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