We've been spending weekends in our Morton-brand post-frame building for the last eight years. It was originally going to be a shop, but delays in moving to the woods have slowed us down. I didn't put much thought into the construction, but it has been very energy efficient and zero maintenance.

We're located in central Missouri, zone 4.

We're now ready to start building a small home on our Ozarks land. I'd like to upgrade the insulation to the highest level that's economical, but Morton seems to be stuck on their 7.5 foot wide, 6 inch thick fiberglass blankets between the posts.

The walls will be (from the outside in):
--- High-rib steel siding
--- 1" of Thermax (polyiso foam with foil face), taped
--- 2x4 horizontal girts 24" OC
--- 6x6 laminated posts 8' OC
--- Air barrier (to be determined)
--- 2x3 horizontal nailers, 24" OC
--- Drywall.

The gap between the air barrier and the drywall will provide space for wiring without penetrating the barrier. I'm planning on using a flash of closed-cell foam to seal the gaps that are inevitable with the steel siding (top, bottom, windows, doors, etc.) This is to keep critters out as well as to stop air movement in the wall. The balance of the insulation would be blown-in cellulose, filling the space between the air barrier and the steel siding.

When I run a WUFI simulation, it indicates that I'll end up with damp insulation against the outside during the winter. Changing the air barrier to poly stops the moisture buildup in the sim, but now I've got a wall that can't dry if water DOES get in.

The Morton engineer says they always spec a poly barrier for interior finished structures (with their big blankets), and they have built tens of thousands this way. Of course he also says that the air leakage through the gaps in the siding is good for the building.

The building will have an insulated slab (2" XPS) and perimeter, and cellulose blown into the attic. I anticipate it being tight enough to need controlled ventilation.

Does anyone have any thoughts or feedback on my plan? Thanks!

I've seen examples of barrier on both sides working far better than outside only, but that was in colder climates. Can you move the iso to the interior and get some air flow behind the siding (ie, a rain screen)?

House,
are the 2x4 horiz. girts bypassing the 6x6 posts or set in between the posts which are actually 2x6-2x8-2x6, with the girts bypassing the outside 2x6 and bumping into the 2x8 center lam? Complicated question, but simple reason.
If the second option is correct, have Morton (or Stockade) or whoever build your posts in reverse, so that the center lam extends to the interior of the building. This puts the girts on the interior of the wall, replacing the 2x3 nailers. Now, order some 5.5" nailbase panels (OSB on one side only) from your local SIPS supplier. Order 4x8 or 8x8 panels, install from the outside horizontally between your posts. This gives you an OSB sheathing just as you would have on conventional construction. Install a tyvek type rainscreen and then your steel siding. Finish the interior to your specs. This is the simpliest, most economical method for this type conversion.

wes is brilliant, but my shed is up.

I think I will 2# foam all to two inches, blow cellulose in the walls and dropped ceiling.

Thanks, Wes. The posts are 3 2x6s as I recall. I've got a couple of SIP suppliers not too far away.

The foil facer of the iso on the exterior impedes the ability for any of it to dry toward the exterior. Using something semi-permeable like 1-1.5" of XPS would help, as long as you also have at least 1/4" (6mm) of rainscreen gap between the rigid foam and the steel siding.  Use mesh-type rainscreening material such as gets used behind stucco  as a critter-barrier rather than foam.  eg: http://www.spycor.com/Mortairvent_Stucco_Rainscreen_p/mav.htm  (there are many others.)

See what the sim does for you if you used 3/8" fan-fold XPS, or 1/4" OSB painted with vapor retardent latex primer (~0.5perms) as the interior air barrier vs. poly.  I suspect you'll get satisfactory results with even a modest rainscreen, ~0.75-1-perm XPS on the exterior and 0.5-1.5 perm goods on the interior air-barrier.

Your plans on making green building is very good plan and designing.