We are framing 2 houses in Vancouver, BC — a wet coastal climate — and are trying to decide on insulation methods.
We have had some discussion on using a Roxul mineral-fibre semi-rigid board instead of SM due to the Roxul’s permeability qualities. Roxul is interested in getting some residential market for what is traditionally a commercial product. May be a little more difficult to use as the density is quite a bit less and we would need to use 2 inch to get an R-value of 8. We like the fact that it may be better at drying and perhaps it is a “greener ” product, but are still puzzling through it and looking for other thoughts. We are also thinking air tight drywall, have an Arxx basement, radiant heat with an Eneready HRV, and still finalizing a solar thermal system for domestic hot water and radiant heat assist.
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I'm not sure what you mean by SM so it's hard to figure out what your comparison is to. I like the air tight drywall approach esp if you use the header wrap detail where you run a strip of house wrap or poly under the mud sill at the top of the foundation and tape it to the exterior house wrap and then run another strip between thr top plate and upper top plate and tape it to the top of the house wrap and staple your drywall gasket to it on the top plate. We've used these details quite a bit and had good success with them. These days we're spray foaming our roof decks and conditioning out attics so we don't bother with the gaskets anymore but the header wrap details still make sense because we're using JM Spider micro-filament fiberglass in our walls. Spider would make sense for your damp climate out in Vancouver do you have installers for that product out there and have you checked the price? .
I suggest that you download and read the Best Practices guide for marine climates
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/38449.pdf