I'm trying to figure whether to have my weather barrier sheet in my rain screen be permeable or not. Here is the stack up from the interior. Latex paint, drywall (or possibly skimcoat plaster), 7 inches of cellulose (wall is double 2x4 with interior 2x4's horizontal), 1/2" CDX (perhaps zip system?), 2" XPS, weather barrier, 3/4" vented furring space, painted vertical board and batten cedar. My primary question is the material for the weather barrier but I snuck a couple others in there too (Zip system and skimcoat). The primary air barrier will be at the sheathing plane and I will also work to make the drywall plane as airtight as possible. Should the weather barrier be something like Vapro Shield with a perm rating >20 or should it be a vapor retarder like Grace TriFlex with a perm of <1? I have the same basic question for the roof which will be and unvented cathedral - paint/drywall/10" cellulose (2x10 rafters)/sheathing (CDX or Zip?) 4"XPS/underlayment/furred vent space/standing seam metal. On the roof I was considering going permeable with either SlopeShield by VaproShield or GAF Deck Armor due to the condensation under the metal. Thoughts?

A lot depends on your exact climate zone, but unless you live in a cooling dominated climate the exterior layers of wall assemblies needs to be >5 perms to allow the structural sheathing to dry.

Using vapor-impermeable underlayments on roofing is fine- the vapor permeance of even 30# felt and composite shingles is <0.3 perms.  If the standing-seam were purlin/furring mounted above a wooden roof deck on 2x timber you can let the roof deck dry to the cavity, but if it mounted to at the deck with an appropriate slip-surface underlayment there isn't sufficient ventilation gap to dry into anyway, and an impermeable membrane under the slip-surface material is fine.  With 4" of XPS between the vent gap you only have 0.25 perms of drying capacity, so low-perm membranes between XPS & roof deck is fine. (From a general-greenliness point of view, note that 4" of polyiso would have similar performance, but since iso is blown with pentane rather than the HFC134a used for nearly all XPS in N. America, iso would have only 0.5% the lifecycle greenhouse gas footprint of XPS- 1/200th the impact!)  With any thickness furring over 10mm or 3/8", condensation under the metal is well controlled in that stackup.  From a structural point of view you'll still probably need to use 2x furring, through-screwed to the structural rafters 24" o.c. with timber screws.

Unvented roof assemblies are better off if there is at least somewhat vapor permeable toward the interior in order to have some drying capacity, which your stackup has. But again, the stackup that makes sense is climate-dependent, and R20  outside of ~R35 cellulose would put your roof deck at risk in climate zones 6 and higher.  With metal roofing the risk is somewhat higher than with composite shingles, but if you follow the IRC has prescription for the nominal minimums on exterior R or greater you'll be fine.  But if you went with 2x12 rafters you'd have to bump it up some. (Another reason to go with iso, which would average R5.5/inch even in the coldest zones, though XPS would rise to about R5.2-5.3/inch at those same sub-arctic temps, narrowing the difference.)

Thanks for the thoughtful input Dana. I am technically in zone 5 (RI) but out on Block Island with Montauk 16 miles west of me in Zone 4. Our micro climate is probably closer to 4 than 5. I find climate ignores state lines but codes do not. I used XPS in my stack up description knowing that was the worst case - I am going to try for a load of used ISO for the greenliness and the cost savings - both of which I care about. The roofing will be in strapping above the foam - I had planned on using ripped ledger board at a full 1" instead of 3/4 strapping or 1 1/2" 2x stock. Curious why you feel the 2x would be required structurally? I do need the full 1" to get enough screw penetration to meet cladding and components requirement for 120 mph wind zone. Don't want my roof in the North Atlantic!! I am investigating roof underlayments now and will review the impermeable products. What are your thoughts on using Zip in the wall stack up instead of OSB or CDX? I like having the Zip/tape system to tighten up early on in construction and since that will be my primary air barrier plane seem zip is easier to seal up at windows/doors and penetrations. The the Zip coating introduce a vapor barrier in the wrong plane though?

For Block Island R15 would probably be (barely) enough, but it never hurts to go higher.

The furring needs to be thick enough to both retain the fasteners for the metal roofing , but also not to buckle/break under extreme uplift conditions when tied to the structure at 24" o.c. fastener spacing. If you tighten up the spacing of the timber screws for more rigidity it adds a lot of thermal bridging via the screws.
The best compromise is usually more rigid 2x furring and keep the screw spacing wide, thermal bridging low. (Steel is 1000x more thermally conductive than iso or XPS.)

Zip works, and goes up fast- it's a real labor saver, and thus much preferred by a lot of contractors in high labor-cost markets. Zip is not a vapor barrier, it's a class-III vapor retarder. At 2-3 perms it's no different from other OSB sheathing. The permeance of OSB the moisture content- and is 5+ perms when it's really soaked, but under normal moisture conditions it'll run 2-3 perms. Plywood is similar, but becomes more vapor-open than OSB can when the moisture content is high, so it dries a bit quicker, but with a 3/4" rainscreen gap you won't have any issues with soaking the wood except in a hurricane, and it would dry pretty quickly with your siding flying out in the Atlantic, eh? :-) (Seriously, the rainscreen is huge for keeping it dry in the first place and drying quickly when it does get wet.)

Thanks Dana! Sounds like I'm pretty much on the right track. I'd be really happy to use the Zip - as you say, big labor saver.
I'll look at the furring rigidity issue - the added thickness of the 2x would wrinkle a few details but should be doable.
By the way, do you have any info on relative permeance of skimcoat plaster on blueboard vs. drywall and tape? I can't seem to find that anywhere.

The permeance of either plaster or drywall is determined almost exclusively by the paint. Unpainted they're both north of 50 perms, but with standard latex interior paints it'll come in between 2-5 perms.

With the moisture buffering of 7" of cellulose cavity fill and drying capacity of rainscreened siding you don't need to lower permeance of the interior in your location. Robert Riversong builds ~12" cellulose double-stud/truss walls without sheathing or rainscreen- just shiplap siding nailed to the exterior truss wall in VT and the cooler parts of MA, and they seem to do OK. Your siding & sheathing will stay drier than his ship-lap with your rainscreen, and 7" of cellulose still buffers quite a lot.

Great! I'm pretty hyper sensative to the moisture issue as you can imagine the peas soup fog we live in all spring and summer mornings out here in the North Atlantic. Many thanks for your wise counsel. Now if I can convince my wife the wall color doesn't need to be changed every year.....

...tinted glasses can work wonders...

:-D