Here is the truth concerning floors as I see it. You don't want to trap the moisture between the metal floor and anything. The moisture should be allowed to travel up and out. Plastic on top of metal is just wrong, for that reason. Foil between the metal and the floor is just wrong for that reason. People don't realize why the vinyl was laid down to begin with. It is to provide a non-slip surface for people to walk on and to keep water from going onto the plywood or metal when school bus drivers have to hose the actual puke and caca out of their bus when little Joe has his accident. Why in the world doesn't anyone on this board notice that no manufacturer has ever put a barrier between the wood and the metal. The manufacturers are not stupid, and they're not that stupid not do do it if within the realm of physics it were proper to do so. Does anyone have any idea how rusted a floor would be if water from a bus driver's hose got caught between a plastic barrier and the floor? Does anybody know that that is the reason you see rust at all? Does anybody know why bus floors are usually rusted at the sides and where the seat bolts go through? Well, it is because the side is where the water that doesn't run off the top(out the back door because most drivers park on a slope when hosing out their bus), runs out.
Now most skoolie people aren't going to be hosing out their bus every week, they have fancy floors to protect. The proper thing to do is lay 'red rosin' paper between the metal floor and the plywood. It is a hydro-absorbent paper that wicks moisture away from the metal and allows it to migrate in a timely manner through the wood floor and out, while providing a conduction barrier(insulation ya) between the metal floor and the plywood or unfaced insulation material. All builders of houses with metal roofs laid down directly onto plywood use this, so their fancy roofs won't rust out within a year. Sheet metal stored outside, one on top of the other will rust out this way also. Little do the people here know that if they put plastic between the metal floor and and their foam or plywood the only way that trapped moisture can escape is through the screws they fastened everything down with. I also note that it is proper to put pinholes in the foam, here and there, for the moisture to escape. And LOL I read here how they want to seal the tops of the screws.
Wick that moisture out T.C. and then you can reside in a non-stinky, mildew free castle on wheels. Here now I give you two proper formulas, which I may pull off the board if arguments ensue.
1) metal floor to red rosin paper to plywood.
2)metal floor to red rosin paper to non-faced foam to plywood.
Dang. Here is one more, I like;
3)metal floor to red rosin paper to salvaged one hundred year old oak flooring.
Oh yeah, one more;
4)metal floor to plywood.
Moisture is the enemy, be it in your socks or where the sun doesn't shine, unless you're in the desert or running a wood stove in the dry northern winter.
Remember the metal floor IS your moisture barrier FROM THE BOTTOM OUTSIDE.
All of the above is just my opinion. Only take advice from a communist with a 'license', because they have been anointed.
Funniest thing I ever saw was an old boy insulating the ceiling in his house in Port Aransas. The sapper used a vapor barrier pink in the ceiling
. His A.C. had to run twice as long to give the same comfort level because he had trapped the moisture in the rooms. Plus after a while you could see the sheetrock tape letting loose on his ceiling. Being the stubborn type, he just got angry if you would bring it to his attention that he was an idiot.
Ok, I've said my piece concerning this topic, and T.C. , Ezbme and Smitty thank you for your kind words in my bus adventure post, they mean a lot to me.