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Community and Q&A

Double Stud Assembly Layering

Matt Thiel | Posted in Green Building Techniques on

Hello,

Our builder recommended a double stud wall instead of 2×6+5.  We are building a single level cabin in northern Alberta, Canada.  As we are looking to use metal siding, and our build package jumped 20% since our initial quote a couple months ago. Our builder suggested this with no change in labour cost and we will adjust dimensions to work for the same interior. The two systems are the same cost for materials with 2 layers of insulation right now.

Double Wall assembly (Inside to outside)
Gypsum – 6mm poly – 2×4 16oc R14Rockwool – GAP – 2×4 16oc with R14Rockwool – 1/2Plywood – WRB – Siding – Tied together with a 2×8 on top

Questions
1. GAP size, I was thinking of leaving 1/2″ between the assemblies to match 2×8 for a total R~25. Does this work and breathe properly? We plan to tape and seal Poly, Sheathing and WRB completely for a low air change value.

I then read “Lstiburek’s Ideal Double-Stud Wall Design” He Implemented the mid wall sheathing and Dense blown cellulose. We will be using batts due to ease and value.  

2. Is the mid-wall sheathing required for vapour issues? If so, could this be replaced with a Vapour permeable WRB?  Sheet Goods are the highest cost per sqft in the wall.
3. Is it worth it to build 1 layer wider insulation and aim for the R40? 3.5in gab to be filled with R14.  
4. Is the rain screen required for condensation? Alberta is fairly dry.

Thank you, I have been pouring over the Q&A for days end throughly enjoying the perspectives.

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Replies

  1. plumb_bob | | #1

    Are you heating with wood heat? If yes, I would say the $ and effort for a super insulated wall for a small structure like a cabin will have minimal returns. My $.02

    1. Matt Thiel | | #2

      This is turning into a full house build, a walk out bungalow, with an ICF basement. Large West and South decks with a straight gable roof.

      I'm trying to seal and insulate enough that I could use electric only heating and loads.
      Withh a large PV on the roof.
      So far HOT2000 is saying about 16000kwh/year total load at $0.16 each from the grid.

      We need continuous heat for the plumbing. But plan to optimize ventilation and temperature while we are away.

  2. Expert Member
    Malcolm Taylor | | #3

    Matt,

    - What you are proposing will work fine and be easy to build.
    - The 1/2" gap won't be a problem if you air-seal both the inside and exterior of the wall.
    - In Alberta a rain-screen usually isn't necessary. The only worry is that some metal siding profiles are pretty vapour-tight, while some like corrugated sheets form their own rain-screen, especially if you use a perforated base flashing. You want a double stud wall with interior poly to be able to easily dry to the outside. If the siding you choose is too flat, you should be furring it out away from the sheathing and providing some ventilation.
    - You are fine using poly rather than a variable perm membrane, and also don't need mid-wall sheathing.
    - Whether to boost the wall to R-40 is a judgement call based on preference rather than any hard data. For a cabin I'd be inclined not to.

  3. Expert Member
    Akos | | #4

    One thing that makes seasonal much simpler is keeping all your plumbing including water tank in the core of the house. Even better if you can get all your P traps there as well. This way you can quickly drain down the system and only heat this one small room.

    As for walls, the gap adds about R1 to your wall assembly, so nothing wrong with the gap. Make sure the studs are staggered a bit (they don't have to be exactly 1/2 stud spacing off, just a couple of inches is enough). Walls with air barriers in the middle are hard to build, I would stick to a simple double stud. Rain screen is simple to add, unless you are going with corrugated, I would add it.

    Lot of times, in remote areas your power delivery costs can really shoot up unless you are using a lot of power. The $0.16 can easaly increase to $0.70, something to keep in mind when modeling energy costs.

    I didn't go with a heat pump for the cottage and regret it. The cost of electric heat would have already payed for a budget cold climate heat pump a couple of times over.

    1. Expert Member
      Malcolm Taylor | | #5

      That's a great point about consolidating things you worry about freezing in a small contained area for a seasonal dwelling.

  4. Matt Thiel | | #6

    Thanks Guys, That was the confirmation I was looking for.
    1. Our mechanical room is in the back(submerged) corner of the basement, and I have already thought about making the lines easy to drain and isolate. We re building at the bottom of a slope, so I am considering keeping the plumbing above the basement floor for maintenance and because I will need to lift it to the holding tank anyways.
    2. We are going to be installing roll shutters for security, but the added insulation will help when we are not using it, and turning the heat to 10-15degC. R-40 is 800kwh saving per year. I think I would have really liked to use a 12 in wall and keep the inner cavity 100% for services. but the framing offset still allows 1/2" for electrical. And the Plumbing is very minimum.
    3. For siding, we are just looking for a cost effective, fire resistant siding(not vinyl). I think a few of the architectural metal ones will leave gaps behind each lap but may still seal tight to the next level above and below. So I will look for rain screen.
    4. Our electrical costs are pretty fixed in Alberta, I used my current bill and that is the average all in number for 2 years. (0.15-0.18) depending on usage. Still with micro-generation PV the pay back is within 5-6years. I would really like to add Solar Thermal one day, but for now that is an added expense.
    5. I am looking at a cold climate heat pump, but i still need supplemental heat for the coldest days. We will have a fireplace, but we will not be there all the time.

    Thanks again!

    1. Dick Russell | | #7

      If the intention is to provide supplemental heat, I'd say skip the fireplace in favor of a properly sized wood stove, with directly ducted combustion air if the structure is very tight. An open hearth makes for far more air draw, even when the fire has mostly died but the damper can't be closed. Fireplace dampers typically don't close too tightly anyway. Further, if the fireplace would be located on an exterior wall, air-sealing the joints between walls and masonry is hard to do properly, and when there is no fire burning, the conductive masonry constitutes a large heat leak.

      1. Matt Thiel | | #8

        Sorry Dick,
        That was me staying up to late last for getting up before 5. We are doing a wood stove, absolutely, for the reasons you stated. And that reminds me, I going to go search the past posts for efficiency ideas/ brands. So far we are pretty set on a Princess from Blaze King.

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