We should sticky the many posts we've made about how to easily (and inexpensively) take a pole barn from an ag. shed to a permanent structure.
Short and sweet:
- protect the posts (either perma-columns [which will add about $200 per post], or less expensiveily with post sleeves)
- protect the skirt board (composite wraps... sold by same companies as post sleeves)
- To finish interior walls: horizontal studs face fastened to inside of posts at 24" OC, R-19 kraft faced batts will pressure fit between interior and exterior horizontal studs, run electric/plumbing etc. and finish interior walls same as in any build.
- To finish ceiling: Specify trusses intended to carry weight of ceiling finishing. Furring strips (I prefer 2x4's) to bottom cord. Insulate and finish as with any type of build.
- To finish exterior: Steel will be cheapest (doesn't require sheathing followed by siding) and lasts a long long long time. But, you can add sheathing and wrap and siding with whatever other material you want, same as any other build. If you go with steel roof, snow jax are worth considering to prevent snow slides. I always recommend gutters.
If cost is a driver, a pole barn built on the agricultural shed side of the spectrum is by far the least expensive option in almost all situations. As you approach a highly appointed product with high end finishes, the cost advantage vs. other building methods decreases (but should still be about 10% cheaper due to speed of construction style).
Built at the high end, post frame buildings (aka pole barn) are extremely strong, permanent, energy efficient, and flexible style of construction. Built at the low end, they'll still usually give you 20+ years of service.