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Flogging the half dead vapor barrier horse…

L.M. | Posted in General Questions on

At the risk of being heaped with much deserved abuse I have a yet more vapor barrier questions.

I am in the process of designing my new home and have settled on a double stud wall construction with a load bearing inner shear wall, a sheathed 2×3 framed outer wall and dense cellulose insulation.

The home will be built in south-eastern British Columbia Canada in an area with about 7000(F) HDD. I am certain that I will be required to install a vapor barrier. Due to rather limited approved choices it will be either the much vilified 6 mil poly or Certainteed Membrain. To avoid penetrating the barrier I would like to install it either inboard or outboard of the inner wall structural sheathing.

I recently happened across a publication from the 1980’s
( ) which contains a few cross sections of walls built in a similar manner to that which I am considering. I realize that these walls were designed for a colder, drier climate than mine but given appropriate exterior rain screen/unimpeded outer sheathing drying ability and well controlled indoor humidity I don’t anticipate any problems.

Is anyone aware of how these 1980’s wall assemblies have faired over the past 35 years?
Am I unwittingly staring down the barrel of a plastic wrapped gun?

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  1. GBA Editor
    Martin Holladay | | #1

    Harold Orr and Rob Dumont's work on superinsulated houses in the late 1970s and early 1980s was groundbreaking and significant. However, I would not use their 1982 recommendations for wall assemblies in 2015, knowing what we know now.

    Polyethylene sheeting was used in the early 1980s as an "air/vapor barrier" because it was widely available and inexpensive. "Smart" vapor retarders had not yet been invented. And building scientists during the 1980s did not fully appreciate the benefits of inward drying during the summer months.

    Moreover, the use of air conditioning during the summer was much more rare in the early 1980s than it is now. Air conditioning raises the chance that an interior polyethylene vapor barrier will cause problems.

    To answer your question, most of the double-stud walls built in the prairie provinces of Canada during the early 1980s are still doing fine, as far as I know. However, in other climates, especially when built by builders who weren't paying close attention to airtightness and moisture management (good flashing details), houses built according to these principles didn't fare as well.

    I advise you to install a smart vapor retarder like MemBrain rather than polyethylene.

  2. Expert Member
    Dana Dorsett | | #2

    If you sheath the exterior wall with fiberboard (about 15 -35 perms) or exterior grade gypsum board (20+ perms) per rather than plywood or OSB (about 1 perm when dry), when combined with rainscreened siding you wouldn't even need MemBrain in your climate. (But it wouldn't hurt, if that's what it takes to get it through inspection. )

    Summertime outdoor dew points aren't very high in your location, and even if you used polyethylene on the interior the risk of condensation from using air conditioning is pretty low, much lower than it would be in Quebec or Ontario. But with a changing climate it's not clear if past experience is going to be the best predictor over he lifecycle of a house- your summertime humidity levels could rise a bit (and probably will.) If you can skip the polyethylene, you should.

  3. Expert Member

    I don't know if they fit into your plans, but the B.C. building code does allow more approved choices of material for use as a vapour barrier than you describe. The drywall or sheathing can be used if coated with a paint that reduces it's permeance to less than 60 ng/(Pa.s.m2) as long as it is sufficiently close to the warm side of the wall assembly.

  4. L.M. | | #4

    Thanks for the response Martin.
    After further consideration I began leaning toward the "smarter" choice but thought I would run it by the experts on GBA anyway!

  5. L.M. | | #5

    Thank you for the responses Dana and Malcolm. More food for thought!
    Dana - I haven't really considered the sheathing possibilities beyond plywood and OSB. I will likely go with plywood but I'm checking into the gypsum sheathing just out of curiosity. I haven't gotten a price for it locally yet, do you know how it compares to plywood in terms of general cost? On the climate side of things my region is moving toward a much hotter drier trend in the summer months. The lower combustibility of gypsum sheathing is appealing in that regard...
    Malcolm - I haven't worked up the nerve to consult the local building deities yet. I was assuming the worst re: their views on vapour barriers. We are looking at a variety of interior wall cladding so the painted interior drywall approach is not feasible, however the possibility of vapour barrier paint on the structural sheathing of the inner wall does bear further investigation.

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