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What should I use instead of polyethylene?

fallwanderer | Posted in Energy Efficiency and Durability on

I am about to start a house addition and the wall section I have settled on is wood siding, vertical furring, roxul comfortboard 1.25 inch, delta vent SA air barrier, sono ecoclimat 4 sheathing (r4, 26 perms),2×6 stud with blown in cellulose (to protect the framing from moisture by acting as a moisture sink) The house is in Montreal so there are cold winter, but also hot and humid summers which complicates thing. I have air conditioning in the house. I intend to soak the framing with bora-care and mold-care product as an extra protection. My question is what should I use instead of the polyethylene? I have heard about some product called Membrain but I am not sure I understand it enough to risk using it.

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  1. GBA Editor
    Martin Holladay | | #1

    The quick answer: MemBrain is a good product for this application. It is a vapor retarder with a variable permeance -- a so-called "smart" retarder. It makes more sense than interior polyethylene, especially if you plan to install air-conditioning for use during hot summer weather.

    Many building inspectors in Canada insist on interior polyethylene, even when a wall assembly would be better off without any interior vapor barrier. If you find yourself in that situation, installing MemBrain usually keeps the inspector happy (and saves your wall from possible problems).

    Here is a link to an article with more information on "smart" vapor retarders: Smart Vapor Retarders for Walls and Roofs.

    If anyone reading this thread is wondering about brand names, Delta Vent SA is a vapor-permeable housewrap. I'm not sure what Sono Ecoclimat is -- it rings a bell, but I didn't find any good results when I tried to Google it.

  2. fallwanderer | | #2

    Thank you Martin,
    I had the name slightly wrong, it's Sonoclimat ECO4. Here is the link below but I hope I am not breaking any rule on product promotion, I have no link with this company, I only like their product because of it's vapour permeability.

  3. GBA Editor
    Martin Holladay | | #3

    Thanks for the correction. I knew that the name rang a bell -- it's the wood-fiber insulation panel / sheathing product that many people have been talking about. Thanks for straightening things out.

    Bonne chance.

  4. Expert Member
    Dana Dorsett | | #4

    The high vapor permeance of the fiberboard sheathing and the rainscreened siding means it probably doesn't really need the MemBrain but it won't hurt.

    It's also unlikely that you'd need it during the summer in a Montreal climate. Summertime dew points north of 23C/73F are both rare & brief in that location, and unless your interior walls are cooled to below the weekly average outdoor dew point temperature not much moisture can accumulate in the finish wall (or condense, if using polyethylene). See the "Humidity Comfort Levels" graph of dew points a bit more than half way down this page:

    Study the graph carefully- there is a lot of information there. Even in the worst weeks of the cusp of July/August the outdoor dew points exceeds 70F/21C (a very comfortable air conditioning temperature) less than 3% of the time, and would be above 23C probably less than 1% of the time during any given heat-wave. The resolution of the graph isn't high enough to see if 75F/24C or higher occurs even 1% of the time, but since the "miserable" label (indicating 75F & greater dew point) doesn't appear on the graph, it's probably << 1%.

    So MemBrain's main function would be to appease a less-persuadable inspector.

    If this were a brick clad wall rather than wood siding there might be another rationale, since brick can store a lot of moisture that can be released in intense bursts when heated by the sun.

  5. fallwanderer | | #5

    Thank you Dana. It was very informative post. I am just wondering now, if I was to replace the poly with a 1 inch foil face foam on the inside, would the dew point when operating the ac be in the foam and hence this would be an even better solution from a technical standpoint although not as economical as the membrain? I’m probably getting carried away...

  6. GBA Editor
    Martin Holladay | | #6

    Interior rigid foam will work, if that's what you want to do. Here is a link to an article on the topic: Walls With Interior Rigid Foam.

  7. Expert Member
    Dana Dorsett | | #7

    Dew point is a temperature, not a location. In Montreal the dew point (temperature) of the outdoors is below temperature of even an air conditioned indoors 99% of time, unless one is air conditioning the interior to well under 23C. That means that condensation and moisture accumulation of outdoor air moisture will not occur in the walls, since there is no location within the wall that dwells below the outdoor dew point for any amount of time.

    Running the AC lowers the temperature of the interior, but it lowers the dew point temperature of the interior even more, to well below the dew point of the outdoor air on hot humid days. If the interior surfaces of the wall are allowed to be somewhat vapor permeable (to water vapor, not air), the dew point (temperature) the lower dew point of the interior air willl reduced the moisture contend of the entrained air in the cellulose, since water vapor will be drawn through the wall toward the interior. Foil faced foam will prevent that drying toward the interior, but that is of no consequence, since the temperature at any layer in the wall is still going to average well above the outdoor dew point temperature.

  8. fallwanderer | | #8

    Thank you very much for your help.

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