If you have wood heat, you will be fine. The stove will provide dry radiant heat, and it will also constantly change the air. I haul a tool trailer around, its one of those, well actually you can't fling a dead cat without hitting one now... It has plywood floors, just painted. I am constantly tracking in water, plus hanging up my wet rain gear, I built a tiny wood stove for it, and it dries the trailer right out. When we have freezing, or near freezing weather, lighting the stove for a half an hour will keep the trailer above the dew-point inside, and voila, no more condensation on my beloved tools.
I would try and find some closed cell foam (rigid) to lay between the sleepers, if you seal in the air gap, you will have condensation in there.
Or, you can do like we do in wood boats, leave the ends of the sleepers exposed, under cabinets or furniture, and not use any insulation. The air gap you leave will give you some insulation, and leaving room for the air to move through, will guarantee that you won't be likely to ever have problems, as any moisture will be able to evaporate. Of course we also vent the cabinets low and high for the same reason.
I think that trying to seal this up could be a problem. I see a lot of mould on the job. Often from this sort of thing, trapped condensation.
If you want to stay warm, I'd be tempted to just lay some of that foil backed bubble wrap insulation on the bottom of your sub floor, staple it down, turn it over and screw it down. Your best defense against moisture is the wood stove you are using. That's my two cents.
Sorry if I rant on and on, but this does get me going. Its similar in my mind to the new tight house full of mould and mildew, and the hundred year old house that's drafty, but will never have moisture problems.
Feel free to ignore me, and or drown me out by turning up the radio, it happens a lot!!
