I didn't have the budget to completely finish my last barn. I did get my ceiling in and had cellulose blown in. For the walls, I used 2" Styrofoam insulation. I'd suggest calling an insulation wholesaler in your area instead of a box store, just for your own peace of mind. You'd be money ahead to insulate that barn. Have you ever worked in one where the sun is shining on the wall? The way the heat radiates inward is remarkable. With R40-ish in the ceiling and the 2" styrofoam insulation on the walls, the cool of the cement floor was enough to keep the whole building cool in the winter. I took the time to silicone between the sheets and spray expanding foam in all penetrations and gaps. You'd never regret insulating.