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Interior air barrier in walls

Quinn Sievewright | Posted in General Questions on

Hello again

Thanks to help from those here last year I have built a holiday home cabin/garage close to Vancouver, Canada, (Zone 4C I recall). The exterior is complete and my project this summer is to complete the interior.

Firstly, this ‘outbuilding’ is not considered by the inspector to be a habitable building, so many ‘code’ rules don’t apply. I believe this means we do not need to install an interior vapour barrier as per the norm in the area. Moreover, my reading on here suggests I should not need an interior vapour barrier anyway. I’m I correct in this assumption?

So that moves me to the need for an interior air barrier, specifically on the walls.

We’ve already purchased Fiberglass Batts for the wall cavities, although not ideal, due to a mix up with the main contractor its what we have.

I know Martin has said double air barriers aren’t really needed, but: “To a large extent, double air barriers are an attempt to respond to the inherent performance problems of fiberglass batts.” So don’t use fiberglass batts.”

I DIY air tested with a large extractor fan installed in a window and the building seems pretty tight having used can foam to seal a few leaks I found with a smoke tester.

I know the local drywall guys won’t be able to do an “airtight” drywall approach. But does it matter? Given my exterior air barrier appears tight, do I just drywall like normal and be done with it?

Or do I need to think about installing something like CertainTeed Membrain under the drywall where I can be careful about interior air sealing to base and top plates and openings? It seams fairly expensive and frankly would prefer not to unless necessary.

Also, this is a cabin, it won’t ever be occupied full time, but I do want to build it right.

Thanks!

Context:
Two storey 800 sq ft garage ground floor. Living area, 2 bedrooms and bath above. 1:12 shed roof.

Wall build up: 2×6 studs, Zip system sheathing taped, 2″ rockwool comfortboard, strapping, Cedar/Corrugated metal siding.

Ceiling building up: T&G truck decking over 4X10 douglas fir beams, modified bitumen vapor barrier (I think it was SOPRAVAP’R), 2 x 4″ layers polyiso, 2 x 4 battons/strapping, 1/2″ ply, EPDM membrane. Unvented hot roof essentially.

Floor build up: OSB subfloor, fiberglass batts 12″, 1″ Silverboard foil-faced EPS (airsealed as much as possible). This is the ‘ceiling’ of the garage underneath.

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Replies

  1. Calum Wilde | | #1

    If your exterior sheathing is relatively air tight than the wall should be able to dry to the inside, especially in new construction. In this case I don't think you'd want an an air tight drywall approach or a vapor barrier of any kind.

  2. Expert Member
    Malcolm Taylor | | #2

    Calum,

    I think you are confusing air and vapour tightness. The air tightness of the sheathing or drywall isn't what determines the ability to dry, it is the permeability of the materials.

    Quinn,

    Unless you intend to cool the building in the summer, I don't think it makes any difference what you use on the interior. Air-tight drywall, a smart membrane and poly VB will all work fine.

    One caution about code concerns. Your inspector may have decided this isn't a habitable structure (even though it seems to have all the components that would make it one), but there is a legal onus on owners to meet code whether it is enforced or not. To me that means the safest path is to conform to the code whenever possible.

  3. Calum Wilde | | #3

    Malcolm,

    Is sheathing, either OSB or plywood, vapour permeable? Quinn spoke of installing a vapour barrier on the inside, when the outside is air tight. I'm assuming that the air tight barrier was made of non-vapour permeable sheathing.

  4. Expert Member
    Malcolm Taylor | | #4

    Calum,

    Both OSB and plywood are permeable. In fact they are variable-perm vapour-retarders, much like a smart membrane.

    Houses in Canada still usually include an interior vapour-barrier (6 mil poly) and are designed to dry to the outside through the sheathing. Houses designed to dry to the inside do so through the drywall. Whether that drywall is detailed as being airtight makes no difference to the rate of drying. It may however make a difference to the rate of wetting. Making every layer inside the WRB you can airtight is a good idea.

  5. Calum Wilde | | #5

    Thanks Malcolm.

  6. GBA Editor
    Martin Holladay | | #6

    Quinn,
    Malcolm gave you good advice. In your climate zone, you have a wide variety of options. I usually advise against the use of interior polyethylene, however. MemBrain or a similar variable-permeance product would be better. (You never know whether the building will be air conditioned in 30 years.)

    In all cases, attention to airtightness is a good idea. There really is no downside to paying attention to airtightness.

  7. Quinn Sievewright | | #7

    Thanks everyone!

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