In zone 4a, I am building a post frame shop.

CEILING ASSEMBLY:
I am looking at ceiling options and was considering OSB or metal. 

I will have a vented attic and blow in Cellulose insulation.

My thinking:
I think metal seams  would be simple to air seal with some of the advanced polyurethane sealants such as Vulkem 116; and, the assembly would be able to dry on the roof side of ceiling with the air flow from eve to ridge.

Is my thinking correct; or , would I be wiser to use the OSB on ceiling as well? 

WALL ASSEMBLY:
As for wall, I will likely use the OSB, smooth side showing since it it will need to be able to dry to the interior as far as I can tell and the smooth side in will allow a better look with some paint.

Wall insulation will be recycled polyiso boards with staggered joints. I have a ton on hand.

Rainscreen 
If I added 1/4″ or 3/8 thick vertical pieces of wood every 9″between metal siding and housewrap. Housewrap would be over the horizonatal 2×6 girts. Since, the metal siding would not allow drying to the outside, I thought a rain screen would allow drying to the outside through the chimney affect.

Making the 1/4″-3/8″ rain screen material would be about an hour of my time to produce with some pine logs I have on hand with my sawmill.

Thank you for any sanity you can offer to my thinking.

-Mike