I have an 18x45 from Alan's and did spray foam (not DIY) with 2.5" on the walls and 3" on the ceiling. I covered the walls with OSB and have been happy with it.
I have a Carolina Carport (18x45x12) and did closed cell with 2" walls and 3" ceiling and can really tell the difference. One day I'll add a mini-split and I'm hoping that's when I'll see the biggest difference.