Hello, looking on advice on these 2 wall options
Cost and labour time is important.

This wall is to be air-tight as possible - interior and exterior tape detail, relative humidity kept in check in winter (bathroom exhaust fan... dry winters here as well), HRV in house.. interior barrier required.. but need to ensure no 6mil poly.

1. 2x8 wall

Drywall
2x3 uninsulated service wall
Membrain (need confirmation from town that inspectors will ok this)
2x8 stud wall filled with dense pack cellulose (how important is it to be borate treated)
Cross bracing for lateral strength (no sheathing)
2" Type 2 EPS (PlastiSpan® HD) -- OR -- 2" IKO Ener-Air (*I know I'm below dewpoint with this.. not ideal so looking at 2x6 wall)
Tyvek WRB
Furring Strips (airgap)
Hardiboard Siding

2. 2 x 6 wall

Drywall
2x3 uninsulated service wall
Membrain (need confirmation from town that inspectors will ok this)
2x6 stud wall filled with dense pack cellulose (how important is it to be borate treated)
Cross bracing for lateral strength (no sheathing)
3" or 3.5" Type 2 EPS (PlastiSpan® HD) (*Proper R-value in zone 6, 2x6 wall to keep dew point in foam)
Tyvek WRB
Furring Strips (airgap)
Hardiboard Siding


Thanks so much!
Kyle

All of those assemblies are R28-ish whole-wall give or take, depending on actual framing fraction, which isn't bad. Insulating the service chase stud bays takes it up to R34-ish, and probably worth it.

With the 2x6 assembly you have sufficient exterior R to use latex paint on wallboard as the interior side vapor retarder, but since you're going to need something between the structural studs and service chase, MemBrain would be a good choice. Air-tight half-inch OSB on the interior side of the structural studs would also work there. OSB is a "smart" vapor retarder, and it would allow you to skip the cut-in bracing. It runs about 1-perm or a bit less when dry, but hits 5 perms or so when the moisture content is high- similar in character to #15 felt. It's more expensive, but also more rugged, harder to puncture or tear than MemBrain (which is only 2-mils thick nylon, which is fairly stretchy, and won't hold back dense-pack very well on it's own.) A mid-layer OSB air barrier/vapor retarder is an approach taken in many PassiveHouse wall designs.

Without wood sheathing on the cold side of the assmbly to worry about the relative advantage of cellulose is small- neither fiber insulation nor EPS is affected by modest and temporary condensation events. Done correctly, using high density rock wool batts (R30 in the 2x8 or R23 in the 2x6) would give you another R1-ish boost in whole-wall performance over cellulose, and is likely to cost less.

Caulking the framing to the EPS at every stud bay will be worth it (more so with rock wool or wet-sprayed cellulose than with dense pack).

Insulating the 2x3 service stud wall with split /compressed batts bringing it up to R34-ish whole-wall performance yields about a 20% reduction in wall losses, which is right in the financial sweet zone. BSC's BA-1005 guesstimate is that in most zone 6 locations R35-ish whole-wall wall assemblies are still financially rational on long-term energy cost savings, if you look at the zone-6 line in Table 2:

http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

Why do all that when a Sips panel construction can do it better?

Once you are considering 8" studs, I'd also look at staggered 4" studs (spaced as far apart as you want). Rigid foam is not required.

Can you get straight 2x8 studs in your area? In my area, once you get to 2x8 size, everything is Southern Yellow Pine (SYP), and mostly new fast grow pine that is rarely straight. 2x4 and 2x6 studs are typically Spruce-Pine-Fir (SPF) and relatively straight. Plus, 2x4 and 2x6 can be had in pre-cut stud lengths. I've never seen 2x8 in pre-cut stud lengths. If your framing crew has to cut every stud to length, that will add to the building cost.  If considering a 2x8 thickness wall, I would definitely consider double 2x4 stud walls with the inner and outer studs off-set to reduce thermal bridging.

EZ SIPS are simple low cost EPS panels routed out to build into standard framing, and will give you an R-25 2x8 thick wall with 2x6 lumber. Our built in 1.75" thick thermal break eliminates thermal bridging to significantly increase insulation efficiency. Please inquire at www.structuralinsulatedpanels.com