I am making a new shop in a metal building (like a polebarn but all metal) I was thinking of going spray foam but for about 1/3 the cost I can get 2in rigid foam with the tinfoil on outside. I have seen people use that and sprayfoam with little cans around outer edge/seams. This will only be heated when I am out there. Not sure if spray foam is worth while, any issues with putting rigid foam board up against outside wall? Is it better to keep it tight to the wall or have a little air gap?