I’m converting a garage to a bedroom and I have to level the floor. I’m going to use sleepers (pressure treated) and lay a deck on top. Should I put visqueen under the sleepers to prevent moisture wicking (this is in northwest Washington-15″ rain/yr and occasional snow). I plan to lay sytrofoam between the sleepers for insulation.
That's what I've done, not far from you. Hopefully this is a slab that is dry during the winter, with good drainage around the foundation and grade sloping away from the building.
It seems to me you could also use continuous XPS over the floor, tape the seams, and use sleepers OVER the foam, fastened thru, and get less thermal bridging. How much would it improve... who knows. No matter what you do the fasteners penetrate the plastic.
I am a firm believer in letting moisture evaporate into air.
I would put a layer of builders felt, tarpaper, under each sleeper, then lay the VB over loosely over them and push it down with the insulfoam so there was an air gap under the visqueen.
I leave the problem of venting to the exterior to you.
There are three kinds of people: Predaters, Prey, and Paladins. For the life of me, I can't see why Prey feels safer from predators by disarming and emasculating Paladins.
Sounds like you would like the Delta-FL and Dri-Floor products, which both provide a vapor barrier that has air space.