My three level townhouse is leaking large amounts of air. I will be doing a blower door test when I take possession. I would like advice on the wall and ceiling assemblies separating the unconditioned attached garage from the living area. I will have access only to the unconditioned side of the assemblies (opening all garage walls and ceiling). I will not be altering the standard 2x4 wall framing, nor the joists (not sure of height). What are high performance options? The home is in Vancouver, BC.

High performance means different things to different people.

Got an R-value in mind? (Or a cfm/50 number you're shooting for?)

"Leaking large amounts of air." usually means things like unblocked uninsulated joist bays or other large holes in what should have been air-tight sheathing/gypsum, some of which may not be fully fixable from the garage side. Blowing cellulose or new-school supefine fiberglass blowing wools into existing wall cavities can tighten things up a LOT without having to open up the walls (beyond the blowing holes.) If in the process you discover that one or more stud bays are taking 5x the amount of material it means you've discovered where some of the BIG holes are.

I don't have an R-value or cfm/50 that I'm trying to reach. I think that I'm pretty limited since I'm not increasing the depth of the joist bays. I will be opening the walls and ceiling up regardless, so that's not a concern. Is blown cellulose still the best option? Open cell spray foam? Polyiso rigid foam, or some combination? I will also be trying to spray foam/caulk all air infiltration areas (between subfloor panels, bottom plates, rim joists). I'm extremely new to this way of thinking, so all suggestions are welcome.

If it's all going to be opened up you should be able to caulk the framing to the interior gypsum (and build inair-barrier blocking where it may be missing from the original construction) before insulating. Be sure to caulk seams between doubled headers as well.

Open cell foam is usually the most air-tight/least convective stud bay cavity-fill (tighter than closed cell foam, if done at full stud depth!)

If you added an inch of rigid foam (any type) under the new finish gypsum it would bring the 2x4 wall's thermal performance up to or exceeding the performance of 2x6 construction. Foil faced polyisocyanurate (aka "iso", "PIR", "polyiso") would be the easiest to air seal, but if the house was built with a polyethylene vapor retarder you SHOULD NOT do that, since foil + poly = moisture trap. Unfaced EPS would be the most vapor-open (important, if there is pre-existing poly) , and greener than XPS from a carbon footprint point of view, since the HFC134a blowing agent used for XPS has ~200x the greenhouse gas potential of the pentane used for blowing EPS and iso.

...(double post)

Can you give me an example of air-barrier blocking?
Why is caulking the stud to drywall connection important? Am I trying to eliminate air movement within the wall cavity? If so, I guess I should seal all horizontal wiring and plumbing penetrations.

Since I've begun learning about green building and superinsualting, I've understood that poly vapour retarders are less than ideal. Can I cut out the poly (width of stud bays, leaving it on interior of studs), then use open cell foam faced with polysio? Does that give me the code required vapour retarder?

I also read on this site that both EPS and XPS are significantly environmentally damaging, with polyiso being the "greenest" of the rigid foams.

It's perhaps easier to point to instances of where air-barriers are absent. It's pretty common in some areas to discover that there is nothing but air between a tub-surround and naked batt insulation on exterior walls, leading to big air leaks/thermal bypass into floor or ceiling joists, etc.  If you find something like that when you peel the exterior side, put SOMETHING in there to block the free passage of air.

Yes, you SHOULD be air-sealing all electrical & plumbing penetrations between stud-bays, as well as where they exit/enter the wall. This is more important when using fiber insulation, but even with open-cell foam the air seal isn't always perfect.

While there's a huge delta in environmental damage between EPS and XPS, it's not clear that iso is dramatically greener than EPS. Both EPS and iso are blown with pentane (~7x CO2 on greenhouse power) compared to XPS which is blown with HFC134a (at ~1400x CO2 greenhouse power.) 

By some estimates the lifecycle greenhouse potential per unit-R of EPS only 13-14% higher than that of polyiso, which is not a dramatically higher hit.

By comparison XPS has more than 500x that of iso (according to that source.)

Open cell spray foam delivers about ~44% more lifecycle greenhouse punch than iso, 26% more than EPS.

These are pretty squishy numbers, and it will vary by exact manufacturer & exact product, but in my mind open cell foam, EPS and iso are all pretty much in the same class, a class VERY DISTINCT from XPS  or 2lb closed cell polyurethane spray, both of which use HFC blowing agents with high greenhouse gas potential. The chemical feedstocks for the different polymers vary somewhat, but there's nothing distinctly worse about styrene vs. polyurethane vs. polyisocynurate.

The biggest environmental hit with EPS is it's ubiquitous use in one-time-use "disposable" packaging or packing materials and it's low scrap value, resulting in a large amount of it in the biosphere (with a millennium long half-life in the environment).  But that has little bearing on it's relative impact when used as a more permanent building insulation.  (Iso is a more stable polymer, with a much longer half-life.)

Finding thin-stock standard densities of iso without foil facers is difficult (and you don't even want to know what unfaced or fiber faced 4lb density rigid-board iso costs!), but unfaced Type I &  type II EPS is widely available in reasonable thicknesses.