Subfloor, insulation, vapor barrier, thermal bridging

[MiLochiNya]

Hey Guys,

I am trying to figure out the best flooring options:

I need to keep thickness down. I found 3/8" fan fold, polystyrene, R3.6. I was planning on liquid nailing this down to the metal (Rustoleum Enamel, 2 coats) and then using 11/32" sheathing plywood on top of this. All of the metal is sealed from the outside.

I was curious to figure out how to put a vapor barrier under the insulation while still using the liquid nails to firmly secure the insulation and subfloor. Do I need a vapor barrier here? I read about sweating but the polystyrene is not affected by moisture.

Do we think that 11/32" sheathing plywood is too thin for subfloor? I MUST keep the insulation, subfloor, and flooring under 7/8" in order to have enough headroom.

I do not want to screw in subfloor in an attempt to mitigate any thermal bridging throughout the interior of the bus.

[superdave]

any thermal break is good. I screwed the hell out of my subfloor, the thermal bridging from the screws is nothing compared to the factory windows I'm leaving in. the fanfold will act as a vapor barrier

[WhiteWhale]

I would not use any additional vapor barrier. The metal floor is exactly that, and if you add one you will trap vapors between the two.

I used 1/2" foam board and 3/4" dry ply structural plywood. I HIGHLY recommend a 3/4" T&G structural plywood if you are going to screw anything at all to the floor, and NOT using OSB as it will be ruined by moisture instantly.

I glued my foam down first then plywood to that. I also ran 2x2 pine boards down edges, screwed into the walls and plywood to help keep everything in place. No screws in floor needed.

I realize with 3/4" ply you wouldnt have much room for insulation thickness. Maybe just use reflective? I honestly dont really feel like my 1/2" foam has been very effective anyway...

Good luck.