12 mil sounds excessively heavy to me (I could certainly be wrong about this) - I have a 16x20 "high tunnel" type of GH that is covered with 6 mil greenhouse film and that was a bear to deal with! (Especially compared with the cheap 3-mil plastic film that I bought locally and used on my smaller 7x15 high tunnel...)
I'm sure someone here (Moonshadows?) knows more about this than I do - is thicker better?
I will say, though, that the "anti-condensation" feature would be really nice (assuming it actually works); I opted to not go that route (because I'm cheap) and there are times in the spring and early winter when it is literally raining inside my high tunnel!)
