things including MemBrain. Where you will almost invariably lose the battle is simply omitting the VB, even if it is for good building science reasons. Having a code compliant alternative to the poly is a good idea.
    I would still use poly on the ceiling. The amount of drying to the interior in a vented trusses roof is dwarfed by the drying to the outside.
    A small point about the section you posted which probably means nothing: The drawing shows raised heel trusses, but no provision for containing the insulation at the perimeter of the exterior walls. It may be on some other detail, or on the specs, but something needs to be called out somewhere.

  6. Expert Member
    Dana Dorsett | | #6

    Interior side poly on the ceiling is potentially a problem if you air condition the place down to 20C during the highest humidity summer days, but wouldn't be the disaster that it can create in hot humid climates.

    If the wall-foam is XPS, you might consider changing that to polyisocyanurate on environmental grounds. Wintertime performance would be about the same, but shoulder season performance would be a bit higher with polyiso. The issue is that all XPS in North America is blown with HFC134a, which has a 100-year global warming potential nearly 1400x CO2, and as the gas seeps out over a handful of decades it's performance will slowly decline to about R10.5 before it stabilizes. (That is still sufficient for dew point control for your climate & stack-up.) Polyiso is blown with pentane (about 7x CO2), and while it's mid-winter performance is well below it's ~R15 labeled R-value, 2.5" would still provide huge dew point margin at the sheathing even with the derating factored in.

  7. Regin8r | | #7

    Great thanks for the info Dana and Martin!

    The city confirmed that while the science behind is effecting changes in the code (proposed changes), what is enforced today is having the vapour barrier over the insulation on the warm side (so looks like MemBrain is in order).

    Malcolm, good point. It is not shown on the cross section, but I am having some kind of baffle put in to maintain the intake vents and contain the insulation. We are overseeing the project ourselves so this website is a great resource and I've learned that the folks here are great at helping and it is good of you to ensure we are aware.

  8. GBA Editor
    Martin Holladay | | #8

    You might ask your local building department whether they would accept a vapor-retarding primer (paint) instead of MemBrain. If they agree that the primer satisfies the code, you would save some money compared to installing MemBrain.

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