A tad complicated - please bear with me. The living room in the house I just bought (built in 1950) has a walnut floor floating over a concrete slab (no basement). The ~3 inches between the walnut and concrete is currently used for air return to the furnace (return vents near the perimeter of the room). I'm probably not intereted in continuing this use - while I can't find any active water issues in the room, the vents have a strong "wet basement" smell that is transmitted to all supply vents in the house when the blower is on. Even if I took up the wood floor in the living room and sealed any possible moisture sources, from there the air return runs under some heavy stonework that I can't take up, on its way to the furnace, so I don't think it's salvageable. If I abandon the supply system in this room (enlarging a return vent/duct in another room to compensate), should I consider putting any sort of loose insulation under the walnut floor? (I live in Wisconsin) If so, what material could I get down there without taking up the whole floor, and ideally resistant to low levels of moisture? THANKS for any help you can offer.