It might not be a great idea. A municipal stadium (near Newport News, VA, if
memory serves) had foam insulation on their roof, and the local fauna
(seagulls) ripped it to shreds. It had, unfortunately, an attractive aroma.
Fish oil in the paint, I believe, was the particular issue.

If this is a simple shed roof, with air circulation over and under, it won't
benefit much from insulation anyhow. And, if there's no circulation underneath,
it'll trap water and cause container rust (or worse, harbor skeeters).