The reason for wanting to use the spray foam is because I thought that the critters wouldn't feel any heat from the tank and not burrow into it. I already have a shed around it with foam board and bubble wrap, not insulating very well though. My thinking was either cellulose or spray foam. Cellulose will be about 200$ and spray foam 450$ for 4". The cellulose would need a very air tight and water tight enclosure (more money) where the spray foam would be more forgiving. I am still unsure which way to go as the cellulose will be alot of work to try to make no air movement in a box outside, where as the foam is sprayed and your done.