I just had the spray foam guy come today for prices. I have a 500 gallon propane tank next to my garage I want foamed, he said $350 for 3" and $450 for 4". I've done some searching here and it seems to me everyone is saying going more than 3" is a waste. any comments. Is it worth the hundred dollars more for 4" or should I have him spray 6" or 8". Anyone with over 3" of foam on tank with some info on this