After months of deliberation, crunching numbers and working with my contractor I think I finally know how my new home will be constructed. I live in Southern Illinois Zone 4. Initially I wanted to put foam on the outside and fill my 2 x 6 walls with dense pack insulation. My contractor wasn't real comfortable with it as he had never tried it and we are using metal roofing and siding to give the look of a pole barn and he worried about the long screws. Next I thought I would go ICF, I really like the safeness and the energy efficiency. When I really set down and started crunching real numbers I found I couldn't afford both the ICF and the Geothermal that I want. My wife, myself and my contractor have all found something we can agree on and we can afford and its the offset 2x4 construction with a 10" top and bottom plate effectively giving me a 9.5 inch cavity to insulate. I can go one of two ways with this. I can used closed cell to spray in 1.5 inches to seal it and then dense pack with cellulose or I can simply fill the entire thing with dense pack cellulose. What are the advantages and pit falls of both methods? Where would I need a vapor barrier in both cases. We will have OSB on the outside of the wall. Do I need house wrap under the metal? Thanks in advance for the advice and suggestions.

I apologize for the formatting, I can't seem to get it to show separate paragraphs.

Will the siding allow the wall to breath to the exterior (ie, is there an air gap)? I'd at least consider Zip sheathing + taped joints for air sealing. Also consider putting rigid foam on the inside.

In a zone 4 climate vapor barriers (and even closed cell foam) can be more of a risk than just dense-packing an offset double-studwall. Adding 1.5" of closed cell foam inserts a ~0.75 perm class-II vapor retarder into the mix, and if it's on the interior side of the exterior sheathing it creates a condensing surface, increasing the wintertime moisture uptake of the cellulose, limiting it's ability to dry toward the exterior, and limiting the ability of the sheathing to dry toward the interior.  While it'll probably still work, it's a net cut in resilience.

It's useful to have a vented rainscreen space (as jonr suggests) between the siding and exterior sheathing, and have at least 1' of roof overhang per story (both at the eaves & rakes) to limit bulk-water penetration.

If you install any vapor retarders at all, an interior side "smart" vapor retarder like Intello Plus or Certainteed MemBrain would offer a modest amount of resilience, but it's not really necessary in your climate.

Code does not require vapor retarders of any type for zone 4A, but if you're putting vapor retardent foam on the exterior side of the assembly it's better if it's R-value is more than 16% of the total R (as in the prescriptive stackup for marine zone 4C) .

Thank you so much for your insight. He will have 1X3's on the OSB to screw the metal down. If I understand correctly that should suffice for the air barrier.

Thank you for the info on the closed cell Dana, I would prefer not to use it. I think I will simply fill the 9.5 inches with dense packed cellulose achieving a 36-38 R value in my walls and I will have R60 in my vented attic. While it certainly won't be in the realm of being a passive home it should provide us an energy efficient home for many years at a reasonable price..

Thank you all very much and I am sure I will have more questions as the build gets nearer.