I'm sure I could do a search and get the answers, but I'm being lazy. About what foam thickness is a diminishing return? In my boiler barn I'm not really space limited so if 4" is much better than 3" I'll do 4 or even 5. AND since it's indoors do you usually use polyurethane closed or open cell? Any other advice I should know before I cover my two, vertical 500 gal tanks is very, very welcome. As usual, thanks in advance. Soon gonna have me 1000 gal storage.....