We used Fiberglass batts at the time but are now encouraging clients to use recycled denim. We are straying away from Polystyrene and expanded polystyrene insulation as much as possible because of the future impact of the material on the environment. Though it is recyclable, it is difficult to do so and would more than likely be thrown in the trash adding to the growing island of plastics floating in our oceans.
We love the idea of a building following the full cycle of life, ending even in its complete decomposition one day in the very distant future. We strive for as tight an envelope as possible but believe that with careful installation we can achieve comparable results. Will your out building be in Northern Arizona?

I appreciate you letting me know about the recycled denim - I never thought about that before. Our out building - more of a poor man's work/workout space will be in Phoenix so we need great insulation. I was inspired by the SIPs because of the insulation properties, ease of construction and affordability. Houzz just published a great article on sheds: https://www.houzz.com/magazine/how-to-add-a-backyard-shed-for-storage-or-living-stsetivw-vs~28804589