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Double stud walls – vapour retarders

Mark Pennell | Posted in Energy Efficiency and Durability on

Good day folks. I am in a cold climate (Halifax, NS, Canada) climate zone 5 or 6 I think???

I am constructing my house in the spring and will be using some variation of the DBL stud wall; either the Riversong DBL bearing wall, or constructing the house conventionally then stand up walls from the inside. I will be going for 10-12″ thick. My main question is about air leakage control, and vapour control. On the inside I am going to likely do ADA, and on the outside is it overkill to use adhesive on every stud and plate before I attach my 1/2″ PLY and then tape all the seams?

Has anyone here used both methods I mentioned above to frame? I really love the Riversong DBL bearing wall as it is a system that seems very stream lined, with how to insulate it etc… but it takes you that one step past unconventional framing which could cost me alot of time in the end.

The ease of the other method is nice, but it throws a wrench in the rim area for air sealing and insulating, as well as thermal bridging for the whole floor assembly and you have to net the well to blow the cellulose now.

I have framed and air sealed two passivehouse cert. homes in the past, so none of this is green to me. I will be doing most of the work myself from the foundation to the trim, and subbing out plumbing, elec, and mudding. Just looking for some pointers to prevent any head scratching!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    Q. "On the outside is it overkill to use adhesive on every stud and plate before I attach my 1/2" plywood and then tape all the seams?"

    A. I would never accuse a builder aiming for airtightness that their methods were overkill. It's true that most builders would choose either adhesive or tape to seal the plywood -- not both -- but adhesive is cheap, so why not?

  2. Marc Labrie | | #2

    Hi Mark, here's a way to do double wall from the inside if it may be of interest. http://blog.postgreenhomes.com/2012/04/04/new-double-stud-wall-assembly-with-rock-wool-smart-vapor-retarder/
    Be sure to read Martin latest article on tapes in this month Fine Homebuilding. He list the best choices for plywood and other surfaces. I'm located in Moncton. Did you worked on Natalie's homes by any chance? Good luck with you project, I know how you feel since we're building too this year.

  3. Jenny Belman | | #3

    Double stud walls are highly insulated wall system that will work in extreme climates, but still has significant risks to moisture related durability issues and premature enclosure failure. An air barrier and a separate vapor control strategy should be employed to ensure air leakage and vapor control.

  4. Expert Member
    Dana Dorsett | | #4

    Halifax winters are closer to the cool edge of US zone 4 or warm edge of zone 5 than a US zone 6 climate, in no doubt due to the moderating effects of being downwind of bodies of water no matter which direction the wind blows (unlike most of Maine or New Brunswick). The mean January temps are very US-zone-5-like (about -3C/27F), but the December & February mean temps are more similar to the cool edges of zone 4. (+2C/36F and -1C/30F respectively.) See:

    http://weatherspark.com/#!dashboard;a=Canada/NS/Halifax

    From a dew-point management prescriptions point of view treating it like zone 4 would likely work, but treating it like zone 5 gives you some margin.

    In US zone 5, as long as the sheathing layer in contact with the insulation is ventilated to the outdoors, you won't need an interior side vapor retarder stronger than standard latex house paint (Class-III vapor retarder). As long as the siding that goes over the exterior plywood sheathing is back-ventilated, air tightness on the interior side (and the buffering capacity of the cellulose) would be more than sufficient.

  5. Mark Pennell | | #5

    Marc- yes I was on a couple of Natalie's projects in 2011. I have seen that video on YouTube some time ago. Looks like a good system. How does the roxul and membrain affect the cost?

    Dana-thanks for the help this is exactly what I was hoping to hear. All siding has to be vented here by code unless it is vinyl siding. I figured that the 12" dbl stud has a huge moisture buffering capacity with all that cellulose.

  6. Marc Labrie | | #6

    Mark, material only retail price in our local market is around $0.065 per sq. ft. per R-1 for Roxul versus $0.027 for Cellulose. I haven't priced Membrain since I'll be using OSB or plywood as a VB but I understand it's pricey. If you are to use ADA you may want to consider vapor barrier paint. https://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/20342/answers-moisture-vapor-retarder-primer-paints

  7. Jerry Liebler | | #7

    Marc,
    Is that cellulose price dense packed or loose? If it's loose it'll easily double for dense pack done right. Around here the big box stores sell Roxul comfort batt at under $0.045/ r sq ft. giving almost the exact opposite result.

  8. Marc Labrie | | #8

    Jerry,
    it's for dense pack, material only. Mark referred to double stud walls, I automatically assumed 3.5 pcf cellulose.

  9. Jerry Liebler | | #9

    Marc,
    Maybe your market is very different. Here the Lowes sells cellulose at $10.15 for 19 # bag, about $0.041/r sq ft. real close to roxul comfort bat at $0.045

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