how would you go about insulating a shipping container?

If they are 8' wide,you instantly lose probably 2" each side with the corrugated walls which brings you to 7'8". The most common insulating material i’m familiar with is closed cell poly-sumthin-shit which has an R value of 7.5 per inch. So 3" of that gets you 21.5. then sheet rock, so that puts you around 7' clear. All options are good options.