After many years in the making, and a couple of rehabs in Houston and Nashville, we've made our way to the mountains of Northern New Mexico where we will break ground this spring on our final house. Two levels into a hill and a level above. ICF for the lower levels on the three sides each that extend into the hill. Frame on the upper level. I'd go for ICF all around but the cost of concrete is extremely expensive here. Design temp here is -25 F. Big diurnal swings with as much as 50 degrees between day and night in the winter. Am planning on 2x6, sealed cavities, 2" EPS on the outside. Blown insulation. Skin will be EIFS and thin stone or cultured stone. I was planning on setting the frame walls so that the surface of its foam matches with that of the ICF where they mate. But I'm a bit confused as to how I should attach the foam. If I was using siding, I'd use furring strips and also create a rain gap at the same time. But how is the best way here?

To meet code in zone 6 you'd need a minimum of R11.25 on the exterior for dew point control, which means 3" of exterior EPS not 2". At 2" it would be somewhat risky, even in the dry desert SW, but it would probably make it if you used a broad-sheet smart vapor retarder on the interior detailed as an air barrier:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

For EIFS finishes you can cap-nail the foam in place, but would have to do something else for stone veneers.

Thanks Dana, that's exactly the information I needed. I was hoping you would reply.

There is much less point loading if you glue and nail foam to furring strips.

Even if you will rely on significant external foam for moisture control, try to stick to "breathable in both directions, more so to the cold side".

Posted By jonr on 17 Feb 2016 04:18 PM
There is much less point loading if you glue and nail foam to furring strips.

Even if you will rely on significant external foam for moisture control, try to stick to "breathable in both directions, more so to the cold side".

Use only purpose-made foam board construction adhesive when gluing furring to foam- the solvents of many adhesives would erode or degrade foam insulation.

At R11+ you don't get a heluva lot of "breathing" of water vapor through any foam insulation. The most vapor permeable would be unfaced EPS, which would come in at about 1 perm or a bit less at 3"/R12.6 for 1.5lb density "Type-II" goods. That's right on the Class-II/Class-III vapor retarder boundary. All polyiso products (even fiber faced) would be less than 1-perm, XPS would be about 0.5 perms @ 2.5"/R12.5.

For more money you can use 3"/R12 rigid rock wool which would be over 25 perms, and in your dry climate that still wouldn't present a summertime condensation risk inside the wall cavities during the peak cooling the way it would in the southeastern US. But of course there's no way to make EIFS stick to rock wool. :-)

After thinking about what has been presented back to me, I think I'll simplify things and continue with ICF for the main level. There is a much smaller top level that is not directly supported by the lower exterior walls. For this I'll stick with the wall stack up I have planned since I'll just be using EIFS Aand not stone. And yes I wish I could stick it to rockwool. So using unfaced EPS, should I use a smart moisture barrier or a more traditional full vapor barrier? I would think a smart barrier as regardless of what the EPS perms are, the EIFS Is far lower perm, yes?