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Community and Q&A

Wall Assembly Question – Green/Cost/Durability Balance

henryrose | Posted in Green Building Techniques on

Paid member here, but mostly silent for several years. We are rebuilding from scratch after our 1.5 storey log home in Ontario, CZ6, was destroyed by fire last year. Fortunately, we all (barely) escaped unharmed and insurance is footing the bill.  However, we have to work with a somewhat limited budget. We are a family of four and intend for our two children, who love the property, to eventually inherit the home. My wife and I are in our mid-sixties and will retire there in a few years, but the four of us were living there full time during COVID and will frequently be cohabiting there for weeks or months at a time before our retirement. The property is on the shore of Georgian Bay in Ontario.

While I was hoping for double-stud wall construction, our builder (who was recommended by the insurance company) had no experience with it.  We then settled on 2 x 8 single wall construction, OSB sheathing, high density blown cellulose and 2″ of ComfortBoard or equivalent on the exterior.  As we are a bit over budget, we have reduced the square footage somewhat to 2900 sq. ft. (this may still seem large for a family of four, but makes sense given the intended uses – our son has a disability, etc.) and we are weighing the builder’s idea of:

1. switching to 2 x 6 construction and possibly switching to Rockwool batts instead of dense pack cellulose and
2. substituting the OSB and ComfortBoard for 2″ of “SilveRboard” with “let in” diagonal steel bracing. As we are on the waterfront and sometimes experience 50 mph winds, we want the building to be quiet and strong. The SilveRboard Graphite XS is made locally and is a double-faced, graphite infused, EPS (not XPS) product with R value 4.4/inch.  Apparently, their XS line is vapour permeable at 2.74 US perm/inch of thickness.  We would use either a 2” or 2.65” thickness.  Recommended attachment is with plastic capped, galvanized ring nails. Here is a link to the tech specs:

https://www.amvicsystem.com/wp-content/uploads/2019/08/Amvic-SilveRboard-Graphite-Product-Data-Sheet.pdf

And the installation manual:

https://www.amvicsystem.com/wp-content/uploads/2019/08/Amvic-Rigid-Board-Installation-Manual.pdf   

I am aware that ComfortBoard and other mineral wool products carry a high level of embedded energy.  On the plus side, ComfortBoard is vapour open, allowing drying to the outside and is fireproof.  The SilveRboard has a flame spread index of ≤25 and smoke developed index of ≤ 450. I don’t know how good or bad this is, but is obviously no match for ComfortBoard.

So the wall assembly has to satisfy our concerns (not necessarily in this order) for:

1. Fire safety (flammability, toxic smoke)
2. Vapour permeability
3. Structural strength
4. Thermal performance
5. Cost
6. GWP (global warming potential)

Arguments:

1. Using 2 x 6 instead of 2 x 8 means potentially smaller trees are cut, less wood overall, less cost but reduces room for insulation.
2. Using Rockwool (or equiv.) batts is a big cost savings, but doesn’t have the ability to absorb and slow down vapour movement the way dense pack cellulose does.
3. The SilveRboard vs. OSB and ComfortBoard may or may not be better from a GWP standpoint.  
4. Since we frequently experience strong winds, especially in winter, will the building be less resistant to wracking and/or noisier if we eliminate the OSB and ComfortBoard?
5. Cost reduction and minimal GWP are both important.

BTW, we are hoping to salvage the 150 year old, somewhat charred, square, white cedar logs and to mill them onsite into 6/4 planks to re-use in creating a faux log home look for the main floor.  There will be a WRB and vertical 1 x 4 rain screen to support the planks. There will be a conditioned crawl space for about half of the footprint and slab on grade for the remainder.  Air source, ducted heat pump, ERV and possible heat pump water heater.

Please chime in with your thoughts of how we might best proceed. Thank you in advance for your time and interest.

Henry Rose

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Replies

  1. Expert Member
    Akos | | #1

    Some of the answers depend on your utility costs.

    The original plan of 2x8+2"MW is about an R31 assembly. 2x6 with 2x6+2.64GPS is R30. Simple 2x8 24" OC with R30 batts is an R24 assembly.

    There is not enough deference between all of these to matter so all would work just as well.

    I would find out what the cost savings would be to skip the exterior insulation use that budget on better windows which would save more in energy use.

    Edit. In a place with a lot of critters, I would be hesitant to use any assembly without exterior sheathing.

    1. henryrose | | #4

      Akos, thanks for those points. It is critter country! Any other solutions come to mind?

      It would be nice to put exterior insulation money towards windows, but apparently our local code requires some exterior insulation if a heat pump is the main heat source.

      Re: R value - that helps me to not stress over that issue.

      1. Expert Member
        Akos | | #6

        Higher R value walls are only required if you have resistance backup for your heat pumps. The modern cold climate units can easily carry a place in CZ6 without any resistance backup. I would size accordingly and go for one of the other compliance packages.

        This doesn't mean you can't have a backup heat source (wood stove or fireplace), it only means it can't be part of the heat pump and needed to carry the place at outdoor design conditions.

        Even if you do want to aim for the higher R values of the electric compliance packages, you can still get there with a 2x8 24OC if you use fiberboard or wood fiber sheathing. Code allows you to comply based on U factor of the whole assembly, so in this case it would be U0.04. This translates to R25.3 which you just squeak by with 2x8 24OC with 1/2" fiberboard. The fiberboard can also go inboards if you use something like sonopan under the drywall.

        1. henryrose | | #7

          Akos, thanks again for your reply. We are planning on a fireplace, so I guess we don’t need the resistance heat backup. We will certainly request a cold temp capable ASHP. It sounds like we should go with OSB sheathing for the critters. I still am drawn to the idea of some external insulation beyond just sheathing to reduce thermal bridging of the studs. I’ll look into wood fibre insulating sheets -not sure if they’re available (and affordable) up here.

  2. Expert Member
    DCcontrarian | | #2

    I'd want to hear more about the let-in steel bracing, that sounds like a significant thermal bridge.

    1. henryrose | | #5

      DC, I’m assuming the bracing is behind the foam, so it doesn’t seem like a huge issue.

  3. StephenSheehy | | #3

    Our contractor had never done a double stud wall. But he found it pretty simple.

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