I have a worn out 8mil cover now, and it's pretty effective at keeping the heat in over night (we can see 40's even in the summer.)
I need to replace it, and have read that because of longevity, buy the cheapest one you can, which would be 5mil. The 8mil is about $8 more for my 15x30, and the website has a graph showing better heat retention/warming with thicker covers. I don't want the really thick ones, too heavy, and a waste of money.
Does anyone have any experience that they can share in terms of effectiveness for different mils?